Microbial community dynamics and coexistence in a sulfide-driven phototrophic bloom

Phototrophic microbial mats commonly contain multiple phototrophic lineages that coexist based on their light, oxygen and nutrient preferences. Here we show that similar coexistence patterns and ecological niches can occur in suspended phototrophic blooms of an organic-rich estuary. The water column showed steep gradients of oxygen, pH, sulfate, sulfide, and salinity. The upper part of the bloom was dominated by aerobic phototrophic Cyanobacteria, the middle and lower parts were dominated by anoxygenic purple sulfur bacteria (Chromatiales) and green sulfur bacteria (Chlorobiales), respectively. We found multiple uncultured phototrophic lineages and present metagenome-assembled genomes of two uncultured organisms within the Chlorobiales. Apparently, those Chlorobiales populations were affected by Microviridae viruses. We suggest a sulfur cycle within the bloom in which elemental sulfur produced by phototrophs is reduced to sulfide by Desulfuromonas sp. These findings improve our understanding of the ecology and ecophysiology of phototrophic blooms and their impact on biogeochemical cycles.