The Seasonal Flux and Fate of Dissolved Organic Carbon Through Bacterioplankton in the Western North Atlantic

Abstract

The oceans teem with heterotrophic bacterioplankton that play an appreciable role in the uptake of dissolved organic carbon (DOC) derived from phytoplankton net primary production (NPP). As such, bacterioplankton carbon demand (BCD), or gross heterotrophic production, represents a major carbon pathway that influences the seasonal accumulation of DOC in the surface ocean and, subsequently, the potential vertical or horizontal export of seasonally accumulated DOC. Here, we examine the contributions of bacterioplankton and DOM to ecological and biogeochemical carbon flow pathways, including those of the microbial loop and the biological carbon pump, in the Western North Atlantic Ocean (∼39–54°N along ∼40°W) over a composite annual phytoplankton bloom cycle. Combining field observations with data collected from corresponding DOC remineralization experiments, we estimate the efficiency at which bacterioplankton utilize DOC, demonstrate seasonality in the fraction of NPP that supports BCD, and provide evidence for shifts in the bioavailability and persistence of the seasonally accumulated DOC. Our results indicate that while the portion of DOC flux through bacterioplankton relative to NPP increased as seasons transitioned from high to low productivity, there was a fraction of the DOM production that accumulated and persisted. This persistent DOM is potentially an important pool of organic carbon available for export to the deep ocean via convective mixing, thus representing an important export term of the biological carbon pump.

Linkages Among Dissolved Organic Matter Export, Dissolved Metabolites, and Associated Microbial Community Structure Response in the Northwestern Sargasso Sea on a Seasonal Scale

“…3 H-leucine incorporation rates are often used as a proxy for heterotrophic bacterioplankton production rates ( Kirchman et al, 1985 ). Duplicate 1.6 mL seawater samples were amended with 20 nmol L –1 of 3 H-leucine (Perkin Elmer, specific activity > 50 Ci mmol –1 ), and incubated at close to in situ temperatures for 3–4 h. Incubations were terminated by 100% trichloroacetic acid (TCA, 6% final concentration) and extracted with 5% TCA and 80% ethanol via the microcentrifuge method ( Smith and Azam, 1992 ; Baetge et al, 2021 ). Seawater plus 3 H-leucine but with 100% TCA (6% final concentration) added at the initiation of the incubation served as killed controls and followed the same incubation and extraction steps.…”

Bacterioplankton response to physical stratification following deep convection

“…Despite NPP outpacing BP in the euphotic zone during the occupation of N2S4, the accumulation of bulk DOC was not resolvable over the timescale of station occupation (Figure 3d and g). Previous work on initial bloom conditions following nutrient entrainment by deep mixing demonstrated that a greater fraction of NPP was partitioned as particulate organic carbon compared to DOC (Carlson et al, 1998;Baetge et al, 2021). Despite our inability to resolve changes in the bulk DOC pool, increases in both the concentration and relative contribution of TDAA to the bulk DOC pool (Figures 3i and 4j) suggest that the quality of the accumulated DOC pool changed to a composition that was less altered diagenetically (i.e., fresh and less recalcitrant) within the euphotic zone throughout station occupation.…”

Characterization of Sea Surface Microlayer and Marine Aerosol Organic Composition Using STXM-NEXAFS Microscopy and FTIR Spectroscopy

“…Alkane-to-hydroxyl ratio of atmospheric particle (top left), SML (bottom left), gPMA (top right), and seawater (bottom right) samples compared to NPP. NPP was integrated and normalized to the depth of the euphotic zone and obtained from Baetge et al Linear fit lines are shown for r > 0.3. The points include measurements above detection for sample types collected on 09/10/17, 09/12/17, 09/15/17, 09/16/17, 03/27/18, and 04/03/18.…”

“…Phytoplankton blooms are associated with an increase in surface-active compounds, particularly lipids, with increasing POC concentrations . Recent contributions from recently produced DOC may explain the changing composition of the SML during these times. Atmospheric aerosol particles could also be influenced by secondary atmospheric processes of organic components, but there is no evidence of such processes changing the alkane and hydroxyl group fractions .…”