he genome of the fuel-producing green microalga Botryococcus braunii has been sequenced by a team of researchers led by a group at Texas A&M AgriLife Research.
The report, in Genome Announcements, comes after almost seven years of research, according to Dr. Tim Devarenne, AgriLife Research biochemist and principal investigator in College Station, TX. In addition to sequencing the genome, other genetic facts emerged that ultimately could help his team and others studying this green microalga further research toward producing algae and plants as a renewable fuel source.
“This alga is colony-forming — a lot of individual cells grow to form a colony,” Dr. Devarenne said. “These cells make lots of hydrocarbons and then export them into an extracellular matrix for storage. And these hydrocarbons can be converted into fuels — gasoline, kerosene and diesel, for example, the same way that one converts petroleum into these fuels.”
Dr. Devarenne pointed to previous studies showing that hydrocarbons from B. braunii have long been associated with petroleum deposits, indicating that over geologic time the alga has coincided with and contributed to the formation of petroleum deposits.
“Essentially, if we were to use the hydrocarbon oils from this alga to be a renewable fuel source, there would be no need to change any kind of infrastructure for making the fuel. It could be put right into the existing petroleum processing system and get the same fuels out of it,” he said.
He said his lab wants to understand not so much how to make fuel, but rather how the alga makes these hydrocarbons, what genes and enzymes are involved and how they function. “Once we understand that, maybe we can manipulate the alga to make more oil or specific types of oil or maybe we can transfer those genes into other photosynthetic organisms to have them make the oil instead of the alga,” said Dr. Devarenne, whose lab in 2016 announced the discovery of the enzyme used by the algae to produce hydrocarbons.
That’s why sequencing the genome was important, he said, because it will help identify all the genes and enzymes in the genome needed for hydrocarbon production and control of this production.
Sequencing the genome means isolating all DNA from the nucleus of the cell, sequencing it into small fragments and then assembling it back together into a complete genome. Think of it as a 166 million-piece jigsaw puzzle, he said, given that the size of the B. braunii genome is estimated to be about 166 million bases.
Dr. Devarenne said that because only portions of the B. braunii genome in this report are “spelled out,” so to speak, it is considered a draft genome, or first attempt at assembling all the pieces.
“It’s not perfect, but it’s still very usable and valuable to the other researchers who are studying this alga,” he said. His own lab plans to do a more in-depth analysis and compare it to other known algae and land plant genomes to see what’s unique and similar.
To assemble the genome, Dr. Devarenne explained, “We send DNA to be sequenced by the Joint Genome Institute, which is part of the U.S. Department of Energy, and they sequence it in lots of very small fragments. These fragments of DNA may be anywhere from 150 to 300 base pairs long. So imagine if we have 166 million bases in our genome, and it is sent back to us in little fragments that have to be assembled back together to arrive at 166 million bases. We used the Texas A&M Supercomputer Center to help.”
As more gaps are filled in, he said, a more complete genome will emerge, and that will help researchers dive deeper into the biochemical processes in this alga. That information will then help them understand how and why the organism makes hydrocarbons in very high quantities, how that process is regulated and what the particular biosynthetic pathways are used to make the hydrocarbons.
“Just like the human genome has been sequenced but isn’t fully understood, there is still a lot to study. It’s really a never-ending process,” Dr. Devarenne said.
The paper can be viewed at http://genomea.asm.org/content/5/16/e00215-17.full.pdf