How fast do these microbes grow?
We know that there are microbes living in the rocks below the seafloor, but very little is known about how quickly they reproduce. Here I talk about one way I am trying to learn the answer to that question.
Sorry it's been longer than usual since my last post! This has been a super busy week on the JOIDES Resolution. We are at a new site (U1374A) and drilling is going very well here. We just changed the drill bit yesterday and are 200 meters into the basement here. I have been very busy collecting samples, inoculating culturing experiments, and starting stable isotope addition bioassays, which I will tell you a bit about today.
Recent work has shown that there are definitely microbes living in the rocks below the seafloor (otherwise, why am I even here?). A very recent article that is free for anyone can be found here. I'll warn you that this is a technical science article, so it may be a bit much for the non-scientists out there (although you can probably understand the Abstract), but I encourage people with a little more training to read it. Anyway, we know a bit about what types of microbes are living down there, but almost nothing about how fast they are growing and reproducing. People who study subseafloor sediments have much more knowledge about growth rates for their environment. This is an important question, because if one wants to understand how fast carbon is moving throughout the planet (this is called the carbon cycle, and is a direct link to climate change through carbon dioxide), then one needs to know how many microbes there are and how fast they are growing.
One of my goals on this cruise is to measure metabolic rates of microbes living in the subseafloor rock habitat. To do this, I am using stable isotope addition bioassays. This may require a bit of an intro (very brief). Basically, many elements have versions with different numbers of neutrons. Typically there is one version that dominates, but there are other isotopes that are also naturally present, but at low abundance. For example, Carbon 12 (a carbon atom with a molecular mass of 12) represents about 99% of all carbon naturally on Earth. However, there is also Carbon 13, which has one more neutron than Carbon 12 and is naturally present at an abundance of about 1%. Got it?
The cool thing about stable isotopes is that one can measure them pretty easily when added to an incubation. So what I am doing here is adding carbon dioxide labeled with Carbon 13 to incubations with subseafloor rocks and then measuring how quickly the Carbon 13 is incorporated into microbial biomass on the rocks. This needs to be done in the Radiation Van on the JOIDES Resolution, here's a picture of me getting ready to work:
There's not much room in there, but it's very quiet compared to the rest of the ship, which is nice. So I collect lots of rock, and therefore hopefully biomass (this is not easy), and place it in vials. Then, I add a seawater media to the vials to mimic the natural waters in the subsurface. Finally, I add some stable isotope labeled compounds. Here I am in action:
As you noticed from the teaser image at the top of this blog entry, there are lots of vials in the refrigerator. I keep these incubations in the refrigerator because it's cold below the seafloor at these inactive seamounts - about 4 degrees Celcius, which is the same temperature as a typical refrigerator. You also notice that there are groups of bottles- that's because I do time series and measure the amount of stable isotope incorporated into microbial biomass over a few time points. This way, I can determine an actual rate for how fast, on average, the microbial community is growing, by measuring the change in the stable isotope on the rocks at each time point. If the microbes are growing, and therefore incorporating stable isotope, there should be a little more isotope label on the rock with each successive time point. Pretty cool, huh? In the shot below, you can see a close up of some of the vials, and you can see the rocks at the bottom of each vial.
This was somewhat technical, I hope I explained it clearly. Please feel free to ask questions if you want to know more. I appreciate the comments that have been posted and it makes me glad to see that there are other fans of subseafloor microbes out there! That's all for now. Once again, I thank Lisa Strong for making photographs of me at work in the rad van. Next time I'll tell you how microbiologists on drilling cruises make sure that their samples aren't contaminated- drilling is dirty business and we microbiologists put a lot of effort into making sure our samples are clean.