Researchers have discovered that despite their static nature, corals do
exert some influence on the microbes in the environment around them.
This was revealed in a paper recently published in the Limnology and Oceanography journal.
Although it has been known that microorganisms in the water influence corals in a variety of ways, this study reveals that this interaction actually goes both ways.
The study was conducted by researchers from the Woods Hole Oceanographic Institution (WHOI), Bermuda Institute of Ocean Sciences (BIOS) and University of California, Santa Barbara (UCSB)
“We’ve known that marine microbes play major roles in moving nutrients and recycling matter into forms that are more usable to the corals. 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,” said WHOI microbiologist Amy Apprill.
Essentially, the corals release organic matter and nutrients which planktonic bacteria known as picoplankton feed on — then they selectively feed on these specific types of bacteria.
In their research, the scientists set up an aquaria-based experiment using nine tanks filled with seawater for 12 days.
Their objective was to track the population of microbes in the seawater. Three tanks were used as controls and were left alone. For three other tanks, they added mucus from the P. astreoides coral (taken from three Bermudian reefs). Corals secrete mucus, which supports an active community of microbes.
For the three remaining tanks, corals were introduced and then removed. The scientists observed that the microbes in these tanks were drastically reduced; particularly affected were the Rhodobacteraceae, Synechococcus and SAR11 bacteria, which were the most abundant groups in the ocean.
When the corals were removed from these tanks, the population of microbes subsequently increased as they were able to replenish themselves. This was accompanied by extremely fast rates of microbial growth.
“The growth rates were quite high, especially in the case of SAR11, which was one of the fastest rates of growth ever documented,” said Apprill. “It suggests that the microbes are growing on something that the corals leave behind in the tank. For the first time, we’re observing important influences that the corals are having on the total surrounding microbial community,” she added.
Another observation made during the experiment was that in the tank with corals, there was remineralisation of nitrogen in the water. This is similar to what takes place in environments with healthy coral reefs: microbes convert ammonia, a toxic waste product produced by most animals. into less toxic substances like nitrate, and this leads to increased nitrogen levels in the water.
Microbiologist at UCSB and co-author Alyson Santoro said of the detoxifying process: “Until now, it wasn’t known exactly where the process was occurring, whether it was happening in the sand or in the water column or in association with corals. This study shows that this detoxifying process is directly and physically associated with the corals.”
Since there were no drastic changes observed in the tanks containing just the coral’s mucus, Apprill concluded that it was “not just the mucus that’s causing the microbes in the water to become more active. It’s actually what the coral is releasing, which is still an undefined group of compounds.”
Co-author Rachel Parsons, a microbial oceanographer at BIOS concluded 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.”
As coral reefs are currently in decline due to climate change and ocean acidification, the researchers asserted that there was an urgent need to find out how specific coral picoplankton interactions contribute to the coral reef ecosystems.
Full Study: Multifaceted impacts of the stony coral Porites astreoides on picoplankton abundance and community composition