Abundant microbes of surface sea waters of the uncharted Engaño Bay at the Atlantic Patagonian Coast: relevance of bacteria-sized photosynthetic eukaryotes

Abstract

Microorganisms play a central role in the structure and function of marine ecosystems. Its vast diversity makes the disentangling of the microbial taxonomic composition an essential task. Engaño Bay, a highly productive temperate region on the Patagonian Atlantic Coast, constitutes a remote uncharted area. Here, metabarcoding analyses revealed that the microbiome community (cells < 5 µm) of surface water is dominated by a few bacterial taxa along with diverse low frequency groups. A substantial number of sequences of photosynthetic picoeukaryotes (PPE) were also evidenced, yet no cyanobacterial sequences could be observed. Specific PCR-detection of cyanobacteria, monitoring of viruses specific for cyanobacteria and PPE, microbiological isolation, and molecular diet analyses confirmed these findings. Cyanobacteria were PCR detected in three out of thirteen samples, while virological monitoring indicated the presence of PPE viruses and the absence of cyanobacteria phages. Traditional culture methods allowed the isolation of Ostreococcus and Micromonas spp. from samples belonging to warm and cold seasons, respectively. Molecular diet analyses showed that PPE were grazed by nanoplankters and that grazing pressure on PPE was stronger than on bacteria. Phylogenetic characterization of viral sequences indicated the presence of Ostreococcus virus, four Micromonas virus lineages and a divergent clade of Phycodnavirus belonging to no known viral species, pointing out a complex scenario of its hosts. This work characterizes the marine microbiome from a temperate uncharted region at the Southern Hemisphere, demonstrating the prevalence of PPE as primary producers with a complex scenario which warrant further studies.