Marine cyanobacteria, Crocosphaera watsonii, doubles the effectiveness of a critical, but scarce nutrient by serially sharing the nutrient between two key processes.

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What is an organism to do when it has two tasks to perform but only enough resources to do one? Crocosphaera watsonii solves the problem by doing only one of the tasks at a time and recycling the reources. The proteins that C. watsonii uses to perform photosynthesis and the ones it uses to fix nitrogen gas both require iron; however, iron is a scarce mineral in the open ocean. By using its iron reserves in photosynthetic proteins during the day, then breaking the proteins down and reusing the iron for the nitrogen fixing proteins at night, C. watsonii is able to survive and flourish without much iron, though it does require an input of energy to keep the recyling process going.

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"The marine nitrogen fixing microorganisms (diazotrophs) are a major source of nitrogen to open ocean ecosystems and are predicted to be limited by iron in most marine environments…The daily synthesis and degradation of enzymes in coordination with their utilization results in a lowered cellular metalloenzyme inventory that requires ∼40% less iron than if these enzymes were maintained throughout the diel cycle. This strategy is energetically expensive, but appears to serve as an important adaptation for confronting the iron scarcity of the open oceans...It allows Crocosphaera to inhabit regions lower in iron and allows the same iron supply to support higher Crocosphaera biomass and nitrogen fixation...iron is considered the critical micronutrient for marine diazotrophs due to their use of the iron-nitrogenase protein complex...The coexistence of oxygenic photosynthesis and nitrogen fixation metabolisms presents a unique challenge for diazotrophs due to their high iron demands and the chemical incompatibility of molecular oxygen and the nitrogenase protein complex...Several unicellular diazotrophs, including Crocosphaera watsonii, have been observed to fix nitrogen during the dark period, and this is widely viewed as an adaption for temporal separation of photosynthesis and nitrogen fixation to avoid the oxygen disruption of the nitrogenase complex." (Saito et al. 2011:2184)

"This iron conservation strategy is analogous to the maritime practice of hotbunking, referring to ships that sail with more sailors (metalloenzyme requirements) than bunks (iron atoms), where sailors on opposing shifts share the same bunk—keeping the bunks continually hot (or iron atoms in use)...Iron conservation strategies in Crocosphaera provided an ecological advantage in the low iron environments of the open ocean." (Saito et al. 2011:2187)

"Sharing cellular iron between photosynthesis and nitrogen fixation machinery allowed the model hotbunking diazotrophs to not only expand their habitat, but also to have higher biomass per mole of available iron." (Saito et al. 2011:2188)

Journal article
Iron conservation by reduction of metalloenzyme inventories in the marine diazotroph Crocosphaera watsoniiProceedings of the National Academy of SciencesJanuary 20, 2011
M. A. Saito, E. M. Bertrand, S. Dutkiewicz, V. V. Bulygin, D. M. Moran, F. M. Monteiro, M. J. Follows, F. W. Valois, J. B. Waterbury

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