Copper-Phytoplankton reciprocal interaction: Biogeochemical cycling of copper and its isotopes
Copper (Cu) is involved in multiple metabolic processes in phytoplankton. However, dissolved Cu (dCu) concentrations that exceed a tolerance level can be toxic. Owing to its biological relevance to marine phytoplankton, there is a need to better understand the distribution of Cu in the ocean. The overarching aim of this study was to investigate the feedback interaction Cu has with marine phytoplankton and to understand better the biogeochemical cycling of Cu and its isotopes in the marine realm. Firstly, two Southern Ocean (SO) diatoms, Proboscia inermis and Chaetoceros flexuosus were cultured under various concentrations of Cu and iron (Fe) in order to investigate the effects of Cu-Fe interaction on growth rate, silicon (Si) uptake kinetics and Si isotope fractionation. Overall, divergent responses were observed in Michaelis-Menten kinetic uptake parameters and Si isotope fractionation. Our findings confirmed that Cu has a significant effect on biochemical processes in diatoms and can potentially alter community composition dynamics in the SO, particularly, by way of its interaction with Fe. To address the gaps in our knowledge of the cycling of Cu, we investigated the distribution of dCu and particulate Cu (pCu) in various biogeochemical regimes. The study region targeted a north-south transect categorised by the transition from warm waters of the East Australian Current (EAC) to cold, fresh subantarctic (SA) waters. A simple anion-exchange purification method was developed using AG-MP 1 resin which achieved near full recovery of Cu and allowed for precise measurements of Cu isotope. At the high productivity stations, the upper water column (200 m) had a relatively heavier dCu isotopic signature. This was complemented by lighter pCu isotope values and simultaneous increases in pCu concentration [pCu]. Below the euphotic zone, decreases in the [pCu] are coupled with a heavier pCu isotope value and a lighter dCu isotope signature. This distribution was largely attributed to the biological uptake of the lighter isotope at the surface and remineralisation of Cu from sinking particles in the deeper waters. At the station located east of the South Tasman Rise, the dCu isotope value in the upper water column was relatively heavier when compared to the pCu isotope signature. This was attributed to organic complexation of the heavy isotope in the dissolved phase. In contrast, this station had a significantly lighter dCu isotope signature in the deep ocean (>2000m), suggesting a potential benthic supply of isotopically light Cu from the South Tasman Rise. At another SA station, high variability in the Cu isotope composition is observed throughout the water column, particularly below the euphotic zone. The heavier dCu isotope signature is complemented by a lighter pCu isotope signature (vice-versa), and peaks in [pCu] are associated with heavier isotopic value for pCu. The mid-depth non-homogeneity is reflective of the continual exchange of Cu between the particulate and dissolved phases (reversible-scavenging). Lastly, an 8-day shipboard mesocosm experiment was conducted at a high productivity site to investigate the important influences of photochemical processes on the biogeochemical cycling of Cu in the natural waters of the EAC. Phytoplankton grew at different rates with variable lag phase durations under different irradiance levels. It was inferred that shifts in phytoplankton community composition influence the distribution of Cu through a feedback interaction via processes such as biological uptake and scavenging. The variability observed in the dCu isotope values confirmed the significant effect irradiance level has in controlling phytoplankton growth and the cycling of Cu. Overall, isotopic measurements proved to be an effective tool in identifying the key processes that affect the cycling of Cu. This warrants further investigation into the distribution of Cu isotopes, encompassing various biogeochemical regimes.
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