Whither the extended Redfield Ratio? Decoupling of cellular, particulate, and dissolved micronutrient trace metal stoichiometries across the global ocean

Phytoplankton require trace metals for numerous physiological processes and actively accumulate iron, manganese, cobalt, nickel, copper and zinc from surrounding waters. It has been suggested that the stoichiometric framework first proposed by Alfred Redfield for carbon, nitrogen and phosphorus in marine plankton may be extended to these trace metal micronutrients, based on similarities in trace metal:biomass stoichiometries initially observed for surface ocean particulate matter and cultured phytoplankton. However, three important differences between metals and macronutrients may limit the biogeochemical utility of applying any ‘average’ stoichiometry measured or assumed for phytoplankton. First, as metals generally play catalytic rather than structural roles in phytoplankton physiology, metal concentrations in cells can vary by orders of magnitude rather than a factor of 3 or less for most macronutrients. Second, metals often have non-negligible associations with abiotic particulate material, and thus cellular stoichiometries are often distinct from the stoichiometry of the overall particle pool. Third, abiotic uptake (i.e., scavenging) of metal dissolved metals is known to occur in both surface and subsurface waters, potentially decoupling stoichiometries of particulate and dissolved pools. We compiled a dataset of global phytoplankton trace metal stoichiometries representing approximately 70 stations across 5 ocean basins. We compared cellular data to co-occurring stoichiometries of surface ocean bulk particulate matter, as well as stoichiometries of metal uptake and remineralization in the underlying water column, to examine the applicability of the extended Redfield ratio concept to micronutrient metals. There is consistent decoupling of iron stoichiometries in the three fractions across environments, with dissolved < cellular < particulate ratios. Comparisons of manganese, cobalt, nickel, and zinc stoichiometries are also being investigated and will be presented. The varied relationships found between trace metal fractions demonstrate the need for new models of trace metal uptake and internal cycling in the ocean.