Eiichi Furusato

Tolerance for Prolonged Darkness of Three Phytoplankton Species, Microcystisaeruginosa (Cyanophyceae), Scenedesmusquadricauda (Chlorophyceae), and Melosiraambigua (Bacillariophyceae)

Phytoplankton community dynamics are affected not only by atural events such as overwintering but also by artificial events such as artificial circulation and related darkness conditions. In order to clarify the effect of tolerance for prolonged darkness on community succession, laboratory cultures of three phytoplankton taxa, Microcystis aeruginosa(Cyanophyceae), Scenedesmusquadricauda(Chlorophyceae), and Melosiraambigua(Bacillariophyceae) were carried out in darkness. The period of darkness was varied: 5, 10, 15, 20 days, and the control. Thereafter, all samples were reilluminated. After more than 10 days of darkness, M. aeruginosa decreased markedly with the length of the darkness period and was reduced to only 1% of the initial cell number after 20 days darkness. In contrast, S. quadricauda and M. ambigua retained their biomass even after 20 days of darkness. After restarting the light–dark cycle, however, all three species similarly increased exponentially and reached their maximum biomass levels. These results suggest that differences in tolerance for prolonged darkness may cause the succession of the phytoplankton under certain conditions.

Dynamics of particulate phosphorus in a shallow eutrophic lake.

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.

A dynamic model of darkness tolerance for phytoplankton: model description

To analyze various effects of prolonged darkness on phytoplankton population dynamics, we developed a dynamic model of darkness tolerance for phytoplankton and investigated its characteristics. To construct the basic concepts of the model, we categorized various changes in abundance of phytoplankton both during prolonged darkness and after reillumination into several patterns, and then considered the physiological processes producing these patterns. The nature of darkness tolerance was considered to incorporate previously experienced light conditions, including darkness, as a physiological activity, and members of the same phytoplankton species exhibit different dynamics even in identical light conditions due to such career effects. Taking this into consideration, we developed a cell quota model in relation to darkness tolerance. State variables for abundance were indicated by cell numbers, and physiological condition by three intracellular carbon pools with different physiological functions. Using our model, we analyzed the various changes in abundance of phytoplankton in relation to exposure to prolonged darkness. Various responses in terms of phytoplankton abundance to prolonged darkness and after reillumination were successfully reproduced by the model that simply assumed that deterioration of physiological mechanics, such as photosynthetic functions, was due to a prolonged dark condition. On the basis of the results of calculation and assumptions for the model, we discuss the limitations, problems, and effectiveness of the model.

Considerations of Optical and Hydraulic Conditions of Cyanobacterial Growth Based on Absorption Characteristics of Antenna Pigments Using Field Observations-Antenna Pigments Buoyancy Frequency Hypothesis-

We investigated the optical and hydraulic conditions of cyanobacteria growth using the field observation data for two reservoirs in Japan. When the buoyancy frequency (N2) is greater than O(10-4)(s-2) - O(10-3)(s-2), the populations of PC type Phormidium and Microcystis increase, however, for N2 values less than that, the population of PE type Phormidium increases. From a theoretical analysis using the Wedderburn number (W), it is considered that the above level of N2 is estimated as the critical condition for vertical mixing of density stratification. Therefore, it is assumed that two antenna pigments types, that is PC and PE, of Phormidium have different suitable hydraulic conditions. Furthermore the hydraulic stability requirement for large-colony cyanobacteria that form water bloom can be estimated based on N2.

Role of organic phosphorus in sediment in a shallow eutrophic lake

We tested the hypothesis that mineralization of molybdenum unreactive phosphorus (MUP) in pore water is the major pathway for the changes in the concentration of molybdenum-reactive P (MRP) in pore water and inorganic P in sediment particles. The concentration of inorganic P in the sediment particles increased from December to April in Lake Kasumigaura, whereas concentrations of organic P in the sediment particles and MUP in pore water decreased. These results suggest that MUP mineralization plays a key role as the source of MRP, whereas desorption of inorganic P from the sediment particles into the pore water is a minor process. One-dimensional numerical simulation of sediment particles and the pore water supported the hypothesis. Diffusive flux of MUP was small in pore water, even in near-surface layers, so mineralization was the dominant process for changing the MUP concentration in the pore water. For MRP, diffusion was the dominant process in the surface layer, whereas adsorption onto the sediment was the dominant process in deeper layers. Researchers usually ignore organic P in the sediment, but organic P in sediment particles and the pore water is a key source of inorganic P in the sediment particles and pore water; our results suggest that in Lake Kasumigaura, organic P in the sediment is an important source, even at depths more than 1 cm below the sediment surface. In contrast, the large molecular size of MUP in pore water hampers diffusion of MUP from the sediment into the overlying water.