In a discovery that adds a new component to the ocean’s carbon cycle — a major contributor to the earth’s carbon cycle on which all life depends — a team of scientists led by Rutgers Professor Zbigniew S. Kolber has discovered that strange, plant-like bacteria capable of a certain type of photosynthesis are far more numerous in the ocean than previously thought.
Kolber is a researcher at the university’s Institute of Marine and Coastal Sciences
(IMCS) at the Cook College campus in New Brunswick In a study published in this
week’s Science magazine, Kolber and his colleagues from Rutgers and five other
universities found that so-called aerobic photoheterotrophic bacteria, or bacteria that
sometimes act like plants and use photosynthesis to satisfy their metabolic energy
needs, may constitute 11 percent or more of all microbes near the surface. Science
magazine is published by the American Association for the Advancement of Science in
Washington, D.C.
Previously, it was generally believed that the ocean’s carbon cycle was founded
almost exclusively on phytoplankton, the tiny plants that serve as the basic food or
oceanic “prairie grass” on which ocean life relies.
Kolber said that the bacteria, which unlike phytoplankton use a photosynthetic
process that doesn’t produce oxygen, appeared in virtually every sample of ocean
water taken by the researchers during their three-week scientific expedition off the
coasts of Oregon and Washington last July.
“These bacteria appear to be a significant component and have to be considered
when scientists look at what’s happening with the carbon cycle and how much is
‘fixed’ or combined with other elements in organic matter and how much is ‘respired’
or returned to the environment by organisms in the open ocean,” says Kolber.
The Rutgers researcher cautioned that at present the impact of the photosynthetic
bacteria on the carbon cycle is unknown: “We don’t know how they affect the amount of
carbon actually sequestered (captured in the form of organic matter) by the ocean, and
whether the discovery of their widespread presence will increase that amount or not.”
The carbon cycle begins when plants use sunlight and carbon dioxide in the air or
dissolved in water to make carbohydrates, fats and proteins. Most of these are
consumed by humans and animals or respired by microbes, and ultimately converted
back to carbon dioxide and other elements which plants use to start the process all
over again.
Scientists first discovered the photosynthetic bacteria more than 20 years ago, mostly
in organically-rich environments such as seaweed masses, beach sands, and
cyanobacterial mats, small, purplish-red mats of bacteria that are often found in
nutrient-rich environments.
More recently Kolber found the photosynthetic bacteria in ocean surface waters, a
discovery which was detailed last year in the science journal, Nature, published in
London, England. “However,” notes the scientist, “this is the first time that they’ve been
found in such abundance in the upper ocean and that their distribution has been
measured vertically at various depths.”
During their expedition last year, Kolber and his team originally sought to verify the
presence of the strange bacteria at hydrothermal vents on the ocean floor in the
northeast Pacific. “It was postulated that the light generated by the superheated water
in thermal vents and chemical processes taking place around them would provide a
good environment for the photosynthetic bacteria,” says the researcher. “But to our
dismay, we found none.”
However, sampling water at different levels from the surface on down to the bottom,
they were astonished to find an abundance of the creatures just a few dozen meters
below the surface; “The maximum number were observed at 30 to 40 meters deep,”
says the scientist. “Below 150 meters, their concentration decreased below our
detection limits, but we found them in virtually every surface water sample. They
appeared to be everywhere.”
Kolber said that the photosynthetic bacteria the researchers found may even be a new
species, different from similar bacteria previously discovered since they are so
widespread and readily adaptable to changing environments.
The bacteria are able to switch from a metabolism based on organic carbon to one
based on photosynthesis, depending on the concentration of organic matter in ocean
water, says the researcher.
When there’s not enough organic matter, they make a bacterial form of chlorophyll
pigment and use photosynthesis to provide them with energy while “fixing” or
converting carbon dioxide into organic matter, although with lower efficiency than
phytoplankton.
When the environment changes, and more organic matter is available, these bacteria
cease to make photosynthetic pigment and start to consume organic matter the way
normal bacteria do.
The scientist estimates that the unusual bacteria are able to supply between 20
percent to 40 percent of their energy requirements from sunlight. “They are not very
efficient photosynthesizers, and photosynthesis alone probably wouldn’t keep them
alive,” says Kolber. “But combined with their ability to obtain nutrients the way other
bacteria do, they are able to thrive.
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Researchers included Paul C. Falkowski, Constantino Vetriani and Michal Koblizek of
Rutgers IMCS; F. Gerald Plumley of the Institute of Marine Science at the University of
Alaska, Fairbanks, Alaska; and Andrew S. Lang and J. Thomas Beatty of the
Department of Microbiology and Immunology at the University of British Columbia,
Vancouver, Canada.
Other research team members were Robert E. Blankenship of the Department of
Chemistry and Biochemistry at Arizona State University, Tempe, Ariz.; Cindy L.
VanDover of the Biology Department, College of William and Mary, Williamsburg, Va.;
and Christopher Rathgeber of the Department of Microbiology, University of Manitoba,
Winnipeg, Canada.
