Temperature excludes N2-fixing heterocystous cyanobacteria in the tropical oceans

Whereas the non-heterocystous cyanobacteria Trichodesmium spp. are the dominant N-2-fixing organisms in the tropical oceans(1), heterocystous species dominate N-2 fixation in freshwater lakes and brackish environments such as the Baltic Sea(2). So far no satisfactory explanation for the absence of heterocystous cyanobacteria in the pelagic of the tropical oceans has been given, even though heterocysts would seem to represent an ideal strategy for protecting nitrogenase from being inactivated by O-2, thereby enabling cyanobacteria to fix N-2 and to perform photosynthesis simultaneously. Trichodesmium is capable of N-2 fixation, apparently without needing to differentiate heterocysts(3). Here we show that differences in the temperature dependence of O-2 flux, respiration and N-2 fixation activity explain how Trichodesmium performs better than heterocystous species at higher temperatures. Our results also explain why Trichodesmium is not successful in temperate or cold seas. The absence of heterocystous cyanobacteria in the pelagic zone of temperate and cold seas, however, requires another explanation.