Protozoan predators are the first consumer trophic level in marine microbial food chains and webs, ingesting both phytoplankton and bacteria. A fraction of this ingested material is passed up the food chain and a fraction is regenerated into the water column and re-utilised by the primary producers. The regeneration of nutrients is fundamental to microbial food web dynamics but is a particularly difficult process to quantify as regenerated nutrients are so rapidly taken up by other cells that then cannot be easily measured.

The major nutrients passed between trophic levels in food webs are carbon (C) and nitrogen (N). Bacterial cells are thought to maintain a relatively constant CN ratio, but phytoplankton CN ratio may vary markedly depending on nutrient availability, and hence exhibit variable “quality” in terms of food items for higher trophic levels.

In this study we sought to determine if the phytoplankton/protozoan trophic transfer and protozoan regeneration of N was influenced by phytoplankton cell nutrient stoichiometry in terms of CN ratio. To do this we conducted a set of laboratory experiments using a metabolic inhibitor L-Methionine sulfoximine (MSX) which has previously been demonstrated to be a glutamine synthetase inhibitor, and hence to be capable of preventing N uptake, for a variety of cells and plants. This inhibitor prevented phytoplankton from taking up regenerated N but did not influence the protozoan predators. We were therefore able to determine the quantity and rate of N regeneration of protozoans ingesting phytoplankton of different CN ratios.

The results of the study suggested that protozoan nutrient regeneration efficiency (NRE) could be explained in terms of the relative CN stoichiometry of prey and predator with significantly greater N regeneration when N replete (low CN ratio) prey were ingested.

Using a mathematical model based on the near classic model of Fasham et al. (1990), we demonstrated that relating the NRE of the protozoan trophic level to the CN ratio of ingested phytoplankton has the potential to markedly modify the predicted dynamics of the simulated microbial food web.

The results are publised in Protist:

Davidson K., Roberts E.C., Wilson A.M., Mitchell E. (2005) The role of prey nutritional status in governing protozoan nitrogen regeneration efficiency. Protist 156: 45-62.