This study was supported by the Czech Science Foundation under the research grant 13-00243S and 19-16554S awarded to K. &#352;. and D. S., respectively. This article was also supported by the project &#34;Biomanipulation as a tool for improving water quality of dam reservoirs&#34;&#160;(No CZ.02.1.01/0.0/0.0/16_025/0007417), funded by the European Regional Development Fund, in Operational Programme Research, Development and Education.

1. Sample collection
<ol>
	<li>Collection of reservoir water sample: the first case study (Exp I; lower natural in situ predator and prey abundance system)
	<ol>
		<li>Collect water samples from the desired location at a suitable depth. Keep the samples in a temperature-controlled cooler filled at in situ temperature (avoiding temperature shock; it should be noted that uptake rates of protists are temperature-dependent) during transport to the laboratory. 
		NOTE: Our sampling was conducted in the meso-eutrophic canyon-shaped &#344;&#237;mov reservoir (South Bohemia, volume 34.5 x 106 m3, maximal depth 43 m, mean retention time 100 days, dimictic). The sampling site was located at a 30 m depth, close to the dam. A mixed sample at a 0.5 m depth was collected by a 2-l Friedinger sampler.</li>
		<li>Continue to section 1.2 as soon as possible.</li>
	</ol>
	</li>
	<li>Collection of fluid from the traps of carnivorous Utricularia reflexa plants: the second case study (Exp II; a system with high predator and prey abundances, extremely small sample volume)
	<ol>
		<li>Gently shake the plants underwater, remove from the cultivation container, and place them on absorbent material to absorb excess water. Depending on the robustness of the plants and size of the traps, choose 8-10 plant individuals.</li>
		<li>Divide each shoot into roughly equal parts by counting leaf nodes bearing trapping organs. Each shoot segment will serve to collect mixed samples representing young, middle-aged, and old traps.</li>
		<li>Attach a thin glass capillary and Eppendorf vial for sample collection to a peristaltic pump. Proceed to insert the capillary tip into the trap opening. Using the vacuum pump, suck out all the fluid from each trap until 900 &#177; 100 &#181;L of trap fluid is collected for each trap age category.</li>
		<li>Use triplicate subsamples of 200 &#181;L of the pooled trap fluid for protistan grazing experiments. Process these immediately as detailed in section 3. Preserve the remaining &#177;300 &#181;L of the sample for all other analyses of microbial components living in the fluid, as detailed below (section 2).</li>
		<li>Proceed immediately to section 2.</li>
	</ol>
	</li>
</ol>
2. Fixation of collected samples
<ol>
	<li>Exp I and II: fix water/trap fluid sub-samples for bacterial enumeration (section 4; approximately 20 mL and 0.3 mL, respectively) with formaldehyde for at least 1 h to obtain a 2% final volume:volume concentration in each sample. 
	NOTE: Handle formaldehyde exclusively in the fume hood, and wear gloves at all times while manipulating samples.</li>
</ol>
3. Sample filtration
<ol>
	<li>Sample dilution (Exp I): no dilution is needed for the reservoir water samples. (Exp II): dilute the trap fluid sample 10x-100x with particle-free MQ water to achieve a suitable distribution of the target microbes on filter surfaces prior to counting via epifluorescence microscopy.</li>
	<li>Filter 1-2 mL (Exp I) of the reservoir water or 10-30 &#956;L (Exp II) of the subsample of the trap fluid for bacterial counting onto black 0.2 &#181;m pore-size filters, using a filtration funnel (25 mm diameter).</li>
	<li>Stain the filters with DAPI (4&#39;,6-diamidino-2&#39;-phenylindole dihydrochloride, 0.2% final concentration) for 4 min. 
	NOTE: Avoid skin and working surface contamination, and wear gloves.</li>
	<li>Place the filter with concentrated microbes onto a drop of immersion oil (for fluorescent microscopy) on a microscope slide. Place another oil drop onto the center of the filter and cover with a coverslip, making sure that the oil is distributed evenly.</li>
	<li>At this point, preferably process samples immediately, or alternatively, store them in the freezer (-20 &#176;C) for several weeks to months until further analysis.</li>
</ol>
4. Enumeration of bacterial numbers on the filters
<ol>
	<li>Place the slide under the epifluorescence microscope (with the filter set corresponding to the fluorochrome DAPI). Place a 10 x 10 counting grid into one of the oculars. Move the slide to a random position.</li>
	<li>Quantify bacterial cells (blue fluorescence) in the area of the counting grid (under the magnification 1000x). In the counts, include the cells crossing the left and upper edges of the counting grid, while excluding those located across the right and lower edges.</li>
	<li>Move to another random position and repeat enumeration on at least 10-15 counting grids, amounting to 500 cells counted in total.</li>
	<li>Establish a conversion factor for a given microscope and magnification, based on knowledge of the ratio of the area of one grid to the total effective filtration area of the filter. Then, divide the total number of cells counted by the number of grids counted, yielding an average number of bacteria per grid.</li>
	<li>Multiply the latter parameter by the conversion factor established and normalize the resulting number per mL of a sample (depending on the volume of sample filtered) to obtain the total bacterial abundance per mL.</li>
</ol>
5. Determining protistan abundance
<ol>
	<li>Fix the water sample (Exp I) or trap fluid (Exp II) subsamples with either glutaraldehyde (1% final concentration, more suitable for samples with the presence of chlorophyll containing particles to be processed days to weeks after fixation) or using the formol-thiosulfate decolorization technique specified below. 
	NOTE: Both preservation techniques prevent egestion of the ingested material from food vacuoles of protists8.</li>
	<li>Regarding the formol-thiosulfate decolorization technique, add 100 &#181;L/1 &#181;L of Lugol&#8217;s solution into 20 mL/200 &#181;L of the water/trap fluid subsample (Exp I/Exp II, respectively).</li>
	<li>Follow immediately with the addition of 0.5 mL/50 &#181;L of borate-buffered formalin then 20 &#181;L/2 &#181;L of 3% sodium thiosulphate (Exp I/Exp II, respectively). 
	NOTE: The sodium thiosulphate decolorizes the yellow color of Lugol to enable observation of cells under the epifluorescence microscope8.</li>
	<li>Filter a known volume of sample (depending on the number of target protists) onto 1 &#181;m pore-size black polycarbonate filters.</li>
	<li>Estimate the number of the protist species is by counting at least 200 cells under magnification 600x using the counting grid (see above).</li>
	<li>Reduce the volume of the sample under a vacuum in the filtration funnel by low vacuum to approximately 2 mL. Then, release the under-pressure and add the DAPI fluorochrome (4&#39;,6-diamidino-2-phenylindole dihydrochloride13, 0.2% final concentration) for 2 min.</li>
</ol>
6. Determining community structure of ciliates in plankton samples
NOTE: Ciliate communities in freshwater habitats are highly diverse14,15,16,18, and their microscopic determination is challenging. Sorting the ciliate groups into functional guilds10,14,16,17 allows for more detailed analysis of different ciliate groups as pelagic bacterivores.
<ol>
	<li>Evaluate ciliate community structure by combining the following:
	<ol>
		<li>DAPI-stained samples in epifluorescence microscopy (to localize the ciliate cells with bright fluorescence of macro- and micro-nuclei of different sizes and morphology) combined with the uptake of fluorescently labeled bacteria (FLB8; for details, see below), tracking the ability of ciliates to feed on bacteria.</li>
		<li>Live sample observation in selected cases17,19. For more details of the above approaches and criteria used for grouping ciliates into different taxonomic categories, see previous publications16,17. 
		NOTE: Cited studies have indicated that among ciliates, omnivorous species from Stichotrichia(genera&#160;Halteria and Pelagohalteria) and Oligotrichia (namely Rimostrombidium spp.) are the most important pelagic consumers of bacterioplankton in a vast majority of freshwater habitats10,17,18.</li>
	</ol>
	</li>
</ol>
7. Estimating ciliate grazing rates
<ol>
	<li>Calculate the uptake rates of ciliates on bacteria based on changes in the average number of tracer [i.e., FLB8 per ciliate related to time of incubation (5-15 min)] and tracer amount of FLB added, accounting at most&#160;for 5%-15% of total bacteria.</li>
	<li>To compare uptake rates among different protistan species, normalize the uptake rates as the number of bacteria per ciliate per hour, with calculations based on the actual incubation time and proportion of tracer FLB added. 
	NOTE: The general scheme of the FLB method application to estimate both cell- or species-specific and bulk bacterivory rates in natural samples is depicted in Figure 1.</li>
	<li>Preparation of FLB from bacterial strains indigenous to a freshwater environment8
	<ol>
		<li>Select a suitable size (mean cell volume = MCV) and morphology of bacteria so that they effectively mimic the typical sizes of bacterioplankton/bacterial cells in the aquatic system being investigated. 
		NOTE: For Exp I, a mixture of isolated strains from the study location of the genus Limnohabitans and Polynucleobacter was used (i.e., typical, highly abundant bacterioplankton in lakes and ponds)20. For details on morphology and sizes of the strains, see previous publications3,17,18,21.</li>
		<li>Harvest bacterial cells from culture by centrifugation (5,000 x g) for 15 min at the early stationary phase and mix them at a numerical ratio that yields MCV &#177; SD of the cells in the mixture corresponding to the typical MCV of bacteria at the chosen location.</li>
		<li>Suspend the pellets in 10 mL of phosphate-buffered saline (PBS; pH = 9).</li>
		<li>Add 2 mg of the yellow-green fluorescing dye 5-(4,6-dichlorotriazin-2-yl) aminofluorescein (DTAF, binds to proteins) to the cell suspension in the phosphate-saline buffer, and incubate in a 60 &#176;C water bath for 2 h.</li>
		<li>After the incubation, centrifuge the cells down, decant the DTAF solution, and wash and centrifuge 3x with PBS.</li>
		<li>After the final wash, re-suspend the cells in 20 mL of the PPi-saline buffer.</li>
		<li>Vortex the FLB suspension and pipette 1.5 mL aliquots into 2 mL cryo-vials, then keep frozen (at -20 &#176;C) in PPi-saline buffer until use.</li>
		<li>Pre-filter PPi-saline buffer through a 0.2 &#181;m polycarbonate filter for use in the next step.</li>
		<li>To determine FLB concentration, transfer a small aliquot (usually 20-40 &#181;L) to 2 mL of particle-free PPi-saline buffer, sonicate at 30 W for several 2 s bursts, and filter onto 0.2 &#181;m polycarbonate black filter for enumeration via epifluorescence microscopy (1,000X magnification) under optical filter settings for DTAF (448 nm/520-540 nm).</li>
	</ol>
	</li>
	<li>Tracer technique for estimation of ciliate bacterivory
	<ol>
		<li>For grazing experiments in natural planktonic habitats, dispense 300 mL samples into well-rinsed 1 L flasks and incubate at in situ temperature for 15 min (to allow protists to recover from the handling shock).</li>
		<li>Add FLB tracers to constitute 5%-15% of total bacteria, with the amounts added depending upon the season and water temperature. 
		NOTE: There is a very broad, season-dependent spectrum in ciliate species-specific uptake rates spanning several orders of magnitude (i.e., from 101-104 bacteria ciliate-1 per hour)10,16,17,18,22,23,24.</li>
		<li>In periods of enhanced occurrence of ciliates with high uptake rates (usually during the summer), also run a parallel incubation with very low FLB additions, constituting only 2%-4% of total bacteria to avoid excessive loading of ciliate vacuoles by tracer FLB (see examples in Figure 2).</li>
		<li>Incubate ciliate/plankton samples with FLB for 5-15 min.</li>
		<li>There are two possibilities for sample fixation to prevent egestion of the ingested material from food vacuoles of protists8. Terminate the incubations by the addition of 1% glutaraldehyde (final concentration that is more suitable for samples with chlorophyll-containing particles, such as algae). Alternatively, use 100 &#181;L/10 &#181;L of Lugol&#39;s solution into 20 mL/200 &#181;L of water/trap fluid subsample, followed immediately by the addition of 0.5 mL/10 &#181;L of borate-buffered formalin then 200 &#181;L/2 &#181;L of 3% sodium thiosulphate (Exp I/Exp II, respectively).</li>
	</ol>
	</li>
	<li>After adding the fixative, let the samples rest for at least 1 h in the dark at 4 &#176;C to ensure thorough preservation of ciliate cells.</li>
	<li>Take natural plankton subsamples from 4-30 mL/10-30 &#956;L (Exp I/Exp II, respectively; the volume depends on ciliate abundance) and stain with DAPI (final concentration 0.2% wt/vol; for details, see step 3.2 above).</li>
	<li>Pass through 1 &#181;m black filters and inspect via epifluorescence microscopy to count ciliates (600x magnification) and enumerate the number of FLB tracers ingested (mostly at 1000x magnification) as detailed in previous publications2,17. Inspect samples within 7 days after preservation.</li>
	<li>To estimate total protozoan/species-specific grazing, multiply average uptake rates of all ciliates, or of only the ciliate species as detected by in situ abundance.</li>
	<li>Example of the calculation of per-cell uptake rates from in situ data from the &#344;&#237;mov water reservoir is described as follows:
	<ol>
		<li>Assume that the bacterial concentration is 3.55 x 106 bacteria/mL and tracer FLB added is 0.25 x 106 FLB/mL, which yields a sum of 3.8 x 106 bacteria/mL of total bacterial particles (natural bacteria + FLB = 100% of prey particles) available to phagotrophic protists in the natural sample. 
		NOTE: The FLB tracers added thus represent 6.58% (a project of 0.25/0.038) of total bacterial particles. The average number of FLB per Halteria sp. is 6.2 FLB in 5 min incubations.</li>
		<li>To normalize uptake per hour, use the following calculation: (6.2 x 12)/(6.58/100) = 1131 bacteria per Halteria cell/h. 
		NOTE: For more examples of distributions of individual uptake rates of Halteria sp. detected under variable water temperatures, amounts (as percents) of tracer FLB added, and different incubation times with FLB, see Figure 3.</li>
	</ol>
	</li>
</ol>

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T., Kratina, P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Diet+tracing+in+ecology:+Method+comparison+and+selection.">Diet tracing in ecology: Method comparison and selection.</a> Methods in Ecology and Evaluation. 9, 278-291 (2018).</li><li>Beisner, B. E., Grossart, H. P., Gasol, J. M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=A+guide+to+methods+for+estimating+phago-mixotrophy+in+nanophytoplankton.">A guide to methods for estimating phago-mixotrophy in nanophytoplankton.</a> Journal of Plankton Research. , 1-13 (2019).</li><li>Dolan, J. D., &#352;imek, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Processing+of+ingested+matter+in+Strombidium+sulcatum,+a+marine+ciliate+(Oligotrichida).">Processing of ingested matter in Strombidium sulcatum, a marine ciliate (Oligotrichida).</a> Limnology and Oceanography. 42, 393-397 (1997).</li><li>Massana, R., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Grazing+rates+and+functional+diversity+of+uncultured+heterotrophic+flagellates.">Grazing rates and functional diversity of uncultured heterotrophic flagellates.</a> The ISME Journal. 3, 588-596 (2009).</li><li>Gruj&#269;i&#263;, V., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Cryptophyta+as+major+freshwater+bacterivores+in+experiments+with+manipulated+bacterial+prey.">Cryptophyta as major freshwater bacterivores in experiments with manipulated bacterial prey.</a> The ISME Journal. 12, 1668-1681 (2018).</li></ol>

The authors have nothing to disclose.

Deciphering trophic interaction in aquatic systems is always challenging28, especially at the nano-plankton scales involving protists and their prey, bacteria. When it comes to nutrient uptake pathways and quantification, the application of methods successfully used at higher trophic levels is less possible, due to the high complexity of biotic interactions. These include, for example, stable isotope labeling approaches. This protocol shows the advantages of using epifluorescence microscopy and fl...

Heterotrophic prokaryotes are a key biological component in aquatic systems and account for a significant fraction of plankton biomass1,2,3. Factors that control their abundance, diversity, and activity are crucial for understanding their role in biogeochemical cycling (i.e., the fate of organic carbon and other nutrients and flow of energy from prokaryotes to higher trophic levels). Protozoan grazing is one of these important factors. Bacterivory&#160;of heterotrophic nanoflagellates and ciliates imposes a strong top-down control over prokaryotic abundance, community function, structure, diversity, and even cellular morphology and growth rate of particular bacterial groups4,5,6. In some systems, protists serve as the major cause of bacterial mortality6,7.
The standard approach used to assess protozoan bacterivory, which has been used for some time now, involves the use of fluorescently labeled bacteria (FLB) as prey analogues and epifluorescence microscopy. Cell-specific uptake rates can be determined by quantifying the number of labeled prey particles in protistan food vacuoles over a selected time course8. There are several advantages to this approach. Tracer is added to natural samples with natural predator and prey assemblages. There is minimum sample manipulation prior to incubation, minimum sample alteration by the added FLB tracer, and incubation times are short to ensure sound results obtained under close to in situ conditions. Alternatively, in environments with low numbers of bacterivorous protists or zooplankton (e.g., offshore marine systems), disappearance rates of FLB added to samples in low amounts (2%-3% tracer) can be detected via flow cytometry in long-term (12-24 h) incubation experiments. Then, numbers of FLB at the start and end points (integrating the impact of all bacterivores) are quantified by flow cytometry (for details, see previous publication9). However, such a parameter only represents total aggregated bacterivory rates that cannot be directly attributed to any particular protistan and zooplankton grazer groups or species.
Overall, quantifying the protistan species- or morphotype-specific bacterial mortality rates in the aquatic environment accurately and with ecological meaning can be challenging. Some protists are selective grazers, and the size and cell shape of the added FLB tracer may distort natural rates of prey ingestion10,11. Moreover, protistan activity and metabolism are highly temperature-sensitive12; therefore, the amount of added FLB tracer needs to be carefully manipulated for each individual sample type (not only based on the natural abundance, size, and morphology of bacteria and prevailing types of bacterivores, but also on temperature). Most studies focus on bulk protistan grazing activity; however, the bacterivory of specific protistan species often holds a much higher information value and may be preferable. In this case, taxonomic knowledge of the protist species present in a sample and understanding of their behavior is needed. Hence, considerable amounts of time and labor are required to obtain sound results on species-specific rates of bacterivory attributable to a particular protistan group or species.
Despite these difficulties, this approach remains the most suitable tool currently available to assess protistan bacterivory in natural settings. Presented here is a comprehensive, easy-to-follow method for using FLB as a tracer in aquatic microbial ecology studies. All of the mentioned problematic aspects of the approach are accounted for and an improved workflow is described, with two experiments from contrasting environments as well as contrasting ciliate species as examples.
The first case study was conducted in an epilimnetic environment from the mesotrophic &#344;&#237;mov water reservoir in the Czech Republic, which shows grazer and bacterial abundances comparable to most surface freshwater bodies (cf.5,7). The second case study was conducted in the highly specific environment inside traps of the aquatic carnivorous plant Utricularia reflexa, which hosts extremely high numbers of both grazing mixotrophic ciliates (Tetrahymena utriculariae) and bacterial cells. Calculations of cell-specific grazing rates and bacterial standing stocks in both sample types are shown. A range of ecological interpretations of the results is then discussed, and examples of possible follow-up studies are finally suggested.

<table border="0" cellpadding="0" cellspacing="0" style="width:1612px;" width="1611">
	<tbody>
		<tr height="19">
			<td height="19" style="height:20px;width:360px;">0.2-&#181;m pore-size filters&#160;</td>
			<td style="width:462px;">SPI supplies, https://www.2spi.com/</td>
			<td style="width:137px;">B0225-MB</td>
			<td style="width:654px;">Black, polycarbonate track etch membrane filters, diameter approprite for filtering apparatus used</td>
		</tr>
		<tr height="19">
			<td height="19" style="height:20px;">5-(4,6-dichlorotriazin-2-yl) aminofluorescein (DTAF)</td>
			<td>Any brand</td>
			<td></td>
			<td></td>
		</tr>
		<tr height="19">
			<td height="19" style="height:20px;">Automatic pipettes with adjustable volumes&#160;</td>
			<td>Any brand, various sizes</td>
			<td></td>
			<td></td>
		</tr>
		<tr height="19">
			<td height="19" style="height:20px;">Centrifuge</td>
			<td>22 000 x g</td>
			<td></td>
			<td></td>
		</tr>
		<tr height="19">
			<td height="19" style="height:20px;">Cryovials</td>
			<td>Any brand, 2 mL size</td>
			<td></td>
			<td></td>
		</tr>
		<tr height="22">
			<td height="22" style="height:22px;">DAPI (4&#180;,6-Diamidino-2&#180;-phenylindole dihydrochloride)</td>
			<td>Any brand&#160;</td>
			<td></td>
			<td>1 mg ml-1</td>
		</tr>
		<tr height="19">
			<td height="19" style="height:20px;">Epiflorescence microscope</td>
			<td>Magnification from 400 x up to 1000 x</td>
			<td></td>
			<td></td>
		</tr>
		<tr height="19">
			<td height="19" style="height:20px;"></td>
			<td>Filters appropriate for viewing in the DAPI and DTAF range</td>
			<td></td>
			<td></td>
		</tr>
		<tr height="19">
			<td height="19" style="height:20px;"></td>
			<td>Counting grid in one of the oculars</td>
			<td></td>
			<td></td>
		</tr>
		<tr height="19">
			<td height="19" style="height:20px;">Filtering apparatus</td>
			<td>Usually with a diameter of 25 mm&#160;</td>
			<td></td>
			<td></td>
		</tr>
		<tr height="19">
			<td height="19" style="height:20px;">Formaldehyde</td>
			<td>A brand for microscopy</td>
			<td></td>
			<td></td>
		</tr>
		<tr height="19">
			<td height="19" style="height:20px;">Glutaraldehyde</td>
			<td>A brand for microscopy</td>
			<td></td>
			<td></td>
		</tr>
		<tr height="19">
			<td height="19" style="height:20px;">Immersion oil for microscopy</td>
			<td>Specific oil with low fluorescence</td>
			<td></td>
			<td></td>
		</tr>
		<tr height="58">
			<td height="58" style="height:58px;">Lugol&#180;s solution</td>
			<td>Any brand or see comment</td>
			<td></td>
			<td style="width:654px;">Make an alkaline Lugol&#39; solution as follows: Solution 1 - dissolve&#160; 10 g of potassium iodide in 20 ml in MQ water, then add 5 g of iodine. Solution 2 - add 5 g of sodium acetate&#160; to 50 ml of MQ water. Add the solution 2 to the solution 1 and thoroughly mix</td>
		</tr>
		<tr height="19">
			<td height="19" style="height:20px;">Methanol stabilized formalin</td>
			<td>Any brand available for microscopy purposes</td>
			<td></td>
			<td></td>
		</tr>
		<tr height="19">
			<td height="19" style="height:20px;">Microscope slides and cover slips</td>
			<td>Any brand produced for microscopy purposes&#160;</td>
			<td></td>
			<td></td>
		</tr>
		<tr height="22">
			<td height="22" style="height:23px;">MQ water for diluting samples</td>
			<td style="width:462px;">Any brand 
			&#160;</td>
			<td></td>
			<td></td>
		</tr>
		<tr height="21">
			<td height="21" style="height:21px;">Phosphate-buffered saline (PBS; pH = 9)</td>
			<td>Any brand</td>
			<td></td>
			<td>0.05 M Na2HPO4-NaCl solution, adjusted to pH 9</td>
		</tr>
		<tr height="24">
			<td height="24" style="height:24px;">PPi-saline buffer</td>
			<td>Any brand</td>
			<td></td>
			<td style="width:654px;">0.02 M Na4P2O7-NaCl solution. Add 0.53 g Na4P2O7 to 100 ml of MQ water plus 0.85 g NaCl&#160;</td>
		</tr>
		<tr height="19">
			<td height="19" style="height:20px;">Sampling device&#160;</td>
			<td>Appropriate for obtaining representative sample&#160;</td>
			<td></td>
			<td>e.g. Friedinger sampler for lake plankton</td>
		</tr>
		<tr height="19">
			<td height="19" style="height:20px;">Sodium thiosulfate solution</td>
			<td>Any brand</td>
			<td></td>
			<td>3% solution is used in the protocol</td>
		</tr>
		<tr height="19">
			<td height="19" style="height:20px;">Sonicator</td>
			<td>Any brand</td>
			<td></td>
			<td>30 W</td>
		</tr>
		<tr height="19">
			<td height="19" style="height:20px;">Vortex</td>
			<td>Any brand allowing&#160; thorough mixing of the solutes and samples</td>
			<td></td>
			<td></td>
		</tr>
		<tr height="19">
			<td height="19" style="height:20px;">Water bath</td>
			<td>Any brand allowing temperature to be maintained at 60 &#176;C</td>
			<td></td>
			<td></td>
		</tr>
	</tbody>
</table>

fluorescently labeled bacteria as a tracer to reveal novel pathways of organic carbon flow in aquatic ecosystems

Elucidating trophic interactions, such as predation and its effects, is a frequent task for many researchers in ecology. The study of microbial communities has many limitations, and determining a predator, prey, and predatory rates is often difficult. Presented here is an optimized method based on the addition of fluorescently labelled prey as a tracer, which allows for reliable quantitation of the grazing rates in aquatic predatory eukaryotes and estimation of nutrient transfer to higher trophic levels.

Example experiment I was run in &#344;&#237;mov water reservoir (South Bohemia, CZ), which is a natural site with lower natural in situ predator and prey abundance. Representative data is reported for the omnivorous ciliate species Halteria grandinella, which is an abundant and efficient grazer of picoplankton (&#60;2 &#181;m) particles10,16,17,18,<sup class...

Watch this Scientific Journal Video about Fluorescently Labeled Bacteria as a Tracer to Reveal Novel Pathways of Organic Carbon Flow in Aquatic Ecosystems at JoVE.com

Fluorescently Labeled Bacteria as a Tracer to Reveal Novel Pathways of Organic Carbon Flow in Aquatic Ecosystems

Presented here is a protocol for a single-cell, epifluorescence microscopy-based technique to quantify grazing rates in aquatic predatory eukaryotes with high precision and taxonomic resolution.

Elucidating trophic interactions, such as predation and its effects, is a frequent task for many researchers in ecology. The study of microbial ...

fluorescently-labeled-bacteria-as-tracer-to-reveal-novel-pathways

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JoVE Journal

Environment

7.0K Views.  Biology Centre CAS. Presented here is a protocol for a single-cell, epifluorescence microscopy-based technique to quantify grazing rates in aquatic predatory eukaryotes with high precision and taxonomic resolution.

Video: Fluorescently Labeled Bacteria as a Tracer to Reveal Novel Pathways of Organic Carbon Flow in Aquatic Ecosystems

Alternative Method of Removing Otoliths from Sturgeon

Extracting the otoliths (ear bones) from fish that have very thick skulls can be difficult and very time consuming. The common practice of making a transverse vertical incision on the top of the skull with a hand or electrical saw may damage the otolith if not performed correctly. Sturgeons (Acipenseridae) are one family in particular that have a very large and thick skull. A new laboratory method entering the brain cavity from the ventral side of the fish to expose the otoliths was easier than other otolith extraction methods found in the literature. Methods reviewed in the literature are designed for the field and are more efficient at processing large quantities of fish quickly. However, this new technique was designed to be more suited for a laboratory setting when time is not pressing and successful extraction from each specimen is critical. The success of finding and removing otoliths using this technique is very high and does not compromise the structure in any manner. This alternative technique is applicable to other similar fish species for extracting the otoliths.

The goal of the protocol is to show an effective method to extract the otoliths from sturgeon carcasses.

Extracting the otoliths (ear bones) from fish that have very thick skulls can be difficult and very time consuming. The common practice of making a ...

Assessment of Labile Organic Carbon in Soil Using Sequential Fumigation Incubation Procedures

Management practices and environmental changes can alter soil nutrient and carbon cycling. Soil labile organic carbon, a readily decomposable C pool, is highly sensitive to disturbance. It is also the primary substrate for soil microorganisms, which is fundamental to nutrient cycling. Due to these attributes, labile organic carbon (LOC) has been identified as an indicator parameter for soil health. Quantifying the turnover rate of LOC also aids in understanding changes in soil nutrient cycling processes. A sequential fumigation incubation method has been developed to estimate soil LOC and potential C turnover rate. The method requires fumigating soil samples and quantifying CO2-C respired during a 10 day incubation period over a series of fumigation-incubation cycles. Labile organic C and potential C turnover rate are then extrapolated from accumulated CO2 with a negative exponential model. Procedures for conducting this method are described.

Labile organic carbon (LOC) and the potential carbon turnover rate are sensitive indicators of changes in soil nutrient cycling processes. Details are provided for a method based on fumigating and incubating soil in a series of cycles and using the CO2 accumulated during the incubation periods to estimate these parameters.

Management practices and environmental changes can alter soil nutrient and carbon cycling. Soil labile organic carbon, a readily decomposable C pool, is ...

Understanding Dissolved Organic Matter Biogeochemistry Through In Situ Nutrient Manipulations in Stream Ecosystems

Dissolved organic matter (DOM) is a highly diverse mixture of molecules providing one of the largest sources of energy and nutrients to stream ecosystems. Yet the in situ study of DOM is difficult as the molecular complexity of the DOM pool cannot be easily reproduced for experimental purposes. Nutrient additions to streams however, have been shown to repeatedly alter the in situ and ambient DOM pool. Here we demonstrate an easily replicable field-based method for manipulating the ambient pool of DOM at the ecosystem scale. During nutrient pulse experiments changes in the concentration of both dissolved organic carbon and dissolved organic nitrogen can be examined across a wide-range of nutrient concentrations. This method allows researchers to examine the controls on the DOM pool and make inferences regarding the role and function that certain fractions of the DOM pool play within ecosystems. We advocate the use of this method as a technique to help develop a deeper understanding of DOM biogeochemistry and how it interacts with nutrients. With further development this method may help elucidate the dynamics of DOM in other ecosystems.

Understanding Dissolved Organic Matter Biogeochemistry Through In Situ Nutrient Manipulations in Stream Ecosystems

Dissolved organic matter provides an important source of energy and nutrients to stream ecosystems. Here we demonstrate a field-based method to manipulate the ambient pool of dissolved organic matter in situ through easily replicable nutrient pulses.

Dissolved organic matter (DOM) is a highly diverse mixture of molecules providing one of the largest sources of energy and nutrients to stream ecosystems. ...

A Flow-through Exposure System for Evaluating Suspended Sediments Effects on Aquatic Life

This paper describes the Fish Larvae and Egg Exposure System (FLEES). The flow-through exposure system is used to investigate the effects of suspended sediment on various aquatic species and life stages in the laboratory by using pumps and automating delivery of sediment and water to simulate suspension of sediment. FLEES data are used to develop exposure-response curves between the effects on aquatic organisms and suspended sediment concentrations at the desired exposure duration. The effects data are used to evaluate management practices used to reduce the interactions between aquatic organisms and anthropogenic causes of suspended sediments. The FLEES is capable of generating total suspended solids (TSS) concentrations as low as 30 to as high as 800 mg/L, making this system an ideal choice for evaluating the effects of TSS resulting from many activities including simulating low ambient levels of TSS to evaluating sources of suspended sediments from dredging operations, vessel traffic, freshets, and storms.

A robust and flexible flow-through exposure system designed to maintain sediment in suspension is presented. The system is used to investigate the effects of suspended sediment on various aquatic species and life stages in the laboratory.

This paper describes the Fish Larvae and Egg Exposure System (FLEES). The flow-through exposure system is used to investigate the effects of suspended ...

Production and Measurement of Organic Particulate Matter in a Flow Tube Reactor

Organic particulate matter (PM) is increasingly recognized as important to the Earth's climate system as well as public health in urban regions, and the production of synthetic PM for laboratory studies have become a widespread necessity. Herein, experimental protocols demonstrate approaches to produce aerosolized organic PM by &#945;-pinene ozonolysis in a flow tube reactor. Methods are described for measuring the size distributions and morphology of the aerosol particles. The video demonstrates basic operations of the flow tube reactor and related instrumentation. The first part of the video shows the procedure for preparing gas-phase reactants, ozonolysis, and production of organic PM. The second part of the video shows the procedures for determining the properties of the produced particle population. The particle number-diameter distributions show different stages of particle growth, namely condensation, coagulation, or a combination of both, depending on reaction conditions. The particle morphology is characterized by an aerosol particle mass analyzer (APM) and a scanning electron microscope (SEM). The results confirm the existence of non-spherical particles that have grown from coagulation for specific reaction conditions. The experimental results also indicate that the flow tube reactor can be used to study the physical and chemical properties of organic PM for relatively high concentrations and short time frames.

This paper describes the operation procedure for the flow tube reactor and related data collection. It shows the protocols for setting the experiments, recording data and generating the number-diameter distribution as well as the particle mass information, which gives useful information about chemical and physical properties of the organic aerosols.

Organic particulate matter (PM) is increasingly recognized as important to the Earth's climate system as well as public health in urban regions, and the ...

Measurement of the Rheology of Crude Oil in Equilibrium with CO2 at Reservoir Conditions

A rheometer system to measure the rheology of crude oil in equilibrium with carbon dioxide (CO2) at high temperatures and pressures is described. The system comprises a high-pressure rheometer which is connected to a circulation loop. The rheometer has a rotational flow-through measurement cell with two alternative geometries: coaxial cylinder and double gap. The circulation loop contains a mixer, to bring the crude oil sample into equilibrium with CO2, and a gear pump that transports the mixture from the mixer to the rheometer and recycles it back to the mixer. The CO2 and crude oil are brought to equilibrium by stirring and circulation and the rheology of the saturated mixture is measured by the rheometer. The system is used to measure the rheological properties of Zuata crude oil (and its toluene dilution) in equilibrium with CO2 at elevated pressures up to 220 bar and a temperature of 50 &#176;C. The results show that CO2 addition changes the oil rheology significantly, initially reducing the viscosity as the CO2 pressure is increased and then increasing the viscosity above a threshold pressure. The non-Newtonian response of the crude is also seen to change with the addition of CO2.

Measurement of the Rheology of Crude Oil in Equilibrium with CO2 at Reservoir Conditions

A method for measuring the rheology of crude oil in equilibrium with carbon dioxide at reservoir conditions is presented.

A rheometer system to measure the rheology of crude oil in equilibrium with carbon dioxide (CO2) at high temperatures and pressures is described. The ...

Characterization of Aquatic Biofilms with Flow Cytometry

Biofilms are dynamic consortia of microorganism that play a key role in freshwater ecosystems. By changing their community structure, biofilms respond quickly to environmental changes and can be thus used as indicators of water quality. Currently, biofilm assessment is mostly based on integrative and functional endpoints, such as photosynthetic or respiratory activity, which do not provide information on the biofilm community structure. Flow cytometry and computational visualization offer an alternative, sensitive, and easy-to-use method for assessment of the community composition, particularly of the photoautotrophic part of freshwater biofilms. It requires only basic sample preparation, after which the entire sample is run through the flow cytometer. The single-cell optical and fluorescent information is used for computational visualization and biological interpretation. Its main advantages over other methods are the speed of analysis and the high-information-content nature. Flow cytometry provides information on several cellular and biofilm traits in a single measurement: particle size, density, pigment content, abiotic content in the biofilm, and coarse taxonomic information. However, it does not provide information on biofilm composition on the species level. We see high potential in the use of the method for environmental monitoring of aquatic ecosystems and as an initial biofilm evaluation step that informs downstream detailed investigations by complementary and more detailed methods.

Flow cytometry in combination with visual clustering offers an easy-to-use and fast method for studying aquatic biofilms. It can be used for biofilm characterization, detection of changes in biofilm community structure, and detection of abiotic particles embedded in the biofilm.

Biofilms are dynamic consortia of microorganism that play a key role in freshwater ecosystems. By changing their community structure, biofilms respond ...

Experimental Protocol for Examining Behavioral Response Profiles in Larval Fish: Application to the Neuro-stimulant Caffeine

Fish models and behaviors are increasingly used in the biomedical sciences; however, fish have long been the subject of ecological, physiological and toxicological studies. Using automated digital tracking platforms, recent efforts in neuropharmacology are leveraging larval fish locomotor behaviors to identify potential therapeutic targets for novel small molecules. Similar to these efforts, research in the environmental sciences and comparative pharmacology and toxicology is examining various behaviors of fish models as diagnostic tools in tiered evaluation of contaminants and real-time monitoring of surface waters for contaminant threats. Whereas the zebrafish is a popular larval fish model in the biomedical sciences, the fathead minnow is a common larval fish model in ecotoxicology. Unfortunately, fathead minnow larvae have received considerably less attention in behavioral studies. Here, we develop and demonstrate a behavioral profile protocol using caffeine as a model neurostimulant. Though photomotor responses of fathead minnows were occasionally affected by caffeine, zebrafish&#160;were markedly more sensitive for photomotor and locomotor endpoints, which responded at environmentally relevant levels. Future studies are needed to understand comparative behavioral sensitivity differences among fish with age and time of day, and to determine whether similar behavioral effects would occur in nature and be indicative of adverse outcomes at the individual or population levels of biological organization.

Here, we present a protocol to examine larval zebrafish and fathead minnow locomotor activities and photomotor responses (PMR) using an automated tracking software. When incorporated in common toxicology bioassays, analyses of these behaviors provide a diagnostic tool to examine chemical bioactivity. This protocol is described using caffeine, a model neurostimulant.

Fish models and behaviors are increasingly used in the biomedical sciences; however, fish have long been the subject of ecological, physiological and ...

Use of Principal Components for Scaling Up Topographic Models to Map Soil Redistribution and Soil Organic Carbon

Landscape topography is a critical factor affecting soil formation and plays an important role in determining soil properties on the earth surface, as it regulates the gravity-driven soil movement induced by runoff and tillage activities. The recent application of Light Detection and Ranging (LiDAR) data holds promise for generating high spatial resolution topographic metrics that can be used to investigate soil property variability. In this study, fifteen topographic metrics derived from LiDAR data were used to investigate topographic impacts on redistribution of soil and spatial distribution of soil organic carbon (SOC). Specifically, we explored the use of topographic principal components (TPCs) for characterizing topography metrics and stepwise principal component regression (SPCR) to develop topography-based soil erosion and SOC models at site and watershed scales. Performance of SPCR models was evaluated against stepwise ordinary least square regression (SOLSR) models. Results showed that SPCR models outperformed SOLSR models in predicting soil redistribution rates and SOC density at different spatial scales. Use of TPCs removes potential collinearity between individual input variables, and dimensionality reduction by principal component analysis (PCA) diminishes the risk of overfitting the prediction models. This study proposes a new approach for modeling soil redistribution across various spatial scales. For one application, access to private lands is often limited, and the need to extrapolate findings from representative study sites to larger settings that include private lands can be important.

Landscape processes are critical components of soil formation and play important roles in determining soil properties and spatial structure in landscapes. We propose a new approach using stepwise principal component regression to predict soil redistribution and soil organic carbon across various spatial scales.

Landscape topography is a critical factor affecting soil formation and plays an important role in determining soil properties on the earth surface, as it ...

Split Point Analysis and Uncertainty Quantification of Thermal-Optical Organic/Elemental Carbon Measurements

Researchers from myriad fields frequently seek to quantify and classify concentrations of carbonaceous aerosols as organic carbon (OC) or elemental carbon (EC). This is commonly accomplished using thermal-optical OC/EC analyzers (TOAs), which enable measurement via controlled thermal pyrolysis and oxidation under specific temperature protocols and within constrained atmospheres. Several commercial TOAs exist, including a semi-continuous instrument that enables on-line analyses in the field. This instrument employs an in-test calibration procedure that requires relatively frequent calibration. This article details a calibration protocol for this semi-continuous TOA and presents an open-source software tool for data analysis and rigorous Monte Carlo quantification of uncertainties. Notably, the software tool includes novel means to correct for instrument drift and identify and quantify the uncertainty in the OC/EC split point. This is a significant improvement on the uncertainty estimation in the manufacturer&#8217;s software, which ignores split point uncertainty and otherwise uses fixed equations for relative and absolute errors (generally leading to under-estimated uncertainties and often yielding non-physical results as demonstrated in several example data sets). The demonstrated calibration protocol and new software tool enabling accurate quantification of combined uncertainties from calibration, repeatability, and OC/EC split point are shared with the intent of assisting other researchers in achieving better measurements of OC, EC, and total carbon mass in aerosol samples.

This article presents a protocol and software tool for the quantification of uncertainties in the calibration and data analysis of a semi-continuous thermal-optical organic/elemental carbon analyzer.

Researchers from myriad fields frequently seek to quantify and classify concentrations of carbonaceous aerosols as organic carbon (OC) or elemental carbon ...

Fluorescently Labeled Bacteria as a Tracer to Reveal Novel Pathways of Organic Carbon Flow in Aquatic Ecosystems

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