Saturday, October 6, 2012

Deep-C Ecology Cruise - October 2012

On the trail of oil-eating bacteria. 

Blog Post (WBII1306) 
October 6, 2012 

On this benthic ecology cruise, we are studying the smallest members of the sedimentary food web, the microbes.  We think that a substantial portion of the oil that was released from the Deepwater Horizon spill remains trapped in deepsea sediments of the Gulf of Mexico.  Our objectives are to understand how oil from a spill impacts the microbial food chain in the deep ocean and how microbes may remove or degrade oil hydrocarbons.

Food is scarce in the deep ocean.  Microbes, along with larger organisms, usually feed on detritus (dead/ dying plant and animal material) falling through the water column to the sediments from the lighted area of the ocean.  When organic matter falls through deep water, the organic matter is mostly used up by microbes in seawater before it reaches the sediments.  Thus, there is less organic matter to eat and it is of less nutritive value because a lot of the juicy stuff is already eaten.  The fluffy stuff on the surface of this deepsea core is full of fresh detritus.

Oil serves as a great carbon and energy source for microbes.  Since oil seeps naturally out of the seafloor in the Gulf and elsewhere in the world’s oceans,  a large diversity of microbes (mainly bacteria and fungi) have evolved to take advantage of this great food source.  Biodegradation by microbes is the ultimate fate of most of the oil that enters the marine environment, either from natural seeps or from accidental discharges.  Here you can see a dark brown layer in this core sampled near where oil was discharged during the Deepwater Horizon accident.  We believe that the dark brown layer indicates that sedimentary microbes are consuming oil in the sediments.

Microbes are sampled at the seafloor mainly by cutting up or sectioning cores and then freezing the sediment sections for later analysis in the laboratory.  Our methods do not allow us to immediately “see” the microbes (they are small by definition, right).  However, one can see evidence of microbial activity in every core.  First of all, the color of the sediment very much indicates active microbes.  The red, gray, and black colors of the sediment come from iron oxide and clay minerals.  The last step in the decomposition of organic matter is mediated by a series of respiratory microbes in the order in which they gain energy from various electron acceptors.  Just like we breathe oxygen, so do lots of microbes and oxygen is used up first in the surficial sediments.  After oxygen, the most plentiful electron acceptors are sulfate and iron oxide minerals.  The red iron oxide minerals turn gray as they are “breathed” or iron-respiring bacteria.  This causes the change in color of deepsea sediments from the top of the core to the mid layers of the core.  If there is sufficient organic matter reaching the seafloor (as oil or natural organic matter), black sediment forms from iron sulfides.  The iron sulfides are produced by bacteria that reduce or respire sulfate to sulfide, that then combines with iron to from iron sulfide minerals.  Thus, you see that we find evidence of microbial action is everywhere in our samples, even before we get back to the lab.

Sediments collected from deeper waters of the Gulf have thicker layers of red sediment, indicating that there is less organic matter available to microbes and it takes longer for oxygen and iron oxide minerals to be used up.

Conversely, at shallower water depths, the red layer is thinner and darker colors are observed, indicating that there is more detritus available and microbes use up electron acceptors faster as the sediment is buried.

One can also observe, that the activity of microbes is coupled to that of larger macrobes.  Here you can see that around the burrows or holes created by polychaete worms in the sediment, sulfate-reducing bacteria are eating organic matter collected by the worms and thereby producing iron sulfides around the worms burrows.

Posted by:
Joel Kostka,
Chief Scientist, WBII1306

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