Effects of Nitrogen Limitation on Dunaliella sp.–Alteromonas sp. Interactions: From Mutualistic to Competitive Relationships

Interactions between photosynthetic and non-photosynthetic microorganisms play an essential role in natural aquatic environments and the contribution of bacteria and microalgae to the nitrogen cyclecan lead to both competitive and mutualistic relationships. Nitrogenis considered to be, with phosphorus and iron, one of the main limiting nutrients for primary production in the oceans and its availability experiences large temporal and geographical variations. For these reasons, it is important to understand how competitive and mutualistic interactions between photosynthetic and heterotrophic microorganisms are impacted by nitrogenlimitation. In a previous study performed in batch cultures, the addition of a selected bacterial strain of Alteromonassp. resulted in a final biomass increase in the green alga Dunaliellasp. as a result of higher nitrogenincorporation into the algal cells. The present work focuses on testing the potential of the same microalgae–bacteria association and nitrogeninteractions in chemostatslimited by nitrogen. Axenicand mixed cultures were compared at two dilution rates to evaluate the impact of nitrogenlimitation on interactions. The addition of bacteria resulted in increased cell size in the microalgae, as well as decreased carbon incorporation, which was exacerbated by high nitrogenlimitation. Biochemical analyses for the different components including microalgae, bacteria, non-living particulate matter, and dissolved organic matter, suggested that bacteria uptake carbon from carbon-rich particulate matter released by microalgae. Dissolved organic nitrogenreleased by microalgae was apparently not taken up by bacteria, which casts doubt on the remineralization of dissolved organic nitrogenby Alteromonassp. in chemostats. Dunaliellasp. obtained ammonium- nitrogenmore efficiently under lower nitrogenlimitation. Overall, we revealed competition between microalgae and bacteria for ammonium when this was in continuous but limited supply. Competition for mineral nitrogenincreased with nitrogenlimitation. From our study we suggest that competitive or mutualistic relationships between microalgae and bacteria largely depend on the ecophysiologicalstatus of the two microorganisms. The outcome of microalgae–bacteria interactions in natural and artificial ecosystems largely depends on environmental factors. Our results indicate the need to improve understanding of the interaction/s between these microbial players.