Four Potential Mechanisms Underlying Phytoplankton-Bacteria Interactions Assessed Using Experiments and Models

Abstract

Phytoplankton growth and death depend on interactions with heterotrophic bacteria, yet the underlying mechanisms remain mostly unclear. We ask whether mathematical models explicitly representing four putative mechanisms of interaction (overflow metabolism, mixotrophy, exoenzymes and ROS detoxification) can recapitulate laboratory co-cultures between Prochlorococcus and 8 heterotrophic bacteria, ranging from synergism to antagonism. Two fundamentally distinct modes of interaction emerge from the models: i) Organic carbon and nitrogen recycling through exoenzymes or overflow metabolism, where high biomass of both organisms leads to more productive systems and more recalcitrant organic matter. ii) Detoxification of ROS, where a small number of "exploited" heterotrophs are sufficient to support Prochlorococcus survival. Cross feeding is likely more common in laboratory co-cultures, and most models cannot reproduce total inhibition of Prochlorococcus, suggesting additional mechanisms such as allelopathy. Key model parameters determining co-culture outcome are related to cell death and biomass recycling, providing priorities for future research.