Scientists at the University of Oregon have discovered a form of blue-green algae that lives independently in California's Salton Sea, using near-infrared light for photosynthesis, according to an article published in this week's online edition of the Proceedings of the National Academy of Sciences (PNAS). "This new strain of Acaryochloris is unique because it is able to live on its own," says UO biology professor Michelle Wood. She obtained samples containing the organism while studying the diversity of blue-green algae in the hypersaline lake as part of the comprehensive study of the Salton Sea coordinated by Professor Stuart Hurlbert, director of the Center for Inland Waters at San Diego State University.
Scott
Miller, the lead author on the paper, grew the organism from samples
collected by Wood and Hurlbert. Now an assistant professor of
biological sciences at The University of Montana, he was working with
Wood as a graduate research associate when he first noticed the
organism's unusual lime-green color.
"I knew right away there was something unusual about its
photochemistry," Miller recalls.
"We purified the pigments from the strain and saw that they were very
similar to those known from a species of blue-green algae called
Acaryochloris marina, but different from any found in other higher
plants or algae. The primary pigment, called chlorophyll d, is only
made by Acaryochloris and it is what enables these species to use
infrared light."
The new microbe is one of only three organisms known to science that
use a combination of near-infrared light and visible light to produce
oxygen by photosynthesis.
"While there are some bacteria that can use infrared light for
photosynthesis, they do not produce oxygen," Wood explains. "Until
recently, we thought it was necessary to use visible light to produce
oxygen through photosynthesis, but now we know there are at least three
organisms that can do this using infrared radiation as well.
"All three of these organi
sms are closely related species of
Acaryochloris, but the other two live in the Pacific and must grow on
or in an animal or plant to survive in nature," Wood says. "This new
microbe opens up a whole new range of possible habitats where oxygen
could be produced by photosynthesis using wavelengths of light that
exist beyond the visible spectrum."
The PNAS article is co-authored by Miller and Wood along with Sunny
Augustine and Jeanne Selker of the University of Oregon and Tien Le
Olson and Robert E. Blankenship of Arizona State University. Augustine,
a postdoctoral associate in Wood's lab, worked with Selker, former
director of the UO electron microscopy facility, to compare the
cellular structure of the new organism with that of the symbiotic form
of Acaryochloris. Blankenship and Olson compared its pigments with
those of A. marina, one of the symbiotic species.
"Chlorophyll d is a pigment that is intermediate between the
chlorophylls found in the more primitive non-oxygen evolving
photosynthetic bacteria and the chlorophylls found in oxygen evolving
photosynthetic organisms. It may have an important place in the
evolution of photosynthesis," says Blankenship, professor and chair of
the ASU Department of Chemistry and Biochemistry.
Miller says another surprising discovery occurred when the scientists
studied the DNA of the new organism. By analyzing sequence data for the
small subunit ribosomal RNA gene, which encodes part of the cell's
protein synthesis machinery, they demonstrated that chlorophyll
d-producing blue-green algae (more technically known as cyanobacteria)
have acquired a piece of DNA from a proteobacterium, a distant relative
that last shared a common ancestor with cyanobacteria more than two
billion years ago.
The small subunit ribosomal RNA gene is widely used by scientists to
infer the relationships among living organisms, in part because it is
generally assumed that it is faithfully transmitted from parent to
offspring. However, in the case of this
new microbe, it appears that
DNA encoding a small portion of the ribosomal gene in a proteobacterium
jumped across the vast evolutionary distance that separates the
proteobacteria and cyanobacteria, and switched places with the portion
of the gene that had originally been inherited from the cyanobacterial
parent.
"This finding shows that even this popular evolutionary chronometer can
be a mosaic of genetic information with radically different origins,"
Miller says.
Using a molecular clock, Miller estimated that the proteobacterial DNA
was obtained by an ancestor of modern chlorophyll-d producing
cyanobacteria between roughly 10 and 100 million years ago.
"The maintenance of this hybrid gene over such a long time period
suggests that it has been favored by natural selection," Miller says.
The foreign DNA encodes a structural feature of the ribosome that makes
large and precise movements during protein synthesis, but its exact
function is still unknown. The next challenge, Miller says, will be to
determine whether this example of instant evolutionary innovation by
genetic exchange has in fact had consequences for ribosome function in
these bacteria.
The new species of Acaryochloris is the latest in a series of new
organisms from the Salton Sea that have been identified as part of
Wood's study of the blue-green algae in the lake. With Miller, UO
emeritus professor Richard Castenholz, and Canadian oceanographer
William Li, she published a paper in the journal Hydrobiologia in 2002
that described five previously unknown species.
The Salton Sea covers a surface area of 376 square miles in
southeastern California. Its current elevation is about 227 feet below
mean sea level, its maximum depth reaches 51 feet and its total volume
is about 7.5 million acre-feet. It was formed in the early 1900s, when
flow from the Colorado River was inadvertently directed to the Salton
Basin. Once the Army Corps of Engineers returned the river to its
normal bed, the lake began to evaporate,
gradually becoming one of the
largest hypersaline lakes in the United States.
Because it also receives the agricultural runoff of the Imperial Valley
and municipal runoff from Mexicali, Mexico, considerable attention has
focused on the fate of the now highly polluted water body. After
massive fish kills and avian mortality in the 1990s, federal agencies
targeted the lake for one of the nation's largest restoration projects.
Wood says the discovery also shows that the Salton Sea, with its high
load of nutrients, may provide an environment that allows this novel
photosynthetic organism to live a free and independent lifestyle. She
notes that the other two species of Acaryochloris live in relatively
pure ocean water, but cannot survive there unless they are growing in
or on another organism.
"I think it is likely that this microbe is descended from symbiotic
relatives who got to the Salton Sea as hitchhikers in water containing
sport fish that were introduced from the ocean many years ago," Wood
explains.
She says the Salton Sea may have fostered this species of
Acaryochloris' ability to live independently by mimicking the
environment created by the original host. In the open ocean, which
itself is nutrient poor, the host animal or plant provided a
nutrient-rich environment with relatively high amounts of infrared
light; in the Salton Sea, the waters themselves create a comparable
niche. Wood thinks that the high availability of nutrients in the
waters of the Salton Sea is what allowed the microbe to survive without
its hosts in the saline lake.
At the University of Oregon since 1990, Wood is a member of the UO's
Center for Ecology and Evolutionary Biology. She has played a key role
in changing the scientific world's understanding of how ocean food webs
are based on much smaller organisms than previously believed.
Wood is an adjunct scientist at the Bigelow Laboratory for Ocean
Sciences and the Harbor Branch Oceanographic Institution. In 2004, she
became a fellow of th
e Cooperative Institute of Oceanographic Satellite
Studies (CIOSS) at Oregon State University.
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Source:University of Oregon
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