SPRING PELAGIC COMMUNITIES OF PHYTOPLANKTON, CYANOBACTERIA, ASSOCIATED HETEROTROPHIC BACTERIA AND VIRUSES IN AN EUTROPHIC SHALLOW TEMPERATE LAKE

摘要

The high viral counts reported in aquatic systems suggest that viral infection may be an important factor controlling bacterial and phytoplankton populations in natural waters. In order to test the importance of viruses on bacterial and phytoplankton dynamics, we have taken advantage of the spring phytoplankton bloom, which occurs regularly in the shallow freshwater Vela Lake. Viral particles were counted by epifluorescence microscopy after staining with SYBR gold. Phytoplankton composition, viral and bacterial densities were determined every second day between April and June of 2006 in surface water samples (32 samples). The concentration of viruses through the spring bloom decrease from 1,27×109 viruses mL~(-1) after the Chlorophyta bloom to 5,40×l0~7 viruses mL~(-1) during the peak of Cyano-bacteria, with a mean of 2,70×l0~8 viruses mL~(-1). Bacterial numbers followed the same pattern of variation, decreasing during the bloom from 3.33×l0~8 cells mL~(-1) to l.01x10~8 cells L~(-1) with a mean of 1.64×10~8 cells L~(-1). Although viruses counts were, on average, higher than bacterial counts, yielding mean virus-to-bacterium ratio of 1.6 (range 0.3 to 7.2), the virus-to-bacterium ratio was frequently low, showing values below 1 in 43 % of the cases. The same pattern of variation of viruses and bacteria during the phytoplankton bloom as well as the significant correlation between viruses and bacteria (p<0.05) suggest that a significant fraction of viruses are bacteriophages. By considering not only the environmental parameters, bacteria and phytoplankton but also viruses as explanatory variable in a canonical correspondence analysis, the variance explained during the study period was increased in about 3% achieving a total of 67.3%, indicating a correlation that may be due to the known regulation of the ecosystem by viruses.