Selenium as a nutrient for freshwater bacterioplankton and its interactions with phosphorus

The influence of selenite on the growth of bacterioplankton present in samples of three lakes was analyzed; these samples were collected in sulfate-rich, oligotrophic Lake Banyoles, moderately eutrophic Lake Erken, and hypereutrophic Lake Vallentunasjön. Experiments were set up in a completely randomized factorial design to analyze the effect of selenite alone and, in the same experiment, the effect of selenite in the presence of phosphate. Cultures of bacterioplankton, free of algae and zooplankton, diluted with filtered natural water, were used in the bioassays. The addition of 100 μg P∙L−1 to samples from Lake Banyoles, collected during the winter, enhanced cell yield 2.7 times; the addition of 10 μg P∙L−1 to samples from Lake Erken, collected during the spring, doubled the yield. Strong effects of phosphate on growth rates were found in samples from lakes Banyoles and Vallentunasjön. When bacteria from Lake Banyoles were exposed to 100 μg P∙L−1, the specific growth rate was 0.08 h−1 (log units), compared with 0.03 h−1 in the control. In spring, Lake Vallentunasjôn contained water with a considerable amount of dissolved organic phosphorus (18 μg P∙L−1); the addition of 100 μg P∙L−1, in the form of phosphate, resulted in a shorter lag phase of at least 48 h and reduced the specific growth rate to about half that in the control. Selenite had a significant positive effect on cell yield in samples from lakes Banyoles (p = 0.0001) and Vallentunasjön (p = 0.020), whereas the effect on cell yield in samples from Lake Erken was slightly negative (p = 0.110). The addition of selenite alone (550 ng Se∙L−1) to samples from Lake Banyoles, collected during the summer, doubled the biovolume of bacterioplankton within 37 h. Among winter bacteria from Lake Banyoles, selenite, at concentrations of 55 and 550 ng Se∙L−1, increased the number of bacteria twofold and threefold, respectively, but only when the phosphate level was high (100 μg P∙L−1). A high inorganic phosphorus level of 100 μg P∙L−1 was also necessary to stimulate the effect of selenite on bacterial growth in samples from Lake Vallentunasjön; 550 ng Se∙L−1 enhanced cell yield 24%. The negative effect of selenite on samples from Lake Erken was most obvious when phosphate (10 or 100 μg P∙L−1) had been added simultaneously (p = 0.030 for selenium and phosphorus interaction). Cell yields were always greater at the highest temperature. With samples from Lake Vallentunasjön, selenite stimulated bacterial growth at 25 °C but had no effect at 10 °C. With samples from Lake Banyoles, the simultaneous addition of phosphate and selenite increased cell yield threefold at 15 °C and only twofold at 30 °C. Key words: phosphorus, sulfate, Lake Erken, Lake Vallentunasjön, Lake Banyoles.