measuring-photophysiology-attached-stage-colacium-sp-cuvette-type

Measuring Photophysiology of Attached Stage of <em>Colacium</em> sp. by a Cuvette-Type Fast Repetition Rate Fluorometer

Fast repetition rate fluorometer (FRRf) is a beneficial method for measuring photosystem II (PSII) photophysiology and primary productivity. Although FRRf ...

National Institute for Environmental Studies

We tested the hypothesis that in shallow, eutrophic Lake Kasumigaura, the concentration of particulate phosphorus (PP) is controlled by biogenic P (P in living or dead phytoplankton and bacterial cells), rather than by resuspension of inorganic P in sediment. Increases in wind velocity and turbidity were associated with bottom shear stress exceeding the critical value for the lake (τc=0.15Nm(-2)); this increased turbidity was due to sediment resuspension. However, concentrations of PP; HCl-extractable, reactive P in PP (P-rP); and HCl-extractable, non-reactive P in PP (P-nrP) were not correlated with wind velocity (PP vs. wind velocity: r=0.40, p>0.05). Rather, the P-nrP concentration accounted for approximately 79% of PP, and the concentrations of PP, P-rP, and P-nrP were correlated with the particulate organic carbon (POC) concentration (POC vs. PP: r=0.90, p<0.01; POC vs. P-rP: r=0.82, p<0.01; POC vs. P-nrP: r=0.86, p<0.01). In our (31)P nuclear magnetic resonance spectroscopy results, mononucleotides accounted for the largest proportion among the detected P compound classes. In addition, concentrations of mononucleotides, orthophosphate, and pyrophosphate were significantly higher in samples with high POC concentrations, whereas the DNA-P concentration was not. These results suggest that biogenic P affects PP concentrations more strongly than does sediment resuspension, and the production of biogenic P creates a pool of mononucleotides, a class of easily degradable P, even in shallow, eutrophic Lake Kasumigaura. 

Dynamics of particulate phosphorus in a shallow eutrophic lake.

Burrow structures produced by various benthic animals in sediments are important components of aquatic ecosystems, allowing the circulation of interstitial water via ingress of fresh bottom water into the burrows upon feeding and intraburrow migration. Although X-ray computed tomography has been used to visualize burrow structures, it could not reveal the structures in the soft mud in Lake Kasumigaura, where evaluation of the water-circulation effect of burrows is an important issue. Here, we describe the first attempt to use magnetic resonance (MR) imaging (MRI) to visualize intact burrow structures in the soft mud sediment cores collected from a eutrophic lake. Our MRI application clarified the dynamic distribution of burrows inhabited by chironomids in the soft mud that previous studies could not visualize. By examining the relationships between the degree of chloride ion depletion in deeper layers and the burrow density calculated from the MR images, we were able to consistently explain the water-circulation effect of burrows, suggesting the higher reliability of burrow density calculated from MR images. In addition, we were able to evaluate the activity of burrows, which is difficult to achieve in sediment core experiments. We observed a smaller water-circulation effect of burrows on ammonium ions than on chloride ions, suggesting the enhancement of ammonium production or release in burrow-rich sediments.

Magnetic Resonance Imaging as a Novel Method for Elucidating Sediment Burrow Structures and Functions.

Direct measurements of gross primary productivity (GPP) in the water column are essential, but can be spatially and temporally restrictive. Fast repetition rate fluorometry (FRRf) is a bio-optical technique based on chlorophyll a (Chl-a) fluorescence that can estimate the electron transport rate (ETRPSII) at photosystem II (PSII) of phytoplankton in real time. However, the derivation of phytoplankton GPP in carbon units from ETRPSII remains challenging because the electron requirement for carbon fixation (Фe,C), which is mechanistically 4 mol e- mol C-1 or above, can vary depending on multiple factors. In addition, FRRf studies are limited in freshwater lakes where phosphorus limitation and cyanobacterial blooms are common. The goal of the present study is to construct a robust Фe,C model for freshwater ecosystems using simultaneous measurements of ETRPSII by FRRf with multi-excitation wavelengths coupled with a traditional carbon fixation rate by the 13C method. The study was conducted in oligotrophic and mesotrophic parts of Lake Biwa from July 2018 to May 2019. The combination of excitation light at 444, 512 and 633 nm correctly estimated ETRPSII of cyanobacteria. The apparent range of Фe,C in the phytoplankton community was 1.1-31.0 mol e- mol C-1 during the study period. A generalised linear model showed that the best fit including 12 physicochemical and biological factors explained 67% of the variance in Фe,C. Among all factors, water temperature was the most significant, while photosynthetically active radiation intensity was not. This study quantifies the in situ FRRf method in a freshwater ecosystem, discusses core issues in the methodology to calculate Фe,C, and assesses the applicability of the method for lake GPP prediction.

Koichi Shimotori

Lake Biwa Branch Office,
Center for Regional Environmental Research

National Institute for Environmental Studies

Measuring Photophysiology of Attached Stage of Colacium sp. by a Cuvette-Type Fast Repetition Rate Fluorometer

Koichi Shimotori

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National Institute for Environmental Studies

Measuring Photophysiology of Attached Stage of Colacium sp. by a Cuvette-Type Fast Repetition Rate Fluorometer

Lake Biwa Branch Office,
Center for Regional Environmental Research