Two-organism model system for the study of marine phycosphere: growth dynamics.

The interactions between phytoplankton (mainly comprised of microalgae and cyanobacteria) and heterotrophic bacteria are some of the most essential and sophisticated relationships in aquatic environments. Phytoplankton serves as the major primary producer in marine ecosystems. Heterotrophic bacteria act as scavengers, responsible for processing the majority of phytoplankton-derived organic carbon. Algal-bacterial interactions occur in the microenvironment surrounding algal cells, known as the phycosphere, the planktonic analogue of the rhizosphere in plants. The phycosphere is a region enriched in organic molecules released by algal cells and represents the key meeting place for the microalgae-bacteria interactions. The interactions between the two groups, cover all possible forms of ecological relationships, spanning mutualism, commensalism, competition and antagonism. One of the most studied microalgae-bacteria interactions involves diatoms and bacteria, but the mechanism of their potential relationships is mostly unknown. Understanding interactions between diatoms and bacteria is essential for deciphering oceanic nutrient fluxes and biogeochemical cycles and this is probably the key to elucidate their biology and evolution. The aim of this work was to unravel the growth dynamics inside a synthetic phycosphere using a simplified system composed by two model organisms, the diatom Phaeodactylum tricornutum and the heterotrophic bacterium Pseudoalteromonas haloplanktis TAC 125. It was analized the population dynamics of P.tricornutum and P.haloplanktis TAC125 in co-culture and as single cultures. We established a reproducible laboratory phototroph-heterotroph system for studying the interactions between the model diatom P.tricornutum and the model bacterium P.haloplanktis TAC125. The microalga and the bacterium co-exist in the same controlled environment, possibly exchanging nutrients and cofactors to grow. These first results suggest further investigations to study the interdependencies between the diatom and the bacterium.