Models and co-culture experiments assess four mechanisms of phytoplankton-bacteria interactions.

Phytoplankton growth and death depend on interactions with heterotrophic bacteria, yet the underlying mechanisms remain mostly unclear. Here we ask whether mathematical models explicitly representing four putative mechanisms of interaction (overflow metabolism, mixotrophy, exoenzymes and reactive oxygen species detoxification) can recapitulate diverse dynamics observed in laboratory co-cultures between the cyanobacterium Prochlorococcus and eight heterotrophic bacteria. Two distinct modes of interaction emerge from our models: (1) organic carbon and nitrogen recycling through exoenzymes or an overflow metabolism, in which the high biomass of both organisms leads to more productivity and recalcitrant organic matter, and (2) reactive oxygen species detoxification, in which a small number of 'exploited' heterotrophs are sufficient to support Prochlorococcus survival. Recycling is probably the main process in laboratory co-cultures. Models do not reproduce total inhibition of Prochlorococcus, suggesting that additional mechanisms such as allelopathy may be involved. The models highlight cell death and biomass recycling as unconstrained, key processes that could enhance our understanding of how interactions impact ecologically and biogeochemically important processes.