The role of prey nutritional status in governing protozoan nitrogen regeneration efficiency

Laboratory experiments were conducted to study nitrogen (N) regeneration by the heterotrophic marine dinoflagellate Oxyrrhis marina when ingesting phytoplankton prey of two different species and of two alternative carbon:nitrogen (C:N) ratios. Experiments were conducted in the presence of L-methionine sulfoximine (MSX) which acts as a glutamine synthetase inhibitor. Utilisation by phytoplankton of N regenerated by protozoans and other organisms drives secondary production in marine food webs. However, the rapid utilisation of this N by phytoplankton has previously hampered accurate assessment of the efficiency of protozoan N regeneration. This phenomenon is particularly problematic when the phytoplankton are nutrient stressed and most likely to rapidly utilise N. The use of MSX prevented significant utilisation by phytoplankton of protozoan regenerated N. Hence, by removing the normal pathway of N cycling, we were able to determine the N regeneration efficiency (NRE) of the protozoan. The results suggested that predator NRE could be explained in terms of the relative CN stoichiometry of prey and predator. Using a mathematical model we demonstrated that changing the method used to simulate the NRE of the protozoan trophic level has the potential to markedly modify the predicted dynamics of the simulated microbial food web.