Spilling the genetic secrets of this absolutely massive bacterium

Under the sea, inside the intestines of surgeonfish from the family Acanthuridae, lives a giant bacterium.

This bacterium – which researchers have just named Epulopiscium viviparus – is huge – a million times the volume of E. coli, and can be seen with the naked eye.

Scientists from the United States have undertaken a deep dive on the giant single celled organism to find out how it works, including describing the full genome, and uncovering just how it can survive while being so big.

“This incredible giant bacterium is unique and interesting in so many ways: its enormous size, its mode of reproduction, the methods by which it meets its metabolic needs and more,” said Professor Esther Angert, a microbiologist from Cornell University.

“Revealing the genomic potential of this organism just kind of blew our minds.”

Giant bacteria have been wowing scientists for years, but questions still remain about just how they can get so big. Regular sized bacteria are small so they can easily move any nutrient from their environment into their cells. Plus making a bacteria so big means there are extra issues with how they can get enough energy.

To track down samples of E. viviparus the researchers had to capture surgeonfish from nearby the Lizard Island Research Station in Australia and then quickly extract the DNA and RNA from the bacteria inside.

Once they sequenced the giant bacterium’s genome they found it to be 3.2 million base pairs long, with 2,635 genes.

A whopping 5% of those genes code for carbohydrate enzymes, which allow the bacterium to harvest more nutrients from the gut of the surgeonfish. This is particularly important because E. viviparus doesn’t have access to oxygen so needs to ferment its food. According to Angert: “fermenting organisms just don’t get as much bang for the buck from nutrients.”

The genes could also answer some questions on the second puzzle – energy. One of the most highly produced enzymes in the bacterium is those which make ATP, which the team call the ‘energy currency’ of cells. It also uses something called ‘sodium motive force’ to drive this creation of ATP – a method that is also used to move.

Finally, a multi-folded membrane on the outer edge of the cell means it’s quicker and easier for those nutrients to be turned into energy. This is surprisingly similar to how mitochondria in humans works.

“Amazingly, these membranes in E. viviparus have kind of converged on the same model as the mitochondria,” said Angert.  

“They have a highly folded membrane that increases surface area where these energy-producing pumps can work, and that increased surface area creates a powerhouse of energy.”

Although this is just interesting for interests’ sake, it does also have some practical applications. Because E. viviparus has such effective strategies to make use of the nutrients found in algae it could be useful for wherever algae is used – like livestock feed, renewable energy and even human consumption.

The research was published in PNAS.