Immunochemical localization of nitrogenase in marine Trichodesmium aggregates: Relationship to N2 fixation potential

Abstract

Colonial aggregation among nonheterocystous filaments of the planktonic marine cyanobacterium Trichodesmium is known to enhance N2 fixation, mediated by the O2-sensitive enzyme complex nitrogenase. Expression of nitrogenase appears linked to the formation of O2-depleted microzones with aggregated bacterium-associated colonies. While this implies a mechanism by which nonheterocystous N2 fixation can take place in an oxygenated water column, both the location and regulation of the N2-fixing apparatus remain unknown. We used an antinitrogenase polyclonal antibody together with postsection immunocolloidal gold staining and transmission electron microscopy to show that (i) virtually all Trichodesmium cells with a colony possessed nitrogenase, (ii) nitrogenase showed no clear intracellular localization, and (iii) certain associated bacteria contained nitrogenase. Our findings emphasize the critical role coloniality plays in regulating nitrogenase expression in nature. We interpret the potential for a large share of Trichodesmium cells to fix N2 as an opportunistic response to the dynamic nature of the sea state; during quiescent conditions, aggregation and consequent expression of nitrogenase can proceed rapidly.