EGU General Assembly 2022
© Author(s) 2022. This work is distributed under
the Creative Commons Attribution 4.0 License.

Why does Trichodesmium fix nitrogen during the day? Special biochemistry linking biogeochemical cycles.

Noelle Held1, John Waterbury2, Eric Webb3, Riss Kellogg2, Matthew McIlvin2, Michael Jakuba2, Frederica Valois2, Dawn Moran2, Kevin Sutherland4, and Mak Saito2
Noelle Held et al.
  • 1ETH Zurich, Zurich, Switzerland (
  • 2Woods Hole Oceanographic Institution, Woods Hole, USA
  • 3University of Southern California, Los Angeles, USA
  • 4Harvard University, Cambridge, USA

Cyanobacteria of the genus Trichodesmium provide about 80 Tg of fixed nitrogen to the surface ocean per year and contribute to marine biogeochemistry, including the sequestration of carbon dioxide and mobilization of particulate iron. Trichodesmium fixes nitrogen in the daylight, despite the incompatibility of the nitrogenase enzyme with oxygen produced during photosynthesis. While the mechanisms protecting nitrogenase remain unclear, all proposed strategies require considerable resource investment. Here we describe a crucial benefit of daytime nitrogen fixation in Trichodesmium spp. that may counteract these costs. We analysed diel proteomes of cultured and field populations of Trichodesmium in comparison with the marine diazotroph Crocosphaera watsonii WH8501, which fixes nitrogen at night. Trichodesmium’s proteome is extraordinarily dynamic and demonstrates simultaneous photosynthesis and nitrogen fixation, resulting in balanced particulate organic carbon and particulate organic nitrogen production. Unlike Crocosphaera, which produces large quantities of glycogen as an energy store for nitrogenase, proteomic evidence is consistent with the idea that Trichodesmium reduces the need to produce glycogen by supplying energy directly to nitrogenase via soluble ferredoxin charged by the photosynthesis protein PsaC. This minimizes ballast associated with glycogen, reducing cell density and decreasing sinking velocity, thus supporting Trichodesmium’s niche as a buoyant, high-light-adapted colony forming cyanobacterium. To occupy its niche of simultaneous nitrogen fixation and photosynthesis, Trichodesmium appears to be a conspicuous consumer of iron, and has therefore developed unique iron-acquisition strategies, including the use of iron-rich dust. Particle capture by buoyant Trichodesmium colonies may therefore increase the residence time and degradation of mineral iron in the euphotic zone. These findings describe how cellular biochemistry defines and reinforces the ecological and biogeochemical function of these keystone marine diazotrophs, particularly their role as a microbial link in the nitrogen, carbon, and iron cycles. 

How to cite: Held, N., Waterbury, J., Webb, E., Kellogg, R., McIlvin, M., Jakuba, M., Valois, F., Moran, D., Sutherland, K., and Saito, M.: Why does Trichodesmium fix nitrogen during the day? Special biochemistry linking biogeochemical cycles., EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9136,, 2022.