Award: OCE-1948104

Phytoplankton living in the surface ocean carry out half the photosynthesis occurring on Earth. Much of the organic matter produced is released into seawater, from which it is then rapidly and efficiently consumed by marine bacteria. This process transfers elements such as carbon, nitrogen, and phosphorus into a microbial-sized food web, driving nutrient cycling and regulating gas exchange between the Earth and atmosphere. Yet we know relatively little about flow through the ocean’s microbial food web because thousands of different microbial species and thousands of different organic compounds are present in each drop of seawater. 

Our project addressed this nearly invisible flow of ocean carbon using both microbial gene expression and chemical analysis methods. In one study, we first acclimated the phytoplankton species Thalassiosira pseudonana at three different growth temperatures (14°C, 20°C, or 28°C) over 100 generations, and then added a marine bacterium (Ruegeria pomeroyi) to the cultures. Chemical analysis indicated that at least 16 compounds were released by Thalassiosira (Figure 1), most of which differed significantly in concentration depending on acclimation temperature of the diatom; all but one of these contained an N, P, and/or S atom bonded to carbon. The movement of these compounds from seawater into marine bacterial cells was rapid, and most were fully consumed within 2 h. This indicated tight correlation of organic compound production and consumption among the ocean’s microbes. Some compounds, however, such as the amino acids leucine and isoleucine, were still not consumed after 12 h. Temperature-influenced changes in the composition of metabolites released by phytoplankton may therefore affect the time scale over which carbon is processed through the marine microbial food web.

Many marine bacteria live near phytoplankton cells in order to compete for the organic compounds being released. These communities are typically dominated by three taxonomic groups of bacteria: Rhodobacterales, Flavobacteriales, Gammaproteobacteria. To determine how these bacterial groups divide up available molecules, we individually cultured a representative from each group with Thalassiosira pseudonana. Gene expression by the bacteria and metabolite chemical analysis indicated that 36 organic molecules were available for bacteria to use. However, there was little overlap in which compounds each bacterium targeted for uptake. Indeed, only three of the 36 compounds were used by all three (Figure 2). This pattern supports the ecological concept of ‘resource partitioning’ in which marine bacteria have come to specialize on different phytoplankton-derived compounds and thereby decrease competition for resources. 

What are the routes by which phytoplankton-produced organic compounds are released into seawater to enter the microbial food web? The methodological challenges of studying the release of diverse pools of metabolites being produced and rapidly consumed by microbes has made this a challenging question to answer. In this study, a new compilation of published data measuring each step in the microbial food web was used to update estimates of the flow of phytoplankton compounds. Data mining and simulation modeling indicated that about half of ocean primary production is released as dissolved organic compounds. The three major sources are: release from living phytoplankton (accounting for approximately 40% of the carbon flow into the microbial loop), release from zooplankton and viruses lysing phytoplankton cells (approximately 40%), and release as waste products excreted from bacteria and zooplankton (20%) (Figure 3). From this study, wh have an improved understanding of the sources and composition of organic compounds driving one of the largest flows in the ocean carbon cycle.

CUREs (Classroom-based Undergraduate Research Experience) are educational tools that give undergraduate students the opportunity to conduct research early in their degree programs. The CURE we developed involved undergraduates in research ongoing for the grant: identification of the compounds being taken up by individual bacterial transporter genes. Students tested bacterial strains whose genome had been disrupted in a single transporter gene by screening them for growth on a suite of compounds. Strains no longer able to grow on a compound indicated that the disrupted transporter was responsible for that compound’s uptake. In collaboration with college and university faculty looking to expand scientific research exposure for their undergraduate students, the Ocean Genes CURE was adopted at four universities/colleges. Instructors met with our group on Zoom each month for Q and A and for obtaining additional assistance for classroom implementation. The NSF project also benefited from the CURE from the increased knowledge of the substrates transporter proteins brought into a bacterial cell, information central to interpreting microbial gene presence and expression in the surface ocean. 

Last Modified: 04/16/2026
Modified by: Mary Ann Moran