The corallicolid (in center) and coral hosts (surrounding photos) show that corallicolids are able to live with Keeling PJ et. al., 2021. PLOS Pathogens. CC-BY 4.0
Long-established facts have shown that coral polyps harbor algal symbionts, which make the majority of their food. It has now been proven that corals can harbor microbes that look like parasites, but don't seem to harm hosts.
It is amazing that we can go to a coral reef which is one of the most studied marine environments and look at corals, which are the most studied, and find a parasite that has infected almost every species of coral that we have seen, something that no one else had noticed. It is amazing to see how much is still unknown.
Each organelle responsible for photosynthesis within plant cells has its very own genome. One-celled parasites, including malaria, that have evolved from algae still possess relics of the organelle and its DNA. Similar sequences were found in metagenomic coral studies, but they were thought to be either contaminants or to bacteria.
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Keeling and his coworkers realized that the sequences belonged to an unknown group of single-celled organisms, with complex cells, about a decade ago. This was unlike bacteria. These organisms were subsequently isolated by the team, which is known as corallicolids.
Keeling says they don't harm corals but almost certainly are parasites. He says it is difficult to see how they could benefit.
The most amazing thing about corallicolids is their ability to capture light energy, but lose the necessary apparatus to use that energy to make food. Keeling says that this captured energy can cause damage to cells if it is not exploited. It is similar to having a bomb in a cell without a way to defuse it.
He says it's difficult to explain how mind-boggling this is. It is a fact that biochemists simply refuse to believe.
Continue reading: Corals could grow faster in warm waters than the climate change.
Metagenomic research suggests that corallicolids may be just one of the 10 previously undiscovered groups of organisms found on coral reefs. Keeling believes there are many others.
He says that the diversity of complex-celled microbes like corallicolids has been greatly underestimated because it is difficult to distinguish their genes and those of multicellular organisms.
Journal reference: PLOS Pathogens, DOI: 10.1371/journal.ppat.1009845
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