The Most Abundant Organisms You’ve Never Heard Of

Plankton is a general term referring to any organism that does not have the ability to swim against prevailing currents. These organisms range from bacteria and viruses all the way to large gelatinous organisms, such as jellyfish.

For the last four years, a team of scientists working on a project known as BIOS-SCOPE has delved into the role that plankton play in the ocean’s biogeochemical cycles. BIOS-SCOPE, for Bermuda Institute of Ocean Sciences – Simons Collaboration on Ocean Processes and Ecology, investigates the microbial ecology of the Sargasso Sea. Some of their most ground-breaking research has been focused on the ocean’s most abundant, yet least understood inhabitants: marine bacterioplankton (bacteria living among plankton) and viruses.

New Discoveries
In 1993, BIOS-SCOPE co-principal investigator Stephen Giovannoni, a microbiologist at Oregon State University, discovered a new marine bacterial species in water samples collected from the mesopelagic region (600 to 3000 feet, or 200 to 1000 meters deep) of the Sargasso Sea. He named the species SAR202 to reflect that it was the 202nd genetic sequence isolated from these samples. These bacteria, which scientists now know originated around 2.8 billion years ago, spent millennia diversifying into the deepest niches of the ocean, called the dark ocean or the aphotic zone (600 to 16,000 feet, or 200 to 5,000 meters deep), where they continue to flourish today.

Since Giovannoni’s discovery, scientists have identified SAR202 in all areas of the aphotic zone, as well as within seafloor sediments and deep lakes. It is estimated that SAR202 represent 10 percent of all plankton cells in the aphotic zone; however, this figure pales in comparison with population estimates of another bacterioplankton: SAR11. Also discovered and described by Giovannoni, SAR11 have the smallest genome of any free-living organism and are the most abundant plankton, comprising 25 percent of the global population.

While SAR202 inhabit the deep ocean, SAR11 occupy the sunlit surface waters and are found throughout the ocean, reaching their largest numbers (more than 500,000 cells per milliliter of seawater) in ocean gyres. Due to their sheer volume, scientists hypothesize that SAR202 and SAR11 play a role in the ocean’s biogeochemistry; specifically, that they metabolize dissolved organic matter, one of the major sources of carbon in the ocean.

Jimmy Saw, a microbiologist working on Giovannoni’s BIOS-SCOPE team, expanded on work done by his colleague Zach Landry on SAR202. He conducted a series of experiments that shed light on the evolutionary history of SAR202 and SAR11, providing further evidence of its metabolic specialization. Within SAR202 genomes, Saw found the presence of enzymes that are believed to have evolved to break down compounds, such as the dissolved organic matter, that resist degradation by other marine microbes.

Long-Term Research
In the early 1990s, Craig Carlson, BIOS-SCOPE program director and professor at the University of California Santa Barbara, studied the close interaction between bacterioplankton that obtain sugars from their surrounding environment (called heterotrophic bacterioplankton) and dissolved organic compounds in the Sargasso Sea. His work demonstrated that bacterioplankton growth and consumption of organic matter partially controlled where and when dissolved organic matter was transformed and utilized in the water column, implying a potential link between baterioplankton and carbon export in the open sea.

This early work did not differentiate one bacterial group from another and, in 1996, Giovannoni and Carlson joined forces and initiated a long standing collaboration—still in existence today—to help resolve how specific microbial groups, like SAR11 and SAR202, respond to and control biogeochemical variability.

Experiments conducted as part of the BIOS-SCOPE project have provided evidence in support of some of the metabolic specialization hypotheses proposed by Saw and other researchers on the team. Shuting Liu, a postdoctoral fellow at the University of California at Santa Barbara, conducted a series of experiments to see if mesopelagic microbes, including SAR202 and SAR11, can use a unique group of compounds called CRAM (carboxyl-rich alicyclic molecules) for growth and metabolism. These compounds are resistant to microbial degradation and adding them to seawater from the Sargasso Sea resulted in an increase in SAR202 cells.

Her results, like Saw’s, show that bacterioplankton have developed special metabolic strategies and niches within the deep ocean. Additional experiments are underway to track the incorporation of dissolved organic matter into SAR202, SAR11, and others. These studies will help scientists understand how bacterioplankton groups in the mesopelagic zone partition and utilize scarce resources.

Under the Microscope
BIOS microbiologist Rachel Parsons found evidence that SAR202 subgroups are indigenous to different ocean depths and geographical regions. Using powerful microscopy tools, Parsons found that one subgroup is more prevalent at the surface and at 3,200 feet (1000 meters) deep, while two others reside predominantly at 650 feet (200 meters) and below 5000 feet (1,500 meters).

“The three subgroups of SAR202 have specialized within different niches of the water column,” Parsons said. “The shallower subgroup harvests dissolved organic matter that is semi-resistant to microbial degradation, while the two other groups have evolved powerful enzymes that break down dissolved organic matter with complex ring structures that are highly resistant to degradation, such as the CRAM compounds.”

Ben Temperton, a microbial ecologist specializing in bioinformatics at the University of Exeter in the United Kingdom, is investigating the viruses associated with SAR11. Marine viruses prey on marine bacteria and the current model is that viruses will infect more dense populations until the host numbers decline, forcing the viruses to adapt or find new hosts. However, Temperton has found that predation of SAR11 by its associated viruses seems to have little impact on host abundance. Instead, a sort of equilibrium between host and virus abundance is maintained.

Temperton postulates that this could be due to surviving SAR11 efficiently recycling the nutrients released by their fallen comrades. Temperton is working closely with a group of researchers from Woods Hole Oceanographic Institution, led by marine chemist Elizabeth Kujawinski, to identify if viral infection impacts the organic materials preferentially utilized by SAR11.

A Look Ahead
In 2020, BIOS-SCOPE scientists will continue their investigations into these organisms to gain a better understanding of their individual metabolic and survival strategies and their collective role in the cycling of organic materials throughout the world’s ocean. This February, BIOS-SCOPE investigators will gather in San Diego, California for their third Data Workshop. This workshop will not only be used to highlight recent discoveries, but will also serve to identify knowledge gaps and how collaborative approaches can help close those gaps. The Data Workshop will also hone the future direction of the project and lay the foundation of the science plan for the next BIOS-SCOPE cruise this coming July.