Scientific progress has led to the introduction of many new chemicals into everyday life. Unfortunately, in addition to their benefits, some of those chemicals have caused problems with toxicity. A group of chemicals that have faced this challenge are called polybrominated diphenyl ethers (PBDEs); they have been widely used as fire retardants but are now restricted due to their toxicity and tendency to accumulate in organisms.
Surprisingly, these complicated chemicals are also made naturally. In some cases, the natural compounds show even higher toxicity than their man-made counterparts. These naturally occurring chemicals are found at all levels of the marine food chain, from cyanobacteria to whales, and they’ve also turned up in humans.
Oddly enough, most of the chemicals come from sponges that live in the tropics. PBDEs can make up more than 10 percent of the sponge’s tissue on a dry weight basis, and these sponges also contain other related polyhalogenated compounds. Although scientists are aware of the natural occurrence of PBDEs in these sponges, little is known about how they are made. In a recent study published in Nature Chemical Biology, researchers have found that the toxic chemicals are not the fault of the sponges. Instead, bacteria living in the sponge produce them.
The PBDEs produced naturally contain some added chemical groups compared to the human version. Because of the unique structure of these chemicals, the paper’s authors suspected that the genes responsible for their production would be different from the genes encoding the proteins that catalyze other pathways in a cell’s modular product assembly lines.
Instead of laboriously searching for the proteins, the authors turned to an approach called metagenomics, which involves sequencing DNA from environmental samples in bulk. The resulting sequences can then be examined for the presence of unusual genes.
So the scientists collected 18 sponge specimens from multiple locations in Guam in 2014 and 2015. Preliminary analysis revealed that the sponges belonged to four different evolutionary clades. Chemical analysis (mass spectrometry and NMR) revealed correlations between these clades and the chemistry involved, but it turned out that it was not the sponges that made the PBDEs.
The DNA isolated from the sponge tissue contained a mixture of sponge DNA and that of many other organisms, including symbiotic, parasitic, pathogenic and/or prey animals. The metagenomic data enabled the scientists to examine all of these organisms to see if they were a source of the genes that code for PBDE biosynthesis.
Genomic sequencing revealed that the sponge itself does not carry the genes necessary to synthesize PBDEs. Instead, a specific type of bacteria that lives in the sponge is responsible for PBDE production, called a cyanobacteria Hormoscilla spongeliae. No other organism seemed to have the right genes. To validate these results, the scientists isolated the genes and put them in another cyanobacterium. Once the gene was inserted, the species could take a chemical precursor and convert it into PBDE.
The bacteria contain a cluster of genes that modify the PBDEs in various ways, which accounts for the group of related chemicals found in the oceans. This is because the exact content of the gene cluster varies a bit, meaning that different bacteria may produce slightly different chemicals.
Now that we’ve figured out exactly why sponges seem to produce so many of these chemicals, follow-up studies may help us understand why these toxic compounds are produced and what benefit they may have for their hosts.
Nature Chemical Biology2017. DOI: 10.1038/NCHEMBIO.2330 (About DOIs).