Daily Changes in Reef Bacteria Synchronized Over Long Distances

Southern Line Island Reef. (Brian Zgliczynski)

(CN) – New research reveals that the smallest members of the coral reef system – bacteria and other microbes – behave much differently at night than during the day, providing insight into a part of an ecosystem literally shrouded in darkness.

Researchers from California State University, San Diego, the University of Hawaii at Manoa and the Scripps Institution of Oceanography published a paper in scientific journal Nature Communications on Friday, saying microorganisms are as susceptible to the rhythms of day and night as other organisms that comprise the coral reef ecosystem.

“This research team is among the first to look at changes in the microbial communities of coral reefs through both day and night,” said Dan Thornhill, a program director in the National Science Foundation’s Division of Ocean Sciences. “These scientists found surprising and remarkable differences in microorganisms depending on the time of day.”

Microorganisms are responsible for breaking down bits of organic material into elements like nitrogen and phosphorus, so critical to photosynthetic creatures that play a fundamental role in the food webs of the coral reef.

While previous studies have showed the dramatic difference in ecosystem behavior in the night versus the day, the study published today is the first of its kind to apply specifically to coral reef microbials.

The researchers found that not only did the bacteria’s behavior change dramatically from day to night, but that behavior appeared to be synchronized across reefs separated by hundreds of miles, as though the microbes were all set to a similar schedule.

“Investigations of day-night rhythms of reef processes are required to holistically understand the functional roles of microbial players in these ecosystems,” said Linda Wegley Kelly, a San Diego State professor and study co-lead author.

The researchers found over the course of their study that the changes in microbial behavior from day into night have implications for the entire ecosystem of both the reefs and the water in their vicinity.

“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,” Kelly said. “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.”

The data was gathered during a recent trip to the Southern Line Islands, a remote equatorial island chain south of the Hawaiian Islands. To avoid having to conduct precarious boating and diving operations during the night, the team built a robot capable of collecting samples from the water overlying the reef systems.

The team not only marked changes in the quantities of microbes at night versus day, but also tracked water chemistry and used genomic tools to show how community changes determine the microbial processes in reefs that differ day and night.

“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,” said Craig Nelson, a professor of oceanography at UH Manoa and study co-lead. “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.”

The difference in microbial abundance is particularly pronounced as it relates to a bacterium called Psychrobacter.

The microbe can comprise between 40% and 70% of the microorganism population during the day and is at least 100 times more abundant at night. Researchers are still attempting to grapple with the implications of the data.

“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,” Kelly said.

Scientists plan to further research whether microbes correlate to the relative stress of coral reefs, which have struggled worldwide to adapt to changes in the oceanic temperatures and acid content as a result of climate change.

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