Tracking the Ocean’s Circadian Rhythm

To a distant observer on the Pacific waters north of the island of Oahu, Hawai’i, two ships seemed to be engaged in an odd dance. One would remain stationary while the other cruised ahead — but to nowhere in particular. Instead, it casually chased two buoys drifting several meters below the surface. After a time, the second ship would adjust its bow thruster, kick in its diesel engines and race to catch up with the leading ship.

But there was sound science behind the maritime jockeying. For nearly two weeks this summer, the 46 scientists onboard were investigating the ‘days in the life’ of the smallest ocean creatures in an effort to fully understand the microbial ecology that underpins the marine food chain. It was the first expedition of the Simons Collaboration on Ocean Processes and Ecology (SCOPE), and preliminary results suggest that the wealth of data obtained will answer many questions about ocean microbiology.

A central hypothesis being tested on the cruise is that, “in complex, seemingly chaotic microbial communities, the dynamics of microbial interactions, matter transformations and energy transfer are actually very orderly, and precisely regulated in time,” says Edward DeLong, a microbial oceanographer at the University of Hawai’i, Manoa, and one of the project’s principal investigators. In other words, if the motions of the microbial world operate like a finely tuned orchestra, the scientists want to find the conductor and understand the instrumental interactions.

The microbes being studied are plankton, the drifters of the sea. Although they are at the mercy of the predominant current, they are not entirely passive. Cilia or tiny whiplike flagella can afford them enough mobility to migrate vertically. The protozoans and animals of this group, called zooplankton, often rise to the surface at night to feed and descend to safer, deeper waters during the day, when predators at the surface such as birds and fish are more active.

For their part, the plantlike drifters of the group, called phytoplankton, need to conduct photosynthesis. Some of these larger microorganisms may be able to ascend at daybreak and descend at dusk, but the smaller ones are simply too tiny to make significant migratory movements and instead have adapted to living at high levels of sunlight near the surface. The most abundant phytoplankton around the waters of Oahu is Prochlorococcus, and “we seek to understand how the behavior of other organisms will coordinate around it,” says Sam Wilson, an oceanographer at the University of Hawai’i, Manoa, and one of the chief scientists on the expedition.

To support the project, the Simons Foundation is now providing SCOPE with $40 million in funding for five years, while the National Science Foundation has contributed $4 million per year over the past 10 years to the Center for Microbial Oceanography: Research and Education (C-MORE). “This summer’s 2015 field expedition was a major element of both programs, combining the last year of our NSF funding with the first year of our new Simons Foundation SCOPE program,” says DeLong, who is also a co-director for SCOPE.

The funding enabled the scientists to use two research vessels, the Kilo Moana and the Ka`imikai-O-Kanaloa (KOK), in tandem for this expedition. The two ships offered flexibility — for instance, while the KOK stopped and conducted multiple sampling operations in one area, the Kilo Moana would continue to follow the microbial community in the current (via two buoys set adrift) and sample the water on a regular schedule. 

The scientists chose that area of the Pacific Ocean because of earlier surveys of the region. “We know this part of the ocean really well,” Wilson says. In 1988, the international Joint Global Ocean Flux Study established long-term ocean projects in the area as well as near Bermuda. The study eventually merged into the Hawaii Ocean Time-Series program, and oceanographers with the University of Hawai’i have been going out every month to the same location — within six nautical miles of 22 degrees 45 minutes north latitude and 158 degrees 0 minutes west longitude. Such steady observation of a region enables researchers to better identify changes when they occur.

The summer 2015 expedition will provide a wealth of new data. “The majority of organisms we study can’t be seen by the naked eye, and trying to detect changes from hour to hour is pushing our levels of detection,” Wilson explains. “Some instrumentation didn’t exist five years ago — for example, we now can detect small changes in oxygen levels every two to three hours thanks to improvements in analytical mass spectrometry and in situ sensors.”

Such field tests are critical to learning about interactions among microbes, which, as scientists are now discovering, have behaviors similar to those of other animals. “They can swim towards nutrient sources and use cell-to-cell communication. They have a repertoire of behaviors which we have not really thought was possible before,” says marine microbiologist Gerhard Herndl of the University of Vienna, who was not involved in the expedition. “We are just beginning to understand the interactions within the microbial world.” He adds that scientists have gained most of their understanding of marine microbes from laboratory experiments. With expeditions, they can evaluate their findings from the lab against the behavior of microbes in their natural environment.

On this mission, sampling of the water column took place at least every four hours over the course of the 11-day cruise. The gold-standard method for taking water samples during oceanographic studies is to use a cluster of water bottles arranged as a rosette. As the bottles collect the water, instruments attached to the rosette measure temperature, depth and electrical conductivity, which is an indication of salinity.

Immediately after collection, most samples were flash-frozen in a bath of liquid nitrogen for analysis back on land. These time-series samples give the scientists a snapshot of the microbial world that is literally frozen in time and space so that the researchers can examine it in detail later. The samples, along with other cruise data, will offer new views into the daily pattern of marine life, including “growth rates of different phytoplankton species in real time; microbial predator-prey interactions; whole-genome gene-expression patterns across multiple microbial species; carbon fixation and primary productivity rates; the production of greenhouse gases; and the production of particulate and dissolved organic carbon,” DeLong says.

Getting the samples back to the labs is not always easy. Dror Shitrit, a graduate student at Technion – Israel Institute of Technology, is counting on 350 pounds of dry ice packed around his water samples to help them survive the flight home. Judging from the number of samples he’s collected, “Dror has about a year of analysis to do,” Wilson estimates.

And as DeLong observes, it is during the analysis stage, as new insights into the ocean’s microbial communities begin to take shape, that the excitement will really begin.