Using 67Cu to Study the Biogeochemical Cycling of Copper in the Northeast Subarctic Pacific Ocean

Abstract

Microbial copper (Cu) nutrition and dissolved Cu speciation were surveyed along Line P, a coastal to open ocean transect that extends from the coast of British Columbia, Canada, to the high-nutrient-low-chlorophyll (HNLC) zone of the northeast subarctic Pacific Ocean. Steady-state size fractionated Cu uptake rates and Cu:C assimilation ratios were determined at in situ Cu concentrations and speciation using a 67 Cu tracer method. The cellular Cu:C ratios that we measured (∼30 μmol Cu mol C −1 ) are similar to recent estimates using synchrotron x-ray fluorescence (SXRF), suggesting that the 67 Cu method can determine in situ metabolic Cu demands. We examined how environmental changes along the Line P transect influenced Cu metabolism in the sub-microplankton community. Cellular Cu:C assimilation ratios and uptake rates were compared with net primary productivity, bacterial abundance and productivity, total dissolved Cu, Cu speciation, and a suite of other chemical and biological parameters. Total dissolved Cu concentrations ([Cu] d ) were within a narrow range (1.5-2.8 nM), and Cu was bound to a ∼5-fold excess of strong ligands with conditional stability constants (K cond 2 ) of ∼10 14 .CuL,Cu + Free Cu 2+ concentrations were low (pCu 14.4-15.1), and total and size fractionated net primary productivity (NPP ; g C L −1 d −1 V μ ) were negatively correlated with inorganic Cu concentrations ([Cu ]). We suggest this is due to greater Cu drawdown by faster growing phytoplankton populations. Using the relationship between [Cu ] drawdown and NPP V , we calculated a regional photosynthetic Cu:C drawdown export ratio between 1.5 and 15 μmol Cu mol C −1 , and a mixed layer residence time (2.5-8 years) that is similar to other independent estimates (2-12 years). Total particulate Cu uptake rates were between 22 and 125 times faster than estimates of Cu export; this is possibly mediated by rapid cellular Cu uptake and efflux by phytoplankton and bacteria or the effects of grazers and bacterial remineralization on dissolved Cu. These results provide a more detailed understanding of the interactions between Cu speciation and microorganisms in seawater, and suggest that marine phytoplankton modify Cu speciation in the open ocean.

Insights Into the Major Processes Driving the Global Distribution of Copper in the Ocean From a Global Model

“…In contrast, phytoplankton uptake represents a sink of DCu of 31 GmolCu/year, with approximately half of the uptake flux being recycled by zooplankton (17 GmolCu/year), and 9 GmolCu/year is remineralized by bacteria. The model suggests a residence time of 3 years for the top 100 m, which agrees with estimates of 2.5–8 years from the North Pacific (Semeniuk et al, ). The remainder 5.8 GmolCu/year sinks into deeper water as particulate organic Cu.…”

“…The average Cu:C ratio decreases progressively from 15 to 10 to 8 μmolCu:molC for phytoplankton uptake, zooplankton recycling, and particle remineralization, respectively, and the ensuing modeled export ratio of 16 μmolCu:molC agrees with Semeniuk et al's () estimations between 1.5 and 15 for the North Pacific region. We calculated the Cu:C ratio in the dissolved phase from the total organic and inorganic dissolved Cu and P and used the Redfield ratio of 106:1 molC:molP.…”

“…The modeled DCu distribution in our reference simulation (REF) is able to closely reproduce measurements from different campaigns and experiments (A. Gourain personal communication compiled data from, e.g., the GEOTRACES database, Schlitzer et al, ; line P transect in the North Pacific, Posacka et al, ; Semeniuk et al, ; and PINTS expedition in the Tasman Sea, Hassler et al, ; see Figures a, d, and g). Average [DCu] in the first 50 m is 0.83 nmolCu/L, with minimal concentrations (below 0.20 nmolCu/L) in the subtropical Pacific along the Indonesian coasts and in the subtropical oligotrophic gyres and maximal concentrations (over 1.5 nmolCu/L) in the Southern Ocean where modeled [DCu] slightly overestimates the measurements.…”

“…We also calculated the total Cu uptake in the first 100 m of the global ocean and found a total uptake of 31 GmolCu/year. Converted into picomoles of Cu per day per liter, results from REF give 2.4 pmolCu·day −1 ·L −1 , which is on the lower end of Semeniuk et al (2009) and Semeniuk et al (2016). These authors also found variability in uptake rates in the northwestern Pacific (between 3 and 125 pmolCu·day −1 ·L −1 ).…”

The biogeochemical cycling of iron, copper, nickel, cadmium, manganese, cobalt, lead, and scandium in a California Current experimental study

“…Biological Cu demand is associated with its role in high affinity Fe uptake systems in diatoms (Peers et al ; Maldonado et al ), though a Cu independent pathway for Fe acquisition in marine phytoplankton also exists (Morrissey et al ). The lack of apparent decrease in Cu in our experiment may reflect rapid cycling of Cu via uptake and efflux as has been hypothesized for observations in the North Pacific (Semeniuk et al ), or the inability of some diatoms to access the exceedingly low Cu 2+ concentrations in this experiment (Sunda and Huntsman ). Indeed, despite the mosaic of phytoplankton Fe limitation occurring in the coastal North Pacific, dissolved Cu profiles here exhibit minimal changes with depth (Supporting Information Fig.…”