Deep sea carbon cycling: microbial action, and mystery

I mentioned this study a while ago, and promised a more detailed explanation. I apologize for how long it has taken, but here it is.

How excited was I to learn that the most recent issue of Nature Geoscience had a special focus on deep sea carbon cycling? I admit it, pretty excited. I was even more excited to learn that one of the 3 papers making up this special focus was about the microbial component of deep sea carbon cycling. This may not be something that you think about every day, but I do... well most days at least. The first two sentences of this paper explain why I find this topic so interesting.

Circulation of hydrothermal fluids in upper oceanic crust may support one of the most extensive, but least understood, of the Earth's biogeochemical systems. However, little is known about non-living organic matter carried in crustal hydrothermal fluids or its possible impact on the carbon cycle of the overlying ocean.

Organic carbon, that is molecules with carbon-hydrogen bonds that have been created by living organisms (ie carbohydrates, proteins, lipids, nucleic acids...), exists in water in two different pools. Particulate organic carbon (POC) is the fraction of organic C that is physically large enough to be filtered out of the water. POC has a shorter lifespan in water (easier to eat, and more reactive) and therefore can tell us something about the local, recent happenings in the water. Dissolved organic carbon (DOC), on the other hand, has a much longer "lifespan" in water, and therefore contains information about the conditions that water has seen over much longer time scales. In the case of the deep sea DOC can tell us about processes that the water carrying the carbon was exposed to as it traveled through the crust.

One of the main ways scientists "read" the carbon to learn about environmental conditions and processes is by analyzing the carbon isotope rations. Carbon isotopes are atoms of carbon with slightly different masses. Certain processes preferentially use "light" or "heavy" carbon, and therefore the overall pool of carbon compounds created by that process becomes enriched in that particular isotope of carbon. The isotopic signature acts as a fingerprint, allowing scientists to determine the processes that likely created the pool of carbon.

Analyzing the type and source of organic carbon that comes out of the Earth's crust, and spills into the ocean can help scientists understand the extent to which the deep ocean processes (microbial and purely geological) influence the broader marine, and global, carbon cycles. This, in turn, is important in understanding the climate system of our planet. If the organic carbon present in vented hydrothermal fluids contains organic carbon that was present in the deep ocean waters before they were entrained in the crust and traveled through it, then the microbial communities in that crust are likely heterotrophic - consuming some of the organic carbon as it passes through the crust. However, if it turns out that much of the organic carbon in these fluids is different from the carbon that went in, that is - if it was produced in the crust rather than carried down into it, it is likely that the crustal microbial communities are at least partially chemosynthetic, and this has much greater implications for the role that the subsurface biosphere plays in the global carbon cycle.

OK... enough background.

In a paper entitled "Chemosynthetic origin of 14C-depleted dissolved organic matter in a ridge-flank hydrothermal system" scientists analyzed the isotopic composition of DOC in hydrothermal fluids (both on and off axis) along the Juan de Fuca Ridge (JdFR). Added bonus (!)- JdFR just so happens to be where my research takes place.

Their data suggest that the organic carbon coming out of these vents is different that the carbon that went in, and therefore there are communities of microbes deep in the crust, turning inorganic carbon into organic carbon. This process may represent an important source of organic carbon into the overlying ocean... or it might not. Yes, its frustrating, but the jury is still out on this one.

The areas where hot fluids that come directly out of the vent chimneys are fairly limited in geographic range, but the warmer fluids that percolate through the ridge flanks represent a very large geographic area and fluid volume. This fluid flow is likely greater the combined fluid flux than from all the world's rivers into the oceans. However, until we know more about how quickly this DOC is degraded, it is difficult to estimate the impact that it has on the chemistry or ecosystems of the deep sea. This study to paints a picture of the deep subsurface as both a DOC scrubber and a DOC producer. According the the author's the deep ocean carbon that gets sucked into the crust with seawater is removed in the crust, and new, different organic carbon is released in the hydrothermal fluids that leave the crust. At this time it is hard to quantify the extent to which deep subsurface acts as a source or a sink for DOC, and as such the influence of vent and subsurface microbial communities on the ocean carbon cycle remains a mystery. However this study is one step towards figuring that out.

McCarthy, M., Beaupré, S., Walker, B., Voparil, I., Guilderson, T., & Druffel, E. (2010). Chemosynthetic origin of 14C-depleted dissolved organic matter in a ridge-flank hydrothermal system Nature Geoscience, 4 (1), 32-36 DOI: 10.1038/NGEO1015

Readers: I am interested in your feedback on posts like this. Are studies like this of interest, even though they don't have a clear and obvious punch line and answer? Are you more interested in the scientific process (ie how did the researchers carry out their study), or the conclusions that were reached (forget the boring details, just tell me what they discovered)?