Adult swim – Old carbon in the DOC pool

We’re starting a summer biological oceanography journal club at LDEO and the inaugural paper is by Lechtenfield et al. (2015), published just a couple of months ago in Nature Communications.  Titled Marine sequestration of carbon in bacterial metabolites, the paper is one of several very interesting articles to come out recently on the nature of marine dissolved organic carbon (DOC).  Marine chemists are on the cusp of something akin to where microbial ecologists were in 2005, when major advances in sequence technology suddenly opened up a world of microbial diversity that hadn’t really been anticipated.  Ten years later we’re still grappling with the paradigm shift, in particular with the question of how necessary understanding all this diversity is to understanding the function of the microbial ecosystem.  Interestingly, because microbial enzymes are involved in the modification of DOC components, the question of DOC chemical diversity and microbial diversity are fundamentally intertwined.

Why all the fuss about DOC?  The ocean’s a big place and contains a shocking amount of carbon – around 38,000 gigatons (or pentagrams, if you prefer).  To put that into perspective consider that the atmosphere contains only about 750 Gt.  Given the size of this reservoir, and the sensitivity of the climate system to atmospheric carbon, the amount of carbon going into and coming out of the ocean keeps a number of people up at night.  Most of this carbon is inorganic; CO₂ and it’s various sister species.  Although this inorganic pool is influenced by biology through photosynthesis and respiration, it is primarily controlled by rather dull and predictable geological cycles*.  A smaller but still very significant amount, roughly equivalent to the amount of carbon in the atmosphere, is DOC.  DOC is interesting because it can be cycled very quickly or very slowly, depending on its chemical structure.  Structures that are easily broken down by microbial enzymes are called labile.  Those that are not are called refractory.

Until recently marine chemists exploring the nature of the DOC pool primarily undertook targeted studies of specific DOC components.  A researcher, for example, might choose to look at a certain class of lipids.  Chemical properties of the target compound class, such as solubility in different solvents, would be exploited to isolate the compounds from seawater, and the compounds would then be analyzed by mass spectrometry and other methods.  Such methods require very precise a priori knowledge of the compounds of interest – a problem for the highly complex and poorly characterized marine DOC pool.

Lechtenfeld et al. used solid phase extraction (SPE) in combination with mass spectrometry, and, more importantly, nuclear magnetic resonance spectroscopy (NMR) to undertake a broad-spectrum characterization of the DOC pool.  By combining these methods they were able to consider both compositional diversity (the various combinations of CHNOP and S that make up organic matter) and structural diversity (the actual arrangements of these elements into meaningful functional groups), without a priori knowledge of what they were looking for.

Using these methods they analyzed the DOC pool produced by marine microbes grown on simple and well defined carbon sources along with the DOC pool of natural seawater.  What they found was, first of all, that the DOC pool is enormously complex, as illustrated in the picture below.  The figure draws heavily on this incredibly detailed work.  I spent a little time with it and I’m still not sure exactly how to interpret the figure – but my best guess is that each point represents a unique molecular configuration, and that (as indicated on the plot) certain regions are diagnostic of specific functional groups.   The plot is in fact a 2D representation of the NMR data, so it helps to think of it in terms of the more familiar independent 1D representations before trying to put it all together.

In this figure the left two frames are DOC from the culture experiments while the right is from near-surface seawater.  While the seawater is clearly more complex, the cultures produced an astonishingly diversity of chemical compounds in a very short amount of time.  Interestingly, roughly %30 of the molecules they produced belong to a class of molecules called carboxyl-rich alicyclic molecules (CRAM).  One example CRAM are the hopanoids, pictured below sans carboxyl-rich side chain.

Due to their cyclic nature CRAM and related molecules can be very resistant to microbial degradation.  I went to a very cool talk recently where it was proposed that these compounds are most readily degraded when ocean water cycles through the aquifer on hydrothermal ridge flanks.  That happens more than one might think, but your average CRAM still has a long wait before its particular packet of water happens to cycle through the aquifer.

So the point is that marine bacteria are efficient little factories for converting simple, labile organic compounds into complex, refractory ones.  This increases the average age of the DOC pool and the amount of photosynthetically fixed carbon that can be sequestered in the deep ocean and sediment.  Why and how, exactly, they do this is an open question, as is exactly how refractory is refractory?

 

 

*I know you geochemists have a sense of humor.