Have you ever noticed your corals covered in mucus and wondered, what’s up with the slime? New research suggests that corals selectively remove and promote the growth of diverse picoplankton (planktonic bacteria and archaea) communities living in coral mucus, and that the mucus has an impact on the microbes in waters surrounding them.
When corals are stressed, they often secrete large quantities of mucus which can be unsightly or worrisome in your tank. However, under normal condition, corals secrete a thin layer of mucus which protects the coral and support an active community of microbes.
In a novel lab observations, researchers at the Woods Hole Oceanographic Institution (WHOI), the Bermuda Institute of Ocean Sciences (BIOS) and University of California, Santa Barbara (UCSB) found that corals are selectively feeding on specific types of bacteria, and that the coral, including coral mucus and it’s associated bacteria, selectively removes and promotes the growth of picoplankton.
“We’ve known that marine microbes play major roles in moving nutrients and recycling matter into forms that are more usable to the corals,” says WHOI microbiologist Amy Apprill, one of the authors of the paper published Oct. 12, 2016, in the journal Limnology and Oceanography. “But this study demonstrates for the first time that corals are having some control on the production of microbes around them, and that the corals are using that to benefit their own growth.”
Coral mucus supports the growth of nutrient-rich, fast-growing copiotrophic-type cells, which the coral in turn feed upon. Corals also release nutrients that promote the growth of these bacteria.
In order to better understand how corals and coral mucus might be interacting with microbes in surrounding reef waters, Apprill, and her colleagues Sean McNally and Rachel Parsons at BIOS, set up aquaria-based experiments using colonies of the coral Porites astreoides obtained from three Bermudian reefs.
“When we put the corals in 30L tanks, the microbes in the sea water were drastically reduced in numbers,” Apprill says. “We observed that the corals were selectively feeding on several types of bacteria—Rhodobacteraceae, Synechococcus, and SAR11, which are most abundant groups of bacteria in the ocean. We then took the corals out of the sea water tanks, and we observed a strong increase in these microbes.”
“It’s not just the mucus that’s causing the microbes in the water to become more active,” says Apprill. “It’s actually what the coral is releasing, which is still an undefined group of compounds.”
“This study shows that corals do in fact influence the picoplankton community by selecting specific lineages of picoplankton for removal and also by potentially using the complex carbon compounds excreted by the coral to promote the growth of these lineages,” adds Rachel Parsons, a microbial oceanographer at BIOS and coauthor of the paper.