Oil spill is boon to bacteria

Last month's blowout of British Petroleum's Deepwater Horizon oil well -- which caused the US Commerce Department to decree today (25th May) that fisheries in three states bordering the Gulf of Mexico are official disasters -- is likely already impacting the Gulf's microscopic denizens, which will, in turn, have long-term effects on commercially important species such as fish and shrimp, scientists say.

Image: National Oceanographic and
Atmospheric Administration

Images of oil-soaked sea gulls and tar coated turtles, which typically follow major oil spills, are starting to materialize in the Gulf, but bacterial populations are likely to boom in response to the release of millions of gallons of oil, linkurl:Monty Graham,;http://www.gulfbase.org/person/view.php?uid=wgraham biological oceanographer at the Dauphin Island Sea Lab off the coast of Alabama, told __The Scientist__. Bacteria could benefit from the oil spill, Graham argued, because some bacterial species and lineages view a massive oil spill as a veritable cornucopia of delicious hydrocarbons, not a catastrophe. The boon to bacteria most likely has ramifications that will ripple throughout marine food webs in the Gulf, especially at prominent nodes where commercially important species such as fish, crabs, and shrimp, reside. What those ramifications are, however, remain as murky as the huge plumes of oil recently discovered hovering just below the surface of Gulf waters. Researchers who study microbial and planktonic ecosystems in the Gulf of Mexico are anxiously anticipating the effects of what may be the biggest oil spill ever. Recently uncovered evidence of oxygen depletion near those plumes (an indication that bacterial respiration is occurring en masse) indicate that a bacterial bloom is ramping up. Normally in the Gulf, as in other oceanic ecosystems, the base of the food web is provided by photosynthetic, one-celled organisms called phytoplankton. Sunlight fuels phytoplankton growth, small crustaceans called copepods dine on phytoplankton, fish larvae (and larvae from other species) dine on copepods, small fish dine on larval organisms, and so forth and so on. Bacteria normally exist in a semi-self-contained food web called the "microbial loop." But if they expand dramatically, these new gobs of organic matter could attract hungry, one-celled heterotrophs called nanoflagellates, which could feed slightly larger ciliates, which would then be fed upon by copepods, thus entering the normal food web that ends in commercially or recreationally important species, such as the Red Drum. In theory, this extra organic matter could help compensate for the loss of phytoplankton, since photosynthesis rates (and thus phytoplankton) may decline due to decreased sunlight penetration through oily water. However, by the time that bacteria-driven energy reaches the upper trophic levels of the food web, it has gone through more steps along the way than the energy stemming from phytoplankton. Since energy is lost at each level, adding extra levels means less energy makes its way to the big eaters. "You really only recover about 10-20 percent of the energy from one trophic level to the next in the food chain," Graham noted, adding that introducing 2 or 3 more steps to the food web "adds a lot of energy loss before it gets to fish." linkurl:Rob Condon,;http://www.skio.usg.edu/?p=news/showarticle&n=126 plankton and microbial ecologist at the Bermuda Institute of Ocean Science, agreed that all the oil floating in the Gulf will likely have big food web impacts. "You could actually see a shift in the food web," he told __The Scientist__. "Typically with oils spills you have a lot of reports on vertebrates and the damage to wetlands, and that's very important, but what's often overlooked are these small components of the food web," such as bacteria. Condon, who studies jellyfish, envisioned a slightly different, though no less disruptive, scenario. Condon said that a boom in bacterial numbers could lead to a mushrooming in populations of jellyfish, which can dine directly on bacteria. Because fish don't typically eat jellyfish, he said, and because microbes can feed on mucus secreted by jellyfish, the oil may create a trophic loop where the surplus bacteria don't enter the flagellate-ciliate-copepod food chain, and instead just fuel big jellyfish numbers. If decreased light penetration knocks down populations of the photosynthetic plankton that form the base of Gulf food webs at the same time, a bloom in jellyfish would prevent the surplus bacteria from making its way into the typical food chain, thus effectively shunting carbon and energy away from higher trophic levels containing commercial fish species. For now, however, all scientists have are hypotheses of what will happen to the food chain, Graham and Condon conceded, and there are many unanswered questions yet lurking in the Gulf. "It's kind of like a natural lab out there where one can test these kinds of hypotheses," Condon said. "We've never seen this sort of localized heavy loading of organic matter for such an extended period of time," Graham agreed. "It's kind of generating a natural experiment." "It looks like there's evidence that microbes are responding to the hydrocarbons," University of California, Santa Barbara, microbial ecologist linkurl:Craig Carlson;http://www.lifesci.ucsb.edu/eemb/faculty/carlson/ told __The Scientist__. "How that's being passed up to higher trophic levels will have to be resolved over the coming weeks and months."
**__Related stories:__***linkurl:New NSF grants for oil spill;http://www.the-scientist.com/blog/display/57441/
[19th May 2010]*linkurl:Scientists brace for oil impact;http://www.the-scientist.com/blog/display/57410/
[17th May 2010]*linkurl:Oil Spill Spawns Alaskan 'Science Rush';http://www.the-scientist.com/article/display/9385/
[12th June 1989]