Don’t Let Their Size Fool You

For nearly a hundred years, scientists have known that plankton—the microscopic organisms that drift and float in the ocean, also known as marine microbes—form the basis of the ocean’s food web. Phytoplankton (literally “plant wanderers”) are photosynthetic, like their terrestrial counterparts, and convert sunlight into energy. Phytoplankton, in turn, are consumed by zooplankton (literally “animal wanderers”), as well as a host of larger marine organisms, including juvenile fish, shellfish, birds, and even whales. However, scientists are now learning that plankton play an even larger role in earth’s complex biogeochemical systems.

In the late 1980s, a research program was conceived to investigate the exchange of carbon between the atmosphere and the ocean. This project, called the Joint Global Ocean Flux Study (JGOFS), established two long-term time-series projects to make sustained observations of ocean chemistry and physics. One of these was the Bermuda Atlantic Time-series Study (BATS), a time-series project in the Sargasso Sea run by scientists at BIOS.

Over the course of the JGOFS program, which lasted through 2003, scientists began to discover that plankton play a significant role in a global process known as the ocean’s “biological pump,” which is part of the global climate system. During photosynthesis, phytoplankton remove—or “fix”—carbon dioxide (CO2) from the atmosphere, converting it into sugars and other carbon compounds that enter the marine food web. CO2 is released back into the system when bacteria and zooplankton consume the organic matter produced by phytoplankton. At the same time, zooplankton and phytoplankton also die and decompose, carrying carbon into deeper waters where it is stored in ocean sediments or transformed by bacteria into forms that can be reused by other organisms.

Despite representing only 1-2% of global plant biomass, primary production by phytoplankton accounts for 40-50% of carbon fixation on earth. Without the biological pump, the atmospheric concentration of carbon dioxide would be significantly higher. As a result, over the last two decades, scientists have turned their attention to investigating how plankton and other marine microbes control the production, removal, and transformation of carbon in the ocean.

BIOS-SCOPE
In 2015, a team of scientists from research institutions in Bermuda, the United Kingdom and the United States began an ambitious five-year research investigation into the microbial oceanography of the northwestern Sargasso Sea. The project, called BIOS-SCOPE, is funded by Simons Foundation International and leverages the expertise of microbial oceanographers, molecular microbiologists, marine chemists, zooplankton ecologists, and physical oceanographers to answer questions that have global significance.

“The collective metabolism of the marine microbial community directly impacts the ocean’s biogeochemical cycles, making microbes fundamentally important to the ocean’s ability to sustain life on earth,” said Craig Carlson, professor at the University of California at Santa Barbara (USCB) and BIOS-SCOPE program director.

After four full years of work, including six at-sea research expeditions, the BIOS-SCOPE researchers have utilized a broad range of approaches—from genomic to chemical to ecological—to describe the marine microbial ecosystem.

One of Earth’s Major Carbon Reservoirs
Dissolved organic matter (or DOM) consists of materials, including carbon compounds, that result from the biological production and decomposition of organic matter—processes that are governed by phytoplankton, bacteria, and zooplankton. As DOM is mixed or transported from the upper levels of the ocean down to the depths, portions of this material resist biological degradation and accumulate as a large reservoir of organic carbon, nitrogen and phosphorous that can persist for millenia.

The BIOS-SCOPE project is working to identify the components of DOM, as well as their relative abundances, to help understand the influence of these compounds on both ocean chemistry and the ecology of the marine microbial community. BIOS-SCOPE scientists at the Woods Hole Oceanographic Institution (WHOI), led by marine chemist Elizabeth Kujawinski, have analyzed over 200 samples of DOM collected during the first two years of the project. They uncovered the chemical signatures of hundreds of unique molecules that comprise DOM, including 100 new compounds, many of which had never before been measured in seawater.

“These new molecules are opening our eyes to the complexity of chemical communication and transfer among marine microbes,” Kujawinski said. “We are excited to find molecules that seem to serve as the currency of the ocean microbiome, connecting microbes into a collaborative network that we observe as the ocean carbon cycle.”

Analysis of samples from the BATS site showed that DOM in the Sargasso Sea exhibits seasonal variation. During the winter, strong winds mix the upper portion of the ocean, bringing DOM from the surface waters to depth, while also carrying inorganic nutrients up toward the surface. These winter mixing events set the stage for an annual burst of photosynthetic activity by phytoplankton known as the “spring phytoplankton bloom.” Since carbon dioxide is consumed during photosynthesis, this makes phytoplankton one of the largest natural carbon sinks, or reservoirs, for storing atmospheric carbon. As a result, these annual spring blooms have far-reaching impacts on the ocean’s biogeochemical cycles and ecology.

BIOS-SCOPE researchers also found that DOM created through photosynthesis is not consumed by microbial communities at the surface of the ocean. Instead, it later becomes available to fuel the growth of microbial communities in deeper waters, shedding light on how marine microbes grow in waters that are typically considered to be lacking in nutrients.

More Than Meets the Eye
Equally important to the BIOS-SCOPE mission is further clarifying and quantifying the respective roles of phytoplankton and zooplankton in the biological pump. Over the course of a day, zooplankton help regulate phytoplankton by consuming them. The resulting waste products are recycled by microbes and, as a result, influence the bacterioplankton population.

Many species of zooplankton exhibit a daily migration pattern in which the animals move to deep waters during the day and rise to the surface at night, with some populations traveling nearly 2,000 feet (600 meters) every day, leading scientists to question if, and how, these daily movements impact the ocean’s biological pump. BIOS-SCOPE experiments led by Amy Maas and Leocadio Blanco-Bercial, BIOS scientists who study zooplankton, have shown that the metabolisms of these organisms exhibit a circadian rhythm—a natural, internal process that regulates biological processes and behaviors over a 24-hour period.

The distribution of metabolites, or the molecules resulting from an organism’s metabolic activities, are intimately connected with the timing of vertically migrating zooplankton. The teams at WHOI and BIOS have demonstrated that certain metabolites are elevated at deeper depths during the day and at the ocean’s surface at night. Accurately measuring and accounting for the production of nutrients, such as metabolites, will help scientists better estimate the impacts of their compounds on phytoplankton abundance and distribution.

“This project has been really exciting because, although we have seen the genes coding for circadian rhythms in zooplankton before, this work is the first to demonstrate that the daily changes in the metabolism of these animals likely has a substantial effect on how much carbon they transport to depth as well as the dissolved metabolites that they release to the microbial community,” said Maas. “It emphasizes how interconnected and complex the ecology and physiology of these animals and microbes truly are.”

Maas and Blanco-Bercial are also conducting experiments using zooplankton fecal pellets to determine how and where particulate organic matter contributes to the oceanic carbon cycle. Understanding the source of this organic matter and the fate of these particles will enable BIOS-SCOPE scientists to determine how organic material fluctuates in the ocean.

Stay tuned for part two of this story, featuring BIOS-SCOPE’s work on marine viruses, appearing in the January issue of Currents.