Award: OCE-1658527

The ocean plays a significant role in the global carbon cycle. It has consistently taken up about a fourth to a third of the carbon dioxide that we emit into the atmosphere through fossil carbon burning each year. The carbon dioxide dissolves in the ocean, and is converted by photosynthetic, microscopic algae (phytoplankton) to organic matter, both dissolved and particulate. While phytoplankton are ultimately the origin of most sinking particulate organic matter in the ocean, they are unicellular and microscopic, and generally too small to sink on their own. But biological particles of phytoplankton origin make up a significant portion of the carbon export from the surface ocean to the depths of the ocean, so how are single phytoplankton cells converted to these larger sinking particles? There are several mechanisms that are hypothesized to be at play in biological aggregate formation: production of transparent exopolymeric substances (TEP) that are known to be sticky and make up the matrix of aggregates, either by the phytoplankton cells or by interaction with bacteria, and/or ballasting of the aggregates by dust (clay) carried to the ocean surface by winds from continents. In addition, planktonic animals (zooplankton) may contribute to particle formation by grazing on phytoplankton and their aggregates, and by producing sinking fecal pellets. When the aggregates become large and dense, in the size of a tenth to a few millimeters, they will then sink out of the surface ocean to greater depths, efficiently removing carbon from the surface ocean and providing nourishment for organisms living deeper in the ocean and on the seafloor. This process is called the Biological Carbon Pump. In this project we combined field and laboratory experiments to test the role of the different mechanisms in marine biological particle formation at the Bermuda Atlantic Time-Series Study station (BATS) in the Sargasso Sea, which is considered representative of the vast nutrient-poor central oceans that are the home of the smallest phytoplankton cells known. In laboratory experiments we found, for the first time, that some of the phytoplankton characteristic of these open ocean regions make their own TEP, particularly a group of abundant cyanobacteria and minute diatoms that have been found associated with sinking particles. We also found that several of these phytoplankton species form sinking aggregates only when ballasting clay is added to the incubations. And some of the bacteria isolated from sinking particles collected in the Sargasso Sea were found to make their own TEP and help in particle formation. In our ship-board experiments carried out in the ocean around BATS, we collected sinking particles with floating particle traps in the upper 300m water depth and observed their composition microscopically and determined their microbial communities through DNA-based analysis. An example of such a particle can be seen in the image. The relative composition of the microbial communities associated with different types of particles was statistically indistinguishable and bacterial taxa known to be associated with the gut microbiomes of zooplankton were indicator species of all the particles. These results point to an important role of zooplankton in generating and modifying sinking particles, by feeding on them or by producing fecal pellets that help aggregate smaller particles. We conclude from our research that phytoplankton, even the smallest cells, can form aggregates through a combination of TEP production, association with bacteria and ballasting by clay, but that the particles that ultimately sink to greater depths have been reworked by planktonic animals. By understanding these processes we will be able to better gauge the consequences of climate change on the Biological Carbon Pump, and the ocean’s future ability to take up and remove carbon dioxide from the atmosphere. Last Modified: 05/21/2021 Submitted by: Susanne Neuer