Anaerobic Protist Survival in Microcosms is Dependent on Microbiome Metabolic Function

Syntrophy is a type of symbiotic cooperation, performed through metabolic interaction between partners. Syntrophic interactions between prokaryotes are common in low-oxygen environments, however, whether such interactions exist between microbial eukaryotes (protists) and prokaryotes is under explored. Previous investigations uncovered a potentially syntrophic interaction between the anaerobic breviate protist Lenisia limosa and the bacterium Arcobacter sp. EP1. This interaction provides benefits to both partners via hydrogen transfer whereby hydrogen produced by the protist is consumed by Arcobacter. Whether this relationship is conserved in other members in the Breviatea remains unexplored and yet critical to understand the scope and impact of these symbiotic associations in oxygen minimum zones (OMZs). Here, we address this knowledge gap, by exploring the metabolism and evolutionary relationship of diverse breviate species. We find that Arcobacteracea species and breviate-related sequences co-occur in the same environmental sequencing projects. These observations suggest that the symbiosis previously described between Lenisia limosa and Arcobacter sp. EP1 might be pervasive in Breviatea. To expand on these data, we present metagenome-assembled genomes of the prokaryotic communities of four undescribed breviate isolates and 16S amplicon sequencing data of anoxic cultures with different electron acceptors. We find that diverse Arcobacter-like species co-occur in all of the cultures and that the abundance of some of the Arcobacteracea species is higher in anaerobic conditions with nitrate. Based on the proteome prediction and the amplicon data, we speculate that some Arcobacteracea species could use the H2 produced by the breviates for dissimilatory nitrate reduction, engaging in a similar metabolic interaction like the one occurring in L. limosa and Arcobacter sp. EP1.