Although the cyanobacterium (Trichodesmium) fixes as much as half of the nitrogen (N2) that supports tropical open-ocean ecosystem, its growth is limited by iron (Fe) availability. A study published in Nature Climate Change performed laboratory experiments to show that the optimum growth temperature of Fe-limited Trichodesmium is about 32°C, which is much higher than for Fe-replete cells (about 27°C). Therefore, the study suggested a large increase in growth and N2 fixation by Trichodesmium under the IPCC-projected warmer and Fe-deplete tropical ocean conditions. The study also showed that the cellular Fe content decreases as ocean temperature increases, which further enables Trichodesmium to much more efficiently leverage the scarce available Fe supplies to support N2 fixation. Applying these new findings to the IPCC RCP8.5 scenario, the study found that the efficiency of Fe-limited cellular iron use could increase by ~76% by 2100, which in turn could alleviate the prevailing Fe limitation across the tropical Pacific and Indian Oceans, and thus profoundly alter the biology and biogeochemistry of tropical open-ocean ecosystems.
Figure 1 from Jiang et al. (2019): Trichodesmium growth responses to Fe availability and warming interactions. C-specific growth rates of T. erythraeum IMS101 in Fe-replete and Fe-limited cultures at four temperatures (22, 27, 32 and 35 °C). Values represent the means and error bars for the standard deviations of biological replicates (n = 3–4); bars marked with different letters are significantly different from each other (P < 0.05).
Jiang, H. B., Fu, F. X., Rivero-Calle, S., Levine, N. M., Sañudo-Wilhelmy, S. A., Qu, P. P., … & Hutchins, D. A. (2018). Ocean warming alleviates iron limitation of marine nitrogen fixation. Nature Climate Change, 8(8), 709. https://www.nature.com/articles/s41558-018-0216-8