Ocean Bacteria Work the Night Shift

New study finds nighttime peaks in bacterial activity and carbon cycling in the ocean

A new study published in Communications Biology is changing how scientists understand the daily rhythm of life in the ocean. Researchers report that ocean bacteria, tiny organisms essential to Earth’s carbon cycle, are far more active at night than previously realized.

The study, led by Shuting Liu, an assistant professor at Kean University (United States), focused on the upper 300 meters of the North Atlantic Ocean near Bermuda. Over several days, the multi-institutional BIOS-SCOPE team tracked bacterial activity and community composition to learn how marine microbes produce, transform, and leave behind dissolved organic material. Basically, how microbial communities wax and wane over a daily cycle.

Dissolved organic matter is produced from the waste and decomposition of the ocean’s plants, animals and microbes, and is so small (a fraction of the size of even the smallest bacteria) that it passes through an oceanographer’s finest filters. While dissolved organic matter is the primary source of energy for the roughly five million bacteria in every teaspoon of seawater, compounds that are indigestible to marine microbes can remain in the ocean for thousands of years.

Researchers found bacterial activity consistently peaked at night. “Our data showed that bacterial activity in surface waters increased at night, responding with a delay to daytime photosynthesis and coinciding with nighttime declines in certain dissolved organic compounds,” said Liu. “This highlights fine-scale fluctuations in microbial activity and their close link to ocean carbon cycling.”

During the day, microscopic phytoplankton use sunlight to convert carbon dioxide into organic matter, some of which dissolves into the water as food for bacteria. Rather than consuming it immediately, bacteria need to spin up enzymes to break down some portion of this material which is most active at night at our site. “Measurements showed nighttime bacterial activity in surface waters was 34 to 47 percent higher than daytime rates under identical incubation conditions,” Liu explained.

This nighttime surge links daytime production of dissolved organic carbon with its consumption by bacteria, showing that key steps in the ocean carbon cycle follow different schedules over the day–night cycle.

Why this matters for climate science

Bacteria play a major role in determining how long carbon stays in the ocean before returning to the atmosphere as carbon dioxide. By consuming readily degradable organic compounds, like sugars, amino acids and other small organic molecules, bacteria influence whether carbon is recycled near the surface or transported deeper into the ocean. Because the deep ocean can store massive amounts of carbon in the form of dissolved organic matter, the microbes that impact this reservoir also influence Earth’s carbon cycle.

“While numerous studies have linked bacteria with the breakdown of organic matter in the ocean,” Liu explains, “Our study extends this work to the day–night scale, revealing consistent daily changes in bacterial groups and their links to specific dissolved organic compounds.”

“The discovery of strong nighttime activity suggests that traditional research methods of conducting this measurement in the early morning or midday may be missing a significant portion of bacterial carbon consumption, potentially underestimating how much carbon bacteria process each day.”

A well-studied ocean laboratory

“The BIOS-SCOPE team includes physical oceanographers, marine chemists, microbiologists, and zooplankton ecologists from ASU BIOS and seven other research institutions in the United States, United Kingdom and Germany, as well as visiting scholars from other institutions like Kean University,” said Craig Carlson, program director of BIOS-SCOPE and director of ASU BIOS. “Each member brings unique expertise and novel technologies to study the problem at hand.”

As part of BIOS-SCOPE, the research was conducted in the Sargasso Sea, a dynamic region of the North Atlantic subtropical gyre. “Bermuda is an ideal study site because the nearby Sargasso Sea experiences predictable seasonal mixing and summer stratification,” Liu explained. “Allowing us to observe day–night biological patterns with minimal physical disturbance.”

A second key strength of the study is the availability of long-term ocean records, which allowed scientists to place their short, high-resolution measurements within a broader environmental context.

“Decades of ocean research at the Bermuda Atlantic Time-series Study provided a strong foundation for this work,” said Carlson. “These long-term and high-frequency data, including observations from gliders, were critical for selecting study sites and interpreting our results.”

A finer look at ocean life

Beyond bacteria, the study also examined interactions with other marine organisms, including microscopic grazers and zooplankton. These relationships help determine how carbon moves through the marine food web and ultimately how efficiently the ocean stores carbon.

By capturing these processes on a day–night timescale, the research highlights the dynamic and responsive nature of ocean ecosystems, often changing hour by hour in ways that can be easily overlooked.

While we may think of the ocean surface as inactive at night, it is anything but quiet. According to this study, some of the most important work in the marine carbon cycle happens during the night shift.