Redox Conditions and Microbial Particle Association: A Multi-Year Study in the Cariaco Basin

Using multiple approaches we have revealed fine-scale complexity in the aggregate association of microbial genes, transcripts, and phylogenetic groups across the redox gradient of the permanently anoxic Cariaco Basin. Spanning 3 years, we examined more than 100 samples from depths between 100 and 1300m. A consistent average of ~10% of all microbes were associated with small aggregates (<2.0µm) or organic particles. However, certain biogeochemically relevant processes mediated by microbes (sulfate reduction, sulfur oxidation, denitrification, anammox) had depth-dependent aggregate associations. While certain targeted processes were most common in the upper part of the anoxic portion of the water column, they also occurred in oxic and hypoxic layers. Anaerobic processes that were observed in oxic environments were preferentially associated with aggregates. Thin layers of intruding oxic water into anoxic water also led to the preferential association of these processes with aggregates. RT-qPCR revealed that genes for these anaerobic processes were actively transcribed within the particulate size fraction in both oxic and hypoxic environments. These results widen the conceivable ecological niche of microbial anaerobic processes in the ocean, particularly those associated with nitrogen and sulfur cycling.