Crock o’ gold in Crocosphaera?

Jack Cook, Woods Hole Oceanographic Institution.

Enzymes are essential to life as we understand it on Earth. But once they’ve participated in whatever reactions they’re involved in, what is the sense in having these expensively synthesised macromolecules lying about the cell, kicking their heels waiting for their next substrate meal? Particularly if those enzymes are also locking up other materials, like iron, in their structure. And what if the enzymes’ host organism exists in a nutrient-limited environment, e.g. HNLC (high nutrient–low chlorophyll) areas of the ocean (which famously have low primary productivity despite adequate supplies of the main limiting nutrients – N and P – but which may be low in Fe)? Wouldn’t it be great then if the non-enzyme component could be shared or recycled into other compounds until those enzymes are needed again..? Science fiction? Maybe not: enter the cyanobacterium Crocosphaera watsonii (yes, I know it’s a prokaryote, but it’s also an autotroph, therefore it’s a plant as far as I’m concerned!). Working with the marine N-fixing (diazotroph) microbe C. watsonii, Mak Saito et al. (PNAS; doi:10.1073/pnas.1006943108) have discovered a large-scale daily cycle of variations in iron-containing proteins, which are involved in photosynthesis (during the day) and N-fixing (during the night). The daily synthesis and degradation of enzymes results in a ‘lowered cellular metalloenzyme inventory that requires approx. 40% less iron than if these enzymes were maintained throughout the diel cycle’. Although this strategy is expensive in terms of energy expenditure, it is viewed as a major advantage in an iron-scarce environment. As a consequence, Crocosphaera can both inhabit regions low in iron and attain higher biomass and nitrogen fixation than it would otherwise achieve there. Surely something so elegant just has to be true. Or, to paraphrase Voltaire, if this iron-scavenging diel cycle didn’t exist we’d have to invent it . Undoubtedly, iron – ‘rusty gold’ to justify this item’s title – is far more precious than its more lustrous auric namesake in these nutrient-poor environments. The current mantra in the world of resource management is ‘the three Rs’ – reduce, reuse, recycle. Crocosphaera watsonii appears to embrace the re-use and recycling of iron, which in turn reduces its dependence upon external sources of this essential nutrient. Three out of three ain’t bad, at all! And something we humans could learn from? Interestingly, on a similar – ‘micro-management of scarce resources’ – theme, Christel Hassler and co-workers (PNAS 108: 1076–1081, 2011) have discovered that other (eukaryotic) marine phytoplankton may enhance the bioavailability of iron by secreting saccharides that chelate the metal. Sweet!