Graduate Student Update from Laura Stevens: Are stromatolites always good biosignatures?


Biosignatures are anything left in the geologic record that could have only formed because life was present, and stromatolites---these dome-shaped, layered rock structures---are thought to form only in the presence of life. Graduate student Laura Stevens is working to build a computer model to understand how stromatolites form and if it’s possible for stromatolites to form without the presence of life.

Image: Cool stromatolite from Wyoming! It’s probably around 40-50 million years old.

(Stromatolite Image Source)

I'm in Nancy Hinman's group, and we work on understanding biosignature formation, using hot springs at Yellowstone as our study sites. It's hot and dirty and it smells really bad, but it's fun!

So, biosignatures are anything left in the rock record that could only have formed because life was present. Fossils are a great example of a biosignature, but there are all kinds of consequences of life that can be geologically preserved, and we still don't fully understand them all.

My big focus is stromatolite formation. Stromatolites are these dome-shaped, layered rock structures that, at least right now, form only in the presence of life.But there are other kinds of layered rock structures that don't need life to form. And, the oldest stromatolites we know about were formed way before cyanobacteria, the organisms that currently form stromatolites, are supposed to have existed. So, could these form through other means that don't involve life? Are stromatolites always good biosignatures?

I'm building a computer model to figure this out, and I need real data for inputs. If stromatolites can form in the absence of life or abioticall we're thinking that water chemistry would be one of the major drivers of the process. So this summer, I gathered chemical data at some different hot springs that I can use in my model. I got lots of pH data, temperature data, and UV data. PH and temperature are big controls on all kinds of chemical reactions, and UV light is super energetic, so it can drive reactions too.

One of our other research targets is seeing whether life in hot springs leaves chemical traces behind. We're specifically looking at the relationships between iron cycling, reactive oxygen species, and the microbes that live in hot springs. Here's a diagram showing everything that happens!


First, microbes poop and die and do their microbial stuff. They put out lots of organic compounds that dissolve in the water (DOC = dissolved organic compounds) that react with light to become highly reactive with other compounds (DOC* = highly reactive DOC).

These DOC* get caught up in the chemical pathway that produces reactive oxygen species (ROS) and hydrogen peroxide (H2O2). ROS are things like the free radicals that you drink all that acai juice to get rid of. They're super toxic to all living things, so living things developed ways to deal with them really early on. Even the most basic microbes have enzymes that break down hydrogen peroxide the progenitor of a lot of ROS into water and oxygen, so water with microbes in it will have basically no hydrogen peroxide in there, since their enzymes break it all down.

The reaction that forms ROS and H2O2 also turns dissolved iron, called iron-II, into insoluble iron-III. Since its not dissolved in the water, it accumulates on the sides of springs, where microbes live, and suffocates them, and they die. It is sad.

This summer, Nancy looked at H2O2 concentration as kind of a proxy for that whole relationship, since it's an end product in that big reaction. Because sunlight is an integral part of this reaction, we had to be at our site when the sun rose and we couldn't leave till the sun set. Our field site was out in the Norris Geyser Basin, where the soil is so acidic that we couldn't put anything cloth down on it for too long, or it would get eaten through. Plants don't really like to live out there although there are some scrubby pines so there's not a lot of roots to hold down the soil, and everything is very dusty. They were long and very windy days but good for science!