The biochemical composition and calorific content of a rotifer and its algal food: comparison of a two stage chemostat and batch culture.

It is often assumed that the use of a two-stage chemostat yields algal food with a well-defined nutritional composition that can maintain herbivores in a steady state of growth. In this study I investigated two bacteriafree culture techniques, continuous flow chemostats and batch cultures, to determine whether the biochemical composition of the rotifer Encentrum linnhei differed in the two cultures. Changes in the biochemical composition and calorific content of the algal food were also examined. In the rotifer reaction vessel only the lipid content of the algal food increased significantly with dilution rates, while significant decreases in protein and carbohydrates were detected at increasing algal densities. A different pattern was observed in the response of the unused algal cells to variables such as dilution, algal input and algal densities in the sump of the rotifer chemostat. In the chemostat the biochemical composition of the rotifers varied as expected with dilution rates, algal input and food availability but significant differences were found in the biochemical composition of the animals growing in the reaction vessel and those collected from the sump. In contrast, the biochemical content of batch-grown E. linnhei varied with time in a way that depended upon food availability and also on the biochemical state of the algal food. However, at the end of the exponential phase of growth, when maximum densities had been achieved, batch-grown rotifers were more biochemically nutritious than chemostat-grown animals in their steady-state phase.