Ocean food source lives by day, dies by night

One of the oceans’ most abundant food sources lives and dies like clockwork.

Hundreds of millions of the cyanobacteria Prochlorococcus grow in every liter of seawater during the day, and about the same number are killed or consumed every night, scientists report online June 15 in Proceedings of the National Academy of Sciences.

The bacteria, each less than 1 micrometer long, convert sunlight into carbon forms that feed other organisms, supporting food webs in the low-nutrient waters of the northeastern Pacific Ocean. Their consistent cycle of growth and death may stabilize marine ecosystems, even under the effects of global warming.

“You’d expect to see more chaotic patterns,” says study coauthor Francois Ribalet, a marine ecologist at the University of Washington in Seattle. But the bacteria’s balanced growth and loss held across thousands of kilometers. This consistent relationship indicates that Prochlorococcus provides reliable food for hungry consumers. “It really suggests that these cyanobacteria are functioning as a conductor for the ecosystem,” says Ribalet.

Viruses and plankton prey upon Prochlorococcus. But in the northern Pacific Ocean in winter, hardly any of the cyanobacteria die during the day. Though previous studies have suggested a tight balance between Prochlorococcus growth and death, this nighttime loss presents an interesting biological situation, says marine microbiologist Adam Martiny of the University of California, Irvine. It is possible that the cyanobacteria’s predators avoid their own enemies by hiding in deeper water during the day and hunting at night, Martiny says. Ribalet says that the organisms that hunt or kill Prochlorococcus may be inhibited by light, or may be able to generate their own food from sunlight during the day.

Prochlorococcus grows faster in warmer waters. But when the cyanobacteria’s reproduction increased, so did their loss overnight. This response means Prochlorococcus’ cyclemay remain stable in waters affected by global warming, Ribalet says.

Martiny says the study’s technology provides an exciting new tool for data collection. The scientists attached a flow cytometer, which identifies and counts individual cells, to the bottom of moving ships. This setup allowed continuous sampling of Prochlorococcus at 5 meters of depth from Seattle to Hawaii and off the California coast.

Based on previous research, the scientists believe the cyanobacteria’s cycle may vary in different ocean regions or during different seasons. Lots of data remain to be analyzed, Ribalet says.

Prochlorococcus’ simple dynamics may make modeling future marine environments easier, Ribalet says. “By understanding the ecosystem, you can finally start to make some predictions.”