Growth dynamics in a diatom-heterotrophic bacterium simplified model system.

Introduction The interactions between phytoplankton (mainly comprised of microalgae and cyanobacteria) and heterotrophic bacteria are some of the most important and sophisticated relationships in aquatic environment. Phytoplankton are the major primary producers in aquatic ecosystems. Heterotrophic bacteria are ubiquitous scavengers, responsible for processing the majority of phytoplankton-derived carbon in the ocean, needed to sustain their growth. Algae-bacteria interactions cover all possible forms of symbiotic relationships, spanning mutualism, commensalism, competition and antagonism. One of the most studied microalgae-bacteria interactions involve diatoms and bacteria, but the mechanism of their potential interactions is mostly unknown. Understanding interactions between diatoms and bacteria is essential for deciphering oceanic nutrient fluxes and biogeochemical cycles. The aim of this work is to unravel the growth dynamics occurring for the diatom Phaeodactylum tricornutum and the heterotrophic bacterium Pseudoalteromonas haloplanktis TAC 125 in co-culture. Materials and Methods The diatom-bacteria coculture was cultivated in a marine salt mix without carbon source for 28 days. Three control single-cultures were set: P.tricornutum, P. haloplanktis TAC 125 supplemented with L-glutamate 10 gL-1 and P.haloplanktis TAC 125 without carbon sources as a negative control. Cultures were kept at 20°C, continuously stirred (100 rpm) and irradiated with a light intensity of 30µmols (photons) m-2s-1. Microalgal growth was determined by measuring chlorophyll a content and by OD600,680,750. Viable count and OD600 were used to evaluate the bacterial growth. The pH of each culture was monitored. Results In the coculture, the bacterial cells showed a significant decrease between day 0 and 7 (from 5,13 x 106 cells ml-1 at day 0 to 9,76 x 105 cells ml-1 at day 7), followed by an oscillatory growth. At day 28 the viable count of the bacterium in co-culture (2,2 x 106 cells ml-1) was significantly higher that the negative control (4,57 x 105 cells ml-1). Indeed, from day 21 to 28, the bacterial cells increased, while the diatom OD in coculture started to decrease, confirmed by a lower pH increase (from 7,67 at day 21 to 8,66 at day 28), compared to the P.tricornutum single-culture (from 8,17 at day 21 to 9,67 at day 28). Conclusions The coculture of P. tricornutum and P. haloplanktis TAC 125 shows a first phase (day 0-7) in which the organism may compete (possibly for inorganic nutrients) with a predominance of the diatom; they then show a second phase (day 7-21) of mutualistic co-existence (carbon-growth factor exchange), and finally (day 21-28) a new competitive phase where the bacterium takes over. This first promising results suggest further investigations to unravel the interdependencies between the diatom and the bacterium.