Scaling down the microbial loop: data-driven modelling of growth interactions in a diatom-bacterium co-culture

Abstract

An intricate set of interactions characterizes marine ecosystems. One of the most important is represented by the so-called microbial loop, which includes the exchange of dissolved organic matter (DOM) from phototrophic organisms to heterotrophic bacteria. Here, it can be used as the major carbon and energy source. Arguably, this interaction is one of the foundations of the entire ocean food-web. Carbon fixed by phytoplankton can be redirected to bacterial cells in two main ways; either i) bacteria feed on dead (eventually lysed) phytoplankton cells or ii) DOM is actively released by phytoplankton cells (a widespread process that may result in up to 50% of the fixed carbon leaving the cell). In this work, we have set up a co-culture of the model diatom Phaeodactylum tricornutum and the model chemoheterotrophic bacterium Pseudoalteromonas haloplanktis TAC125 and used this system to study the interactions between these two representatives of the microbial loop. We show that the bacterium can indeed thrive on diatom-derived carbon and that this growth can be sustained by both diatom dead cells and diatom-released compounds. These observations were formalized in a network of putative interactions between P. tricornutum and P. haloplanktis and implemented in a mathematical model that reproduces the observed co-culture dynamics, suggesting that our hypotheses on the interactions occurring in this two-player system can accurately explain the experimental data.