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Plankton Genome Sheds Light into Making Fish Oils

19 June 2013

UK - Rothamsted Research scientists have been part of an international consortium that sequenced the whole genome and started characterizing the genes of the ecologically important marine alga Emiliana huxleyi, or "Ehux" to its friends.

"Ehux" is a coccolithophore, with an exoskeleton made of calcium carbonate. Even though the process by which the alga's "armor" forms releases CO2, Ehux can trap as much as 20 per cent of organic carbon, derived from CO2, in some marine ecosystems.

Ehux also produce interesting and important fatty acids such as the omega-3 long chain polyunsaturates such as EPA1 and DHA2. Sequenced by the Department of Energy Joint Genome Institute (DOE JGI), the Ehux genome was compared with sequences from other algal isolates and the results reported in the June 12 edition of Nature.

Ehux and its brethren are the basis of most ocean food chains. Phytoplankton biomass exceeds that of all marine animals combined. Activities of Ehux and some other phytoplankton such as diatoms influence climate processes, such as lowering ocean temperatures by reflecting sunlight and through carbon metabolism. Its versatility in either contributing to primary production of organic compounds from carbon dioxide or adding to CO2 emissions makes Ehux a critical player in the marine carbon cycle.

Part of the third most abundant group of phytoplankton, behind the diatoms and dinoflagellates, the Ehux strain was isolated from the South Pacific and is the first reference genome for coccolithophores. Originally estimated to be about 30 million bases, closer to a diatom, but the genome ended up being closer to 141 million bases.

"Carbon dioxide is fixed during photosynthesis and calcification," said Betsy Read, a professor of biological sciences at California State University, San Marcos who led the large international consortium of 75 researchers from a dozen nations exploring Ehux and the first author of this paper.

"It is also released during the process of calcification, but we do not know how this release balances with the amount of CO2 that is buried when Ehux sinks to the bottom of the ocean. This is an important yet unresolved question."

"Ehux thrives in a broad range of physiochemical conditions in the ocean," said Igor Grigoriev, the paper's last author, whose team from the DOE JGI led the genome annotation and analysis.

"It's a complex genome, with lots of genes and repeats, the first reference for haptophytes and fills another gap in the Eukaryotic Tree of Life. It is amazing that while you need a microscope in order to see this elegantly sculptured microbe, you can see from outer space the light reflected from large areas of ocean during Ehux blooms."

The researchers also found that the core gene sets include genes that allow Ehux to produce interesting and important fatty acids such as the omega-3 long chain polyunsaturates such as EPA and DHA.

Professor Johnathan Napier of Rothamsted Research noted that Ehux used an unusual aerobic pathway to synthesise these fatty acids.

"Micoalgae such as Ehux are the primary producers of so-called fish oils, and given the abundance of this coccolithophore in our oceans, it means that this remarkable organism plays a critical and quirky role at the base of the foodwebs which provide omega-3 PUFAs," he said.

Professor Napier's team at Rothamsted Research, who receive strategic funding from BBSRC, has previously used fatty acid biosynthetic genes from algae to generate transgenic plants capable of synthesizing EPA and DHA.

"We will be very interested in testing the capacity of these new Ehux biosynthetic genes to make omega-3 PUFAs in transgenic plants," said Professor Napier. "These genes may allow for more efficient synthesis of these oils that are important for our health and the environment".

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