Swanner E.D, Mloszewska A.M., Cirpka O.A., Schoenberg R., Konhauser K.O., and Kappler A. Modulation of oxygen production Archaen oceans by episodes of Fe(II) toxicity. Nature Geoscience. 5 January 2015. DOI: 10.1038/NGEO2327
Once upon a time long ago, the earth was devoid of oxygen (a.k.a. dioxygen, O2, henceforth “oxygen”) , but one day, about 3.5 billion years ago, the cyanobacteria started to produce oxygen as a byproduct of photosynthesis (using light, water, and carbon dioxide to produce energy). As it happened, oxygen was poisonous to “anaerobic” bacteria, which thrived under oxygen-free conditions long before cyanobacteria. Yet, anaerobic bacteria were spared by the sponge-like capture of oxygen by minerals, like iron, until one fateful day, about 2.3 billion years ago, there simply wasn’t enough iron left to soak up all the oxygen, and – shezam – yes, shezam, oxygen took to the atmosphere for it had no better place to go. The result was a mass extinction, quite possibly the most massive in all of natural history, of most anaerobic life forms, but out of this great explosion of oxygen came the possibility for you and me (try that next Valentine’s day, and let me know how it goes). Eventually, the anaerobes returned, and everyone lived more or less happily ever after (between interspersed mass extinctions).
Now, this is all well and good, but the scientists have long wondered why there was such a lag (a billion years, give or take), between the earliest biological production of oxygen and the so-called “Great Oxidation Event,” when oxygen was introduced into the atmosphere. Why did it take so long for photosynthetic cyanobacteria to exhaust the mineral supply? A recent study by Elizabeth Swanner et al. published in Nature Geoscience thinks that periodic bursts of toxic Iron(II) (or Fe(II)), an oxidized form of iron from volcanoes deep down in the ocean basin might have had a role to play in putting cyanobacteria in check for so many years.
To test their hypothesis, Swanner et al. grew a common free-floating (planktonic) species of the cyanobacteria Synechococccus, which together with another group of cyanobacteria Prochlorococcus, accounts for upwards of an eighth of modern oxygen production. They subjected the cyanobacteria to several concentrations of Fe(II) and monitored their growth and photosynthetic productivity. If left for enough time, cultures eventually reached the same growth densities as a result of the eventual consumption of Fe(II) by oxygen, but they observed a drop off in the initial growth rates of cultures with increasing Fe(II) concentration. They were also able to measure an accompanying increase intercellular (within the cell) accumulation of “reactive oxygen species,” the toxic byproducts of the reaction of oxygen with Fe(II). Additionally, they observed a reduction in fluorescence of the cells with an increasing Fe(II) concentrations, suggesting that the photosynthetic machinery of the cells was also poisoned.
Based on their growth study, Swanner et al. conclude that Fe(II) would be toxic to the particular species of cyanobacterium (the singular of cyanobacteria) grown in their study. As something of a reality check, they performed a literature meta-study to correlate presence or absence of cyanobacteria observed in the field with corresponding site-specific Fe(II) concentrations. Sure enough cyanobacteria were only reported from sites below a minimum threshold of Fe(II).
Having shown that Fe(II) is toxic to modern cyanobacteria, Swanner et al. sought to address its toxicity in ancient species. Due to the scarcity of essential minerals such as iron required for photosynthesis, cyanobacteria would be expected to concentrate in coastal waters, where upwelling (mixing of nutrients from the rising deep cold waters) is maximal. To address the effectiveness of Fe(II) from the ocean’s depths as an oxygen sponge, Swanner et al. modeled Fe(II) gradients for three different upwelling rates against oxygen concentrations from the bottom of the ocean up to the to the “photic zone,” where sunlight is available to photosynthetic organisms. At moderate and high coastal mixing rates they observed reduced release, and no release of oxygen to the atmosphere from the photic zone due to efficient mixing of Fe(II). At a lower rate representative of open ocean mixing, oxygen was efficiently released from the photic zone into the atmosphere. Hence, it would be reasonable to assert that periodic upwellings of Fe(II) might’ve checked oxygen release into the atmosphere by early cyanobacteria.
Swanner et al. conclude that episodic events such as volcanic activity might have modulated oxygen release into the atmosphere, which could provide some explanation to the mysterious lag between the appearance of photosynthetic cyanobacteria, and the much delayed oxidation of the atmosphere. So, next time you take a breath, think of the Sisyphean struggle of the cyanobacteria that eventually enabled aerobic life on earth.
Abrahim is a PhD student at Scripps Institution of Oceanography in San Diego where he studies marine chemical biology.