Friend or foe? Researchers investigate the mysterious microbes living
inside corals
Researchers tell the strange tale of Corallicolida: The harmless parasite
that can't photosynthesize

Date:
September 16, 2021
Source:
PLOS
Summary:
In a new article, researchers describe investigations into an
enigmatic group of coral-infecting microbes.



FULL STORY ==========================================================================
In an article publishing Sept. 16 in the open-access journal PLOS
Pathogens, Patrick Keeling and colleagues at the University of British
Columbia in Canada describe investigations into an enigmatic group of coral-infecting microbes.


========================================================================== Coral reefs provide homes for biodiverse ecosystems including fishes,
mollusks, and crustaceans, and inside their cells, they also harbor a
diverse community of microbes. The best-known are photosynthetic symbionts
in the genus Symbiodiniaceae, which provide essential ingredients to
build the corals' calcium-carbonate skeleton, in return for protection
and fuel for photosynthesis. However, over the past two decades,
researchers have identified millions of new eukaryotic sequences
associated with corals, including a mysterious group of microbial
parasites called Corallicolida. First discovered by accident as artefacts
in molecular ecological surveys, and eventually isolated from an aquarium, Corallicolida live inside coral cells. Corallicolida are colourless and
their genomes lack key genes needed for photosynthesis, indicating that
they are not photosynthetic symbionts like the Symbiodiniacea.

Perhaps, ecologists wondered, the Corallicolida are the descendants of
a once- beneficial photosynthetic symbiont that lost its photosynthetic machinery and became a parasite. However, so far there is no evidence that
they cause any harm to their host. In fact, Corallicolida don't seem to
have any major effect -- good or bad -- on the coral cells they inhabit.

A further twist in the story came with the discovery that, despite
lacking genes for photosystems or photosynthetic reaction centres, the Corallicolida genome includes genes for the four types of chlorophyll. The chlorophyll genes are expressed in Corallicolida cells and are subject
to purifying natural selection -- indications that they are still playing
some role in the cell.

Exactly what that role might be remains unclear, and much more research
is needed to determine whether the Corallicolida are friends or foes to
reef- building corals, the authors say.

"Coral reefs are an important marine habitat and well-studied
biodiversity hotspot, so the fact that so many corals around the
world are infected with an intracellular parasite that had hardly
even been noticed is a testament to how little we know about microbial biodiversity," Keeling adds. "For years these parasites were largely
known only from hints buried in large-scale molecular sequence surveys,
so hopefully by connecting a few dots to link up pictures, sequences,
and host identification we can now start to look more deeply into how they infect, spread between hosts, and what effects they have on coral health." ========================================================================== Story Source: Materials provided by PLOS. Note: Content may be edited
for style and length.


========================================================================== Journal Reference:
1. Patrick J. Keeling, Varsha Mathur, Waldan K. Kwong. Corallicolids:
The
elusive coral-infecting apicomplexans. PLOS Pathogens, 2021; 17
(9): e1009845 DOI: 10.1371/journal.ppat.1009845 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/09/210916142853.htm

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