Bacteria surrounding coral reefs change synchronously, even at great distances

In coral reef ecosystems, among hard corals, algae fronds and schools of fish, microorganisms are essential for nutrient recycling – turning fragments of organic matter into forms of nitrogen and phosphorus. phosphorus, useful for example photosynthetic organisms.

A study published today in Nature Communications Researchers from the State University of San Diego (SDSU), the University of Hawaii at Mānoa, the Scripps Institution of Oceanography, and others have revealed that the Bacteria in the water covering dozens of coral reefs have changed drastically during the night. same day community as observed the previous morning. Moreover, as if these communities were all aware of the same calendar, these changes were synchronized on reefs separated by hundreds of kilometers.

"Surveys of the day / night rhythms of reef processes are needed to comprehensively understand the functional roles of microbial actors in these ecosystems," said Linda Wegley Kelly, Assistant Professor of Research at SDSU and co-lead author of # 39; study.

In 2013, an international team of researchers cruised the Southern Line Islands, a chain of remote equatorial islands south of Hawaii, to measure a series of reef processes. In order to avoid the risks associated with boating and diving activities at night, a standalone sampler was designed to collect a sample of water just above the coral reef at midnight. By collecting samples in this way, researchers measured changes in water chemistry and the types of microbes present compared to the day at many sites. The team also used genomic tools to show how these community changes are determining microbial processes in reefs that differ day and night.

"Previous studies on marine microbes have shown that different functional groups are altering their activity throughout the day, but microbial populations remain relatively constant over daily cycles," said Craig Nelson, assistant professor of oceanography at the University of California. School of Ocean Science and Technology and Earth UH Mānoa. and study co-direct. "Those who study the reefs know that these ecosystems change dramatically from day to night, but this study reveals a hitherto invisible aspect of this change: a dramatic and predictable change – not only in what microbes do – but what groups are the most abundant. "

The team discovered that a group of microbes called Psychrobacter seems to be in the lead. surprisingly, Psychrobacter can account for 40 to 70% of the marine microbial community during the day and is a hundred times more abundant than the night. But what influences Psychrobacter?

"The changes we observed in the composition of microbes during a day-night cycle imply that coral reef habitats manipulate the surrounding seawater – chemistry and microbiology – according to daytime activities and at night from the local community biota, "said Kelly. "This function can modulate the number of microbes in the water, promote the energy cycle across the reef food web or have stabilizing effects on the ecosystem."

"This research team is among the first to examine changes in coral reef microbial communities day and night," said Dan Thornhill, program director at the National Science Foundation's Division of Marine Sciences, who funded l & # 39; study. "These scientists have discovered surprising and remarkable differences in microorganisms according to the time of day.The study also revealed the important role of nocturnal microbes in nutrient recycling on the reefs."

Given the apparent importance of Psychrobacter, the team then seeks to assemble its genome, to isolate the bacteria in culture and to examine further the biology of this organism.

The current work of the authors show how the state of the reef – that is, health or stress, the dominance of corals or algae – can alter the number and type of microbes present in the surrounding sea water. This highlights the environmental consequences of changing benthic reef habitats; where the loss of fundamental calcifying organisms eliminates processes that impact coastal waters and provide connectivity between land and sea.