Thingstad TF, Ovreas L, Egge JK, Lovdal T, Heldal M
Biogeochemistry, cell size, food webs, life strategies, trophic niches, mesocosm, metadata
Coexistence of two organisms competing for the same nutrient is possible if one is an uptake , and the other a predation defence specialist. In pelagic food webs this principlehas been linked to cell size. Small osmotroph cells, with their high surface : volume ratio,have been argued to be uptake specialists, while larger osmotrophs avoiding the intensegrazing pressure from small protozoan predators might represent predation defence specialists. This may seem like an obligatory trade-off situation that necessitates a choiceof either being small or being large, and thus being potentially dominant in oligotrophicor in eutrophic environments, respectively. However, in a more precise form, the theoryfor nutrient diffusion states that it is the surface : cell requirement of limiting element ratio, rather than the surface : volume ratio, that is important. The distinction is crucial,since it opens up the possibility of there being life strategies that use a non-limitingelement to increase size. Hypothesized to maximize uptake and predator defencesimultaneously, such strategies should be particularly successful. We suggest that thisstrategy is exploited by osmotrophs with different size and physiology, such asheterotrophic bacteria, unicellular cyanobacteria and diatoms. Since the strategy implies ashift in organism stoichiometry, the biogeochemical implications are strong, illustratingthe tight relationships between physical micro-scale processes, organism life strategies,biodiversity, food web structure, and biogeochemistry.