Increased Nutrient Levels Enhance Bacterial Exopolysaccharides Production in the Context of Algae.

Microbial aggregation is central in environmental processes, such as marine snow and harmful marine mucilage events. Nutrient enrichment positively correlates with microbial aggregation. This correlation is largely attributed to the overgrowth of microalgae and the overproduction of agglomerating exopolysaccharides. However, recent studies highlight the significant contribution of bacterial exopolysaccharides to algal-bacterial aggregation. Here, using controlled laboratory experiments and environmental metatranscriptomic analysis, we investigate the impact of nutrient enrichment on bacterial exopolysaccharides production, while bacteria are in the context of their algal hosts. Our findings demonstrate a marked increase in bacterial exopolysaccharides production in response to a relative increase of inorganic phosphorus and nitrogen levels, both in the lab and in the environment. These results highlight the interplay between nutrient regimes, bacterial physiology and microbial aggregation in marine ecosystems and emphasise gaps in our understanding regarding the bacterial role in environmental processes that involve microbial aggregation.