New and open access! (for a while). Here's a about this paper. Not because I worked on it for way too long and it's still super boring--but because I think there's something really interesting going on with sulfur in the cryosphere that we might have overlooked… https://twitter.com/GCA_Journal/status/1344669066942943232

Typically S can enter the biosphere thru the dissimilatory or assimilatory pathways. We can talk about S in terms of energy (dissimilatory) where e- acceptors are reduced to yield energy or in terms of biomass (assimilatory) where S is incorporated into S-bearing AA, cys & met.

Some of that biomass gets metabolized into other forms of organic sulfur that can lead to the release of volatiles like DMS (& friends), the most common volatile sulfur emitted from the oceans (tho I don't even dare to call it a biosignature).

Or they can get degraded photochemically or oxidatively to form recalcitrant dissolved organic sulfur (DOS) often seen in aquatic environments (marine or lacustrine) and sediments. Oxidized sulfur is common in DOS, but you need oxygenated or photic environments to form them.

So we were flabbergasted when we found highly oxidized sulfur in an ANOXIC, APHOTIC, hypersaline brine, deep within a frozen lake in Antarctica (Lake Vida). I spent years identifying them using mass spec, and then more years convincing myself that they were artifacts.

Because… it simply couldn't be. But the more I dug and dug, the more evidence was stacked up against me. These were not artifacts. They were indeed real. How could these highly oxidized sulfur compounds form? And what the heck are they doing in a brine like Lake Vida?

We're not sure. But we think it could be the result of an ecosystem legacy that carried some of the signatures of Lake Vida's past. Back when it was exposed to sunlight and the atmosphere. Forming things like this is super easy & legacy is pervasive in Antarctica.

But what's neat is another alternative and, IMHO, more fascinating hypothesis. Maybe these compounds were formed from long term brine-rock reactions that produce high levels of reactive species like O and H radicals from the dissociation of water.

We know this is an important mechanism in the deep subsurface, where H2 and SO42- are known to be produced by radiolysis, supporting enigmatic & isolated communities. A highly desirable prospect in terms of habitability of subsurface environments and icy/ocean worlds.

These water/brine-rock reactions are not unheard of. High N2O levels (the highest ever recorded in the WORLD) in Lake Vida was thought to have formed from a process called chemodenitrification, where N2O is produced from the oxidation of Fe2+coupled to reduction of NO3- or NO2-.

Think kinetic can be a problem? Think again--this process was experimentally recorded at -20 C. Lake Vida is at -13C... So when you go to look for biosigs in places like Europa or Enceladus, the one thing you have to keep in mind is that, yes it's cold. Life is probably slow.

But cold doesn't prevent abiotic reactions from happening. Cold just means that abiotic reactions can likelier compete with biological ones. So you might encounter something of the past. A legacy of an older, more dynamic ecosystem...

Or you might encounter a signature of life that has been changed, slowly but surely. Changed by the ever driving physical forces of the natural world. But hopefully not to a point that it's unrecognizable. Anyway, thanks for coming to my *thrilling* TEDtalk about sulfur :)