Microbes mediating the sulfur cycle in the Atlantic Ocean and their link to chemolithoautotrophy

Abstract

Summary Only about 10%–30% of the organic matter produced in the epipelagic layers reaches the dark ocean. Under these limiting conditions, reduced inorganic substrates might be used as an energy source to fuel prokaryotic chemoautotrophic and/or mixotrophic activity. The apr A gene encodes the alpha subunit of the adenosine‐5′‐phosphosulfate (APS) reductase, present in sulfate‐reducing (SRP) and sulfur‐oxidizing prokaryotes (SOP). The sulfur‐oxidizing pathway can be coupled to inorganic carbon fixation via the Calvin–Benson–Bassham cycle. The abundances of apr A and cbb M, encoding RuBisCO form II (the key CO 2 fixing enzyme), were determined over the entire water column along a latitudinal transect in the Atlantic from 64°N to 50°S covering six oceanic provinces. The abundance of apr A and cbb M genes significantly increased with depth reaching the highest abundances in meso‐ and upper bathypelagic layers. The contribution of cells containing these genes also increased from mesotrophic towards oligotrophic provinces, suggesting that under nutrient limiting conditions alternative energy sources are advantageous. However, the apr A/ cbb M ratios indicated that only a fraction of the SOP is associated with inorganic carbon fixation. The apr A harbouring prokaryotic community was dominated by Pelagibacterales in surface and mesopelagic waters, while Candidatus Thioglobus, Chromatiales and the Deltaproteobacterium_SCGC dominated the bathypelagic realm. Noticeably, the contribution of the SRP to the prokaryotic community harbouring apr A gene was low, suggesting a major utilization of inorganic sulfur compounds either as an energy source (occasionally coupled with inorganic carbon fixation) or in biosynthesis pathways.