It has long been known that coral polyps harbour algal symbionts that make most of their food. Now it has been shown that corals can also harbour microbes that appear to be parasites but don’t seem to harm their hosts.
“The fact that we could go out to a coral reef, which is probably the best studied marine environment, and specifically look at the coral, which is the best studied part of that environment, and find a parasite that infects virtually every species of coral we’ve looked at, that no one had ever really noticed, blows my mind,” says Patrick Keeling at the University of British Columbia in Canada. “How much out there that is still unknown is bewildering sometimes.”
The organelle that carries out photosynthesis inside plant cells has its own tiny genome. A group of single-celled parasites – including malaria – that evolved from algae still have relics of this organelle and its genome. Similar sequences had turned up in so-called metagenomic studies of corals but were thought to be contaminants or belong to bacteria.
Around a decade or so ago, Keeling and his colleagues realised that these sequences actually belonged to a previous unknown group of single-celled organisms with complex cells, unlike bacteria. The team has gone on to find and isolate these organisms, dubbed corallicolids.
They don’t obviously harm corals but are almost certainly parasites, says Keeling. “It’s really hard to see how they could be beneficial,” he says.
What is most mystifying is that that corallicolids appear to make a little chlorophyll that can capture light energy but have lost the apparatus for using that energy to make food. This captured energy can damage cells if there is no way to exploit it – it is like having a bomb inside a cell with no way to defuse it, says Keeling.
“It’s hard to describe how mind-boggling it is,” he says. “Biochemists just flat out refuse to believe it.”
Metagenomic studies suggest that the corallicolids are just one of 10 previously unknown groups of organisms on coral reefs, and Keeling thinks there are many more groups waiting to be discovered.
The diversity of complex-celled microbes such as the corallicolids is massively underestimated, he says, because it is hard to distinguish their genes from those of multicellular animals.
Journal reference: PLOS Pathogens, DOI: 10.1371/journal.ppat.1009845
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