Jennifer Smith, one of the coral reef researchers, with sampling tents. Photo courtesy of UH/Emily Kelly

In coral reef ecosystems, amid stony corals, fronds of algae and schools of fish, microorganisms are essential for recycling nutrients — transforming bits of organic matter into forms of nitrogen and phosphorus, for example, that are useful to photosynthetic organisms.

study published in Nature Communications by researchers at the University of Hawaiʻi at Mānoa, San Diego State University, Scripps Institution of Oceanography at UC San Diego and other institutions revealed that the bacteria present in the water overlying dozens of coral reefs changed dramatically during the night, and then returned to the same daytime community as observed the morning before.

Further, as if these communities were all privy to the same schedule, these changes were synchronized across reefs separated by hundreds of miles, as recently reported by UH.

Said Craig Nelson, assistant professor of oceanography at the UH Mānoa School of Ocean and Earth Science and Technology and study co-lead, “Previous studies of marine microbes have shown that different functional groups change their activity over the day, but microbial populations remain relatively constant over diel (24-hour) cycles. Those who study reefs know that these ecosystems change significantly from day to night, but this study reveals one previously invisible aspect of that change: a dramatic and predictable shift—not only in what microbes are doing—but which groups are most abundant.”

Added Linda Wegley Kelly, adjunct assistant research professor at SDSU and co-lead author of the study, “Investigations of day-night rhythms of reef processes are required to holistically understand the functional roles of microbial players in these ecosystems.”

Revealing the Previously Invisible Rhythm

In 2013, an international team of researchers conducted a cruise to the Southern Line Islands, a remote chain of equatorial islands south of Hawaiʻi, to measure a suite of reef processes. To avoid the hazards of boating and dive operations at night, an autonomous sampler was designed to collect a water sample just above the coral reef at midnight. Collecting samples in this way, the researchers measured changes in the water chemistry and the types of microbes present compared to the daytime at numerous sites. The team also used genomic tools to show how these community changes determine the microbial processes in reefs that differ day and night.

The team discovered that a group of microbes called Psychrobacter appears to be leading the way. Surprisingly, Psychrobacter can make up 40–70 percent of the marine microbial community during the day, and is a hundred-times more abundant than during the night. But what is influencing Psychrobacter?

“The changes we observed in microbe composition over a day-night cycle imply that coral reef habitats manipulate the surrounding seawater—both the chemistry and microbiology — based on the diurnal and nocturnal activities of the collective local biota,” said Kelly. “This function may modulate the number of microbes in the water, promote cycling of energy through the reef food web, or provide stabilizing effects to the ecosystem.”

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

For the full story, see the SOEST website.