Algal host fitness is enhanced by passively assembled bacterial communities.

Microbial communities colonize hosts and impact their fitness. One of the simplest host-microbiome associations exists between photosynthetic eukaryotic algae and heterotrophic bacteria. Algal-associated bacteria have been found to aid the host in nutrient acquisition, morphogenesis, and stress responses in exchange for host-derived metabolites. Despite these examples, it remains unclear how much host fitness varies with microbiome composition or whether host fitness requires the recruitment and retention of specific bacterial partners. The problem of recruitment specificity is especially acute in algal microbiomes and other systems where the microbial community is external to the host.

We ask how algal fitness depends on bacterial community composition. In this system, algae and bacteria are co-cultured in the presence of organic nitrogen that is bioavailable to the bacteria but not the algae. Algae can only grow in the presence of bacteria capable of metabolizing organic nitrogen and producing inorganic nitrogen as a by-product. In contrast to previous studies, we find that many bacteria generically display this trait, obviating the need for specific bacterial recruitment. In these commensal associations, bacterial growth is unaltered by the host. However, a smaller subset of taxa appear to enter mutualist associations in which the host reciprocally enriches the bacteria, potentially through exchange of photosynthetically-derived carbon. These emergent feedbacks distinguish mutualists from passive commensals and we speculate that environmental variables such as total nutrient supply might modulate their relative recruitment. Taken together, our results suggest that fitness-enhancing bacterial communities can assemble passively, without bespoke recruitment mechanisms—implying that some apparent symbioses may simply emerge from metabolic by-products rather than active cooperation.