The Late Arrival of the Heterocyst
Nitrogen fixation is inhibited by molecular oxygen and cyanobacteria have evolved several mechanisms to deal with this inhibition 73, 74. In some filamentous cyanobacteria, heterocysts are specialised nitrogen-fixing cells 43, 75 that have no oxygen-evolving photosystem II. Heterocysts fix nitrogen using ATP from photosystem I cyclic photophosphorylation together with electrons from substrates imported from neighbouring vegetative cells 43, 75, 76, 77. Heterocysts therefore permit cyanobacterial nitrogen fixation under aerobic conditions. Some cyanobacteria without heterocysts can fix nitrogen, but do so only in darkness, temporally separating nitrogen fixation from photosynthesis 78, 79, or under low oxygen conditions 64, 65, 71, often using sulphide instead of water as the inorganic electron donor 68, 71. Yet other cyanobacteria without heterocysts can fix nitrogen, but do so only at low oxygen partial pressures 32, 55, 57, 58, 63, 64, 65, 66, 67 because they have not evolved special O2 protection mechanisms. Non-nitrogen-fixing cyanobacteria obtain nitrogen from ammonia, nitrate, nitrite, or urea [80], and control photosystem stoichiometry to match these substrates’ differing requirements for ATP relative to electrons from reduced ferredoxin 81, 82.
Cyanobacterial nitrogen fixation is ancient [83] (Figure 4). Nevertheless, fossil heterocysts have not been seen in rocks predating the Devonian period [72]. The cyanobacterial lineages that today possess heterocysts also possess spore-like structures termed akinetes 84, 85. Though fossil akinetes have a record that extends into the Proterozoic 85, 86, heterocysts themselves are lacking in the otherwise well-documented Precambrian cyanobacterial fossil record 11, 85, 86, 87. The oldest uncontroversial fossil heterocysts trace to land ecosystems of the Rhynie chert [84], a mere 408 MY old [88]. A Devonian origin of heterocysts (Figure 4) suggests that they arose as cells dedicated to protection of nitrogen fixation from an oxygenated atmosphere and in response to late (Figure 1) oxygen accumulation.
Cyanobacteria have evolved mechanisms besides heterocysts to avoid nitrogenase inhibition by oxygen 43, 50, including N2 fixation in the dark [89], or filament bundles, as in Trichodesmium 55, 58, 90. Critics might counter that such mechanisms could have bypassed O2 feedback inhibition during the Proterozoic. There are, however, two problems with this objection. First, the mechanisms that cyanobacteria use to deal with modern O2 levels appear in phylogenies to have arisen independently in diverse lineages (Figure 4), not at the base of cyanobacterial evolution when water oxidation had first emerged 50, 91. Second, despite cyanobacteria having a fossil record that extends into the Archaean [92], there is no palaeontological trace of Proterozoic heterocysts 93, 94. The oldest uncontroversial fossil heterocysts are merely Devonian in age [84], younger than the first land plants and contemporary with the late atmospheric O2 increase (Figure 1). We propose that filamentous cyanobacterial lineages are ancient, while their heterocysts are not, and that heterocysts emerged to protect nitrogenase from the high atmospheric levels of oxygen generated by land plants. Indeed, a recent phylogenomic investigation of trait evolution in cyanobacteria also indicates that heterocysts arose late in cyanobacterial evolution [95].
Denitrifying bacteria may offer a clue.