Elemental composition (C:N:P) and growth rates of bacteria and Rhodomonas grazed by Daphnia

The elemental composition and growth rate of Rhodomonas and heterotrophic bacteria were studied in batch cultures in the presence and absence of Daphnia and at two different levels of phosphorus limitation. The elemental content of single cells was measured with X-ray microanalysis. Simultaneously, dilution experiments were performed in order to estimate grazing losses, growth rates and dominant nutrient sources for bacteria and Rhodomonas. The phosphorus:carbon (P:C) ratios of the bacteria were generally higher in the experiment with the stronger P limitation of the system. High P:C ratios were taken as an indication that bacteria were carbon limited. The presence of Daphnia resulted in a further increase in bacterial P:C ratios and increased specific growth rates. Thus, grazing increased the availability both of inorganic nutrients and organic substrates, stimulating the growth of the bacteria. P:C ratios of Rhodomonas decreased with increasing P limitation of the system. Only at strong P limitation did the presence of Daphnia result in increased P:C ratios of Rhodomonas compared with the control without daphnids. This study shows that the elemental content and growth rate of heterotrophic bacteria and Rhodomonas are influenced by grazing and nutrient regeneration by daphnids. The response is dynamic and depends on the level of nutrient limitation of the system.