Fig. 3.

A potential evolutionary scenarium for the development of the diazocytic phenotype in Trichodesmium. (1) An ancient nonheterocystous filamentous cyanobacterium, a forerunner of Nostocales and Oscillatoriales (Fig. 1b and c), under nitrogen deprivation conditions develop strings of proheterocysts (as in Anabaena-type spp.; Wilcox et al., 1975). (2) The majority of the proheterocysts revert back into vegetative cells, while one continus the evolution into a thick-walled heterocyst, the dominating nitrogen-fixing phenotype in limnic and terrestrial ecosystems today. (3) In oceans, this proheterocystous phenotype may have been retained and evolved into the strings of diazocytes we see in Trichodesmium spp. today, while the closest relatives (Fig. 1b) either fix nitrogen in the dark (Lyngbya spp.; e.g. Lundgren et al., 2003) or have lost the capacity to fix nitrogen (Arthrospira spp.; Larsson et al., 2011; Latysheva et al., 2012).