Spatial Heterogeneity in Particle-Associated, Light-Independent Superoxide Production Within Productive Coastal Waters

Abstract

In the marine environment, the reactive oxygen species (ROS) superoxide is produced through a diverse array of light-dependent and light-independent reactions, the latter of which is thought to be primarily controlled by microorganisms. Marine superoxide production influences organic matter remineralization, metal redox cycling, and dissolved oxygen concentrations, yet the relative contributions of different sources to total superoxide production remain poorly constrained. Here we investigate the production, steady-state concentration, and particle-associated nature of light-independent superoxide in productive waters off the northeast coast of North America. We find exceptionally high levels of light-independent superoxide in the marine water column, with concentrations ranging from 10 pM to in excess of 2,000 pM. The highest superoxide concentrations were particle associated in surface seawater and in aphotic seawater collected meters off the seafloor. Filtration of seawater overlying the continental shelf lowered the light-independent, steady-state superoxide concentration by an average of 84%. We identify eukaryotic phytoplankton as the dominant particle-associated source of superoxide to these coastal waters. We contrast these measurements with those collected at an off-shelf station, where superoxide concentrations did not exceed 100 pM, and particles account for an average of 40% of the steady-state superoxide concentration. This study demonstrates the primary role of particles in the production of superoxide in seawater overlying the continental shelf and highlights the importance of light-independent, dissolved-phase reactions in marine ROS production.

Keywords: extracellular superoxide; light‐independent ROS; reactive oxygen species.

Figure 1

Sample locations of ROS measurements collected in this study in August 2017. Stations 1, 2, 3, 5, and 6 are on‐shelf sampling locations (max depth 50–60 m), and Station 4 is a deeper (max depth over 2,000 m) off‐shelf sampling location.

Figure 2

Water column profiles (on‐shelf locations) of light‐independent superoxide (pM; upper axis with black filled dots). Also plotted are the dissolved oxygen concentrations (𝜇M; lower primary axis with gray dashed line) and chlorophyll‐based fluorescence (mg m⁻³; lower secondary axis with solid gray line). The error bars represent the standard deviation between two replicates. The dashed horizontal gray line represents the 1% light level defined as the base of the photic zone.

Figure 3

Water column light‐independent, steady‐state superoxide (pM; upper axis with black filled dots) concentrations at Station 4 (left). The fractional contribution of particle‐associated superoxide production to the total superoxide concentration is also shown (particle associated in black, nonparticle associated in white, right image). Dissolved oxygen concentration (𝜇M; lower primary axis with gray dashed line, left only) and chlorophyll‐based fluorescence (mg m⁻³; lower secondary axis with solid gray line) are also shown. The error bars represent the standard deviation between two measurements. The dashed horizontal gray line represents the 1% light level defined as the base of the photic zone. Note the y axis breaks at 900 m (left) and 450 m (right).

Figure 4

Water column profiles of four on‐shelf sampling locations where particle‐ and nonparticle‐associated superoxide concentrations were measured. The total light‐independent superoxide concentrations (pM) are plotted as bar graphs with particle‐associated (black) and nonparticle‐associated (white) contributions shown. Water column chlorophyll‐based fluorescence (mg m³) is plotted in gray. The error bars represent the standard deviation between two replicates for total light‐independent superoxide concentration.

Figure 5

Linear least squares fits for superoxide production rate as a function of nanoeukaryotes (top left), picoeukaryotes (top right), Synechococcus (middle left), bacteria (middle right), and chlorophyll concentration (bottom). Superoxide production rates were determined at all locations where superoxide decay rate constants were determined in unfiltered seawater. Error bars on cell counts represent one standard deviation of triplicate analysis.