Coral reefs set daily rhythms for nearby microbial communities.

A study sampling water every six hours above a Red Sea reef found that coral reefs impose clear daily cycles on nearby microbial communities, with midday peaks in Symbiodiniaceae signals and nighttime increases in microscopic predators.

New research shows coral reefs do more than support visible marine life; they also structure the timing of microscopic organisms in surrounding waters. Scientists collected water above a reef in the northern Gulf of Aqaba and in nearby open waters during both winter and summer. Samples were taken every six hours and analyzed with genetic sequencing, flow cytometry, imaging, and biogeochemical measurements. The study was published in Science Advances and led by researchers including Herdís G. R. Steinsdóttir, Miguel J. Frada, and Derya Akkaynak. Key findings: - Waters above the reef contained consistently fewer bacteria and microalgae than nearby open waters, a pattern the researchers link to removal by reef-associated organisms. - Populations of heterotrophic protists, microscopic predators that feed on bacteria, rose sharply at night, in some cases increasing by as much as 80 percent. - Genetic signals from Symbiodiniaceae regularly peaked around midday in reef waters, suggesting daily cycles of release, growth, or turnover tied to light and coral metabolism. - The daily microbial rhythms observed were as strong as, and sometimes stronger than, seasonal differences between winter and summer. - The team combined high-frequency sampling with multiple laboratory and imaging methods to capture rapid shifts in abundance and community composition. Summary: The results indicate that coral reefs actively impose repeating daily patterns on nearby microbial communities, reshaping which microbes are present and when. These cycles affect the movement of energy and nutrients around reefs and point to daily microbial changes as a potential way to monitor reef function and health. Undetermined at this time.