What is poison to one man is a rare delicacy for another. Living in acid or caustic, extreme heat, and toxins does not sound appealing. However, in hot springs in Yellowstone National Park, Galdieria uses energy from the sun to produce sugars through photosynthesis. In the darkness of old mine shafts in drainage as caustic as battery acid, it feeds on bacteria and survives high concentrations of arsenic and heavy metals. How has a one-celled alga acquired such flexibility and resilience?
The unicellular red micro-alga Galdieria sulphuraria (Cyanidiales) is a eukaryote that can represent up to 90% of the biomass in extreme habitats such as hot sulfur springs with pH values of 0 to 4 and temperatures of up to 56°C. This red alga thrives autotropically as well as heterotropically on more that 50 different carbon sources, including a number of rare sugars and sugar alcohols.
To answer this question, an international research team led by Gerald Schoenknecht of oklahoma State University and Andreas Weber and Martin Lercher of Heinrich-Heine-Universitat (Heinrich-Heine University) in Dusseldorf, Germany, decoded genetic information in Galdieria.
They are three of 18 co-authors of a paper on the findings published in this week's issue of the journal Science.
The scientists made an unexpected discovery: Galdieria's genome shows clear signs of borrowing genes from its neighbors.
Many genes that contribute to Galdieria's adaptations were not inherited from its ancestor red algae, but were acquired from bacteria or archaebacteria.
This horizontal gene transfer is typical for the evolution of bacteria, researchers say. Horizontal gene transfer refers to the transfer of genes between organisms in a manner other than traditional reproduction. Also termed lateral gene transfer, it contrasts with vertical transfer, the transmission of genes from the parental generation to offspring via sexual or asexual reproduction. This transfer has been shown to be an important factor in the evolution of many organisms, including bacteria, plants and humans.
However, Galdieria is the first known organism with a nucleus (called a eukaryote) that has adapted to extreme environments based on horizontal gene transfer.
"The age of comparative genome sequencing began only slightly more than a decade ago, and revealed a new mechanism of evolution--horizontal gene transfer--that would not have been discovered any other way," says Matt Kane, program director in the National Science Foundation's (NSF) Division of Environmental Biology, which funded the research.
Galdieria's heat tolerance seems to come from genes that exist in hundreds of copies in its genome, all descending from a single gene the alga copied millions of years ago from an archaebacterium.
"The results give us new insights into evolution," Schoenknecht says. "Before this, there was not much indication that eukaryotes acquire genes from bacteria."
The alga owes its ability to survive the toxic effects of such elements as mercury and arsenic to transport proteins and enzymes that originated in genes it swiped from bacteria.
It also copied genes offering tolerance to high salt concentrations, and an ability to make use of a wide variety of food sources. The genes were copied from bacteria that live in the same extreme environment as Galdieria.
"Why reinvent the wheel if you can copy it from your neighbor?" asks Lercher.
"It's usually assumed that organisms with a nucleus cannot copy genes from different species--that's why eukaryotes depend on sex to recombine their genomes.
"How has Galdieria managed to overcome this limitation? It's an exciting question."
What Galdieria did is "a dream come true for biotechnology," says Weber.
In the future, genetic engineering may allow other algae to make use of the proteins that offer stress tolerance to Galdieria.
Such a development would be relevant to biofuel production, says Schoenknecht, as oil-producing algae don't yet have the ability to withstand the same extreme conditions as Galdieria.
For further information see Genome.
Galdieria image via Michigan State University.