This is a microbe's world, not a human one, and compared to us, their biological machinations are bonkers. Some snooze in massive crystals, some use irradiated sulfur-rich minerals to live in deoxygenated darkness, and others, as noted by ScienceMag, can quite literally manufacture fuel.
Back in 1986, Swiss microbiologists uncovered a bacterium within Lake Zurich, and found that it synthesizes toluene, a type of hydrocarbon. Now, as reported in Nature Chemical Biology, a team led by the Lawrence Berkeley National Laboratory has worked out how this microbial alchemy takes place.
Toluene is used as an octane booster in gasoline, but that’s not its only use; it’s also a solvent found in paint thinners, glues, varnishes, rubber cement, and it's even involved in the making of TNT. It’s also used by some as a form of recreational drug, but considering it can be neurotoxic, this is inadvisable.
Humanity first synthesized toluene in the early-to-mid 19th century, but it appears that bacteria may have pipped us to the post. Although the bacterial genus found in Lake Zurich, Tolumonas auensis, was the original microbial maestro in this regard, it’s not the only one that has this ability – Clostridium aerofoetidum has the skills too.
Sadly, no-one’s been able to figure out how any of them manage to proceed with this “biochemically challenging reaction”, as the multidisciplinary team note in their paper.
They explain that attempts to use these bacteria to “reproduce toluene biosynthesis” in laboratory conditions have been unsuccessful, and although they suspected a particular enzyme was playing a key role, only indirect evidence was found to date.
Leaving Lake Zurich behind, the new team of researchers had a peek inside Jewel Lake, in a park in Berkeley. Finding samples of toluene in the lake, and a nearby sewage treatment plant, they ran samples through a metagenomic analysis, hoping to identify any biochemical componentry that could help explain where it was coming from.
They succeeded. They found a collection of genes that were always paired with two specific enzymes. The first, PhdB, is an enzyme of bacterial origin, one that catalyzes, or accelerates, a key toluene-manufacturing chemical reaction. The other, PhdA, which activates PhdB, is found in two distinct oxygen-lacking microbial communities.
In order to confirm that these enzymes were responsible, they inserted the relevant genes into a common bacterial species in the lab. Chemically tagging the relevant chemical constituents, they observed the enzymes working to make toluene.
This discovery was certainly reached using perhaps unconventional means, but it appears to have solved a mystery more than 30 years in the making. What isn’t clear, though, is why these microbes take the trouble to synthesize toxic toluene in the first place, although the authors offer several possible explanations.
The first is that it’s used as a form of “negative allelopathy”, a technical term for inhibiting the proliferation of a competing species. An alternative hypothesis is that it allows the bacteria to make physiological alterations to itself, making it more tolerant to acidic conditions. The team speculated that it may even provide a source of energy to the bacteria themselves.
Bacteria may certainly be better than our biotechnology at present, but the team’s paper hints that, should we be able to harness this process ourselves, a new source of renewable fuel may not be out of the question.