Cyanobacteria: Good week, or bad week..? Part III

This is the third of our quartet of posts looking at the newsworthy world of the blue-greens.

Asteroids, bad for dinosaurs, but good for cyanobacteria?

This really good news for cyanobacteria – both benign and bad blue-green species – comes from investigation into the consequences of the Chicxulub asteroid. This is the Yucatán Peninsula (in modern-day México)-denting phenomenon that is implicated in causing a mass extermination event, the K-T (Cretaceous-Tertiary) or, alternatively, the end-Cretaceous, or even the Cretaceous-Paleogene (K-Pg) Extinction, approx. 66 millions of years ago (Ma) (Peter Schulte et al., 2010). Attention-grabbingly, and a sensationalist headline-writer’s dream come true, that collision was bad news for dinosaurs and is repeatedly implicated in their extinction. However, that colossal impact is also widely recognised as contributing to the demise of many other groups of animal and plants that disappeared from the fossil record at about that time*. But, it seems that this life-ending, catastrophic event was actually a boon to cyanobacteria – well, those that weren’t extinguished by the impact itself – according to work by Bettina Schaefer et al. (2020) that examined the microbial ecology of the site in the first four million years after the impact.

How did they come to this conclusion? [Ed. – not that we as plant lovers have any problem with such an autotroph-promoting conclusion, we’re just curious to know how it was reached…] Did they find cyanobacteria at the impact site today? Not stated, but that would only tell you that blue-greens exist there in 2020, and says nothing about when they first occupied/recolonised that site. Did they find fossil cyanobacteria at the collision site? Sort of, but not actual fossils of the organisms themselves. Many – all? – living organisms produce materials which remain in the environment long after the producing entities are dead and gone (our human propensity for producing plastic pollution is a sort of example of that; indeed, the preponderance and proliferation of plastic has led to our modern times being classified as the Plastic Age, or Plastocene). These compounds are referred to as ‘biomarkers’ or ‘molecular fossils’ and literally mark the site of biological activity of living organisms and stand in for the otherwise unavailable physical fossils.

Cyanobacteria have the ability to produce particular organic compounds – such as the lip ids known as 2α-methylhopanes (2α-MeH; Roger Summons et al., 1999; Jessica Ricci et al., 2014) – which can persist in the environment long after – several tens of millions of years after – the synthesiser’s demise, as a sort of molecular memory of the BG’s presence. It is these particular biomarkers that Schaefer et al. found concentrated in a core through the asteroid collision site, at a core depth that matched the first 30,000 years after impact**.

Although evidence for other microbial communities appeared in the core after the time of the asteroid’s collision, molecular fossil evidence for the primacy of cyanobacterial colonisation of the impact site appears strong. Schaefer et al’s work adds a highly-detailed, almost day-by-day, ecological chronology to the post-impact colonisation and recovery work carried out by Christopher Lowery et al. (2018). It therefore appears that blue-greens were amongst the first organisms to colonize the newly-created site, setting the scene for a succession of other microbes and exploiting organisms. In a manner akin to secondary succession in other environments, maybe the cyanobacteria can be viewed as the pioneers of this rather unique succession. In keeping with the seric terminology used for successions, Mr P. Cuttings proposes the term impactosere for this ecological phenomenon.

Having survived such a potential escha tonic event, one can’t help but wonder if cyanobacteria are the botanical equivalent of cock roaches, or tardi grades***. Regardless, this work also provides a good ecological example for the adage that nature abhors a vacuum; when a new site is created, life will exploit it.

This and the preceding post [Botany One Part II URL to insert] were rather blue-greencentric, can we get a more human-focussed dimension to an ostensibly cyanophyte story? This writer does like his plants-and-people connections, after all. Well… see Part IV [Botany One URL needed].

* In all, it is estimated that approx. 75% (“three-quarters of all the species on Earth”) – or “some 76 percent of all species on the planet”, or even “ approx. 80% of all species of animals”, or “>75% of all land and sea animals” (Kunio Kaiho and Naga Oshima, 2017) – present at that time were lost after the Chicxulub asteroid impact. Famously, with the demise of the dinosaurs, this event brought the Age of the Reptiles to an end, and heralded the dawn of the Age of the Mammals – and Flowering Plants(!!!). As devastating as its effects were – and contrasting markedly with the imprecision of how much of life was extinguished by the event – it is also one of the rare times in palaeohistory when the start of a new geological era – “the first day of the Cenozoic” (Sean Gulick et al., 2019) – can be pinpointed ‘to the day’ (although we don’t know which day that actually was, and whether it was 66 or 65 (Carl Swisher et al., 1992) millions, or 66,038,000 ± 11,000 years ago…).

** Coring is a technique that can provide historical data about a site such as past environments or life-forms that were present, e.g. using cores of ice or lake sediment (but you’ll have to excuse the suspect maths in the 2^nd paragraph of that item). Taking a core is similar to the way one uses an apple corer to removes the core from an apple, and usually produces a cylinder of material. Extracting time-relevant data from the core works on the basis that material closest to the top of the substrate – e.g. the surface of the ice where it meets the atmosphere, or the boundary between the sediment at the bottom of the lake and the covering water – was deposited most recently, and material further below the ice/sediment surface is progressively older. Cores effectively permit travel back in time whilst remaining in the present, i.e. they provide a window onto the past. And, if you know the rate at which sediment is deposited – or ice accumulates – at the site, then the distance from the top of the core to any particular location along it [i.e. ‘depth’] will relate to a particular date/time in the past. In that way, material within the core can be dated, often quite accurately. Assuming present conditions are a good guide to past ecology, etc, and that nothing untoward has happened that upsets the sequence of deposition of material in the core with time, then the technique can be very helpful in giving evidence of conditions – both biological, chemical, and physical – a long time ago. For example, pollen trapped within lake sediment can give clues to the history of vegetation and climate at or near the site, and analysis of time-dated slices of ice can reveal much about the composition of the atmosphere at times long ago. Or you can core through the site of an ancient collision between an asteroid and the Earth, as performed by Expedition 364 of the International Ocean Discovery Program (IODP), and use it to investigate the recolonisation ecology of that region millions of years ago (as considered in the blog post above).

*** Or maybe those botanical ‘laurels’ should go to ferns..?