Nitrogen Fixation in the Sea: Why Only Trichodesmium?

Abstract

The relative importance of different nutrients in limiting primary production in the sea continues to be the subject of debate, but it is clear that several nutrients are often in short supply in many regions of the oceans. Nitrogen fixation capabilities should provide an ecological advantage to microorganisms in the oceanic environment, regardless of the primary nutrient limiting productivity. Trichodesmium is a filamentous nonheterocystous nitrogen-fixing cyanobacterium which is a conspicuous component of tropical and subtropical oceans, and appears to play a major role in carbon and nitrogen-fixation in regions where it is found. It is not intuitively obvious which characteristics of Trichodesmium confer an ecological advantage such that it is the predominant organism to exploit nitrogen fixation as a mechanism to obtain nitrogen in nitrogen-deficient oligotrophic oceans. Furthermore, there still remains the question of whether Trichodesmium is truly one of only a few species to fix nitrogen, or whether other nitrogen fixing organisms exist in the open ocean, but have yet to be cultivated. Molecular approaches have provided a way to examine both of these issues: what is the distribution of nitrogen fixing microorganisms in the marine environment, and what are the molecular and biochemical features that determine the ecological success of Trichodesmium in the open ocean environment?