Nitrogen-fixing, photosynthetic, anaerobic bacteria associated with pelagic copepods

Abstract

Purple sulfur bacteria are photosynthetic, anaerobic microorganisms that fix carbon dioxide using hydrogen sulfide as an electron donor; many are also nitrogen fixers. Because of their requirements for sulfide or organic carbon as electron donors in anoxygenic photosynthesis, these bacteria are generally thought to be limited to shallow, organic-rich, anoxic environments such as subtidal marine sediments. We report here the discovery of nitrogen-fixing, purple sulfur bacteria associated with pelagic copepods from the Caribbean Sea. Anaerobic incubations of bacteria associated with full-gut and voided-gut copepods resulted in enrichments of purple/red-pigmented purple sulfur bacteria while anaerobic incubations of bacteria associated with fecal pellets did not yield any purple sulfur bacteria, suggesting that the photosynthetic anaerobes were specifically associated with copepods. Pigment analysis of the Caribbean Sea copepod-associated bacterial enrichments demonstrated that these bacteria possess bacteriochlorophyll a and carotenoids in the okenone series, confirming that these bacteria are purple sulfur bacteria. Increases in acetylene reduction paralleled the growth of purple sulfur bacteria in the copepod enrichments, suggesting that the purple sulfur bacteria are active nitrogen fixers. The association of these bacteria with planktonic copepods suggests a previously unrecognized role for photosynthetic anaerobes in the marine S, N and C cycles, even in the aerobic water column of the open ocean.