Coral reefs have a hidden daily rhythm scientists just discovered

Coral reefs are widely known as hotspots of biodiversity, but this study highlights another important role. Reefs also act as strong regulators of microscopic life in the surrounding ocean. The research was led by Dr. Herdís G. R. Steinsdóttir a postdoctoral researcher working under the guidance of Dr. Miguel J. Frada of the Department of Ecology, Evolution and Behaviour at the Hebrew University of Jerusalem and the Interuniversity Institute for Marine Sciences in Eilat and Dr. Derya Akkaynak from the University of Haifa and the Interuniversity Institute for Marine Sciences in Eilat. The team found that coral reefs impose clear daily rhythms on nearby microbial communities, reshaping which microbes are present and how many of them appear over the course of a day.

Tracking Microbial Changes Across Time and Seasons

Published in Science Advances, the study monitored microbial populations in waters above a coral reef in the northern Gulf of Aqaba in the Red Sea. These measurements were compared with nearby open waters during both winter and summer. By sampling every six hours, the researchers uncovered daily and seasonal cycles that had not been documented before. These patterns affected bacteria, microalgae, and microscopic predators.

"We found that the reef is not just passively surrounded by microbes," said Dr. Frada. "It actively structures microbial life in time, creating daily patterns that repeat across seasons and influence how energy and nutrients move through the ecosystem."

Predation and Nighttime Microbial Shifts

The research team observed that waters above the reef consistently contained far fewer bacteria and microalgae than nearby open waters. This difference suggests that reef organisms are actively removing these microbes. At the same time, populations of heterotrophic protists, microscopic predators that feed on bacteria, rose sharply at night. In some cases, their numbers increased by as much as 80 percent, pointing to predation as a major force driving daily microbial changes.

One of the most notable findings involved Symbiodiniaceae, a family of dinoflagellates best known for their role as coral symbionts. Genetic signals from these organisms regularly peaked around midday in reef waters. This pattern suggests daily cycles of release, growth, or turnover that may be linked to light conditions and coral metabolism.

"These daily microbial rhythms were as strong as, and sometimes stronger than, seasonal differences," said Dr. Steinsdóttir. "This shows that time of day is a critical factor when studying reef-associated microbial communities."

A New Way to Monitor Reef Health

By combining genetic sequencing, flow cytometry, imaging technologies, and biogeochemical measurements, the interdisciplinary team produced one of the most detailed time-based views so far of microbial life around coral reefs. The results suggest that daily microbial cycles could serve as sensitive indicators of how reefs function and how healthy they are in an ocean that is rapidly changing.