The seasonal abundance and vertical distribution of the <3-μm phytoplakton in the north basin of Lake Biwa

Effect of magnetic iron oxide (Fe₃O₄) nanoparticles on the growth and photosynthetic pigment content of Picochlorum sp

Magnetite iron oxide (Fe3O4) nanoparticles (NPs) are key materials applied in many different fields of modern technology. The potential environmental impact of these NPs is of great concern. In this study, initially the effect of Fe3O4 NPs size (20 and 40 nm) as well as bulk (>100 nm) at 200 mg L(-1) on Picochlorum sp. (Trebouxiophyceae, Chlorophyta) is investigated during the different growth phases. The most inhibitory NPs were then chosen to assess their effects at different concentrations. The 20 nm NPs at 200 mg L(-1) were found to significantly reduce the viable cell concentration and chlorophyll a content during the exponential growth phase compared to the other particle sizes. However, the 20 nm NPs at different concentrations were found to promote algal growth during the late growth stages (stationary and decline phases) compared to the control. Additionally, algae were found to accelerate the aggregation and sedimentation of nanoparticles into the medium and therefore can be considered as potential organisms for bioremediation of nano-pollution.

Production Rate of Planktonic Bacteria in the North Basin of Lake Biwa, Japan

Vertical and seasonal variations in the cell number and production rate of planktonic bacteria were investigated at a pelagic site (water depth, ca. 72 m) of the north basin of Lake Biwa during April to October 1986. The [ methyl - 3 H]thymidine uptake rate into a cold trichloroacetic acid-insoluble fraction and the frequency of dividing cells (FDCs) were measured for each sample as indices of the bacterial production rate. The seasonal data of bacterial number, thymidine uptake rate, and bacterial growth rate based on the FDCs were correlated with one another (rank correlation analysis, P < 0.05). These bacterial variables were not correlated positively with the chlorophyll a concentration. Vertically, the maxima of both bacterial number and the thymidine uptake rate were found in the euphotic zone. The direct counting of bacteria and the measurements of thymidine uptake rate combined with the size-fractionation method revealed that more than 90% of the bacterial biomass and production rate were attributed to unattached bacteria throughout the investigation period. The carbon flux estimates of bacterial production were less certain due to the variability of the conversion factor for the thymidine uptake method and that of the calibration for the FDC method, but even when the conservative range of bacterial net production rate was used (5 to 60 μg of carbon per liter per day), it can be suggested that bacterial net production in the investigated area was a significant fraction (ca. 30%) of the level of the primary production rate in the same water basin.