Bioavailability of nitrogen, phosphorus, iron and cobalt to surface water planktonic microbes in the low latitude Atlantic Ocean

Abstract

The effects on bacterioplankton of very low concentrations of nitrogen and phosphorus/iron in the Atlantic were investigated using shipboard enrichment experiments. In the North Atlantic gyre, bacterioplankton abundance and amino acid uptake increased upon combined addition of ammonium and phosphate (9-42% and 120-880% increase, respectively). Outside the gyre, the requirement for phosphate additions was reduced. In the South Atlantic, ammonium additions generally caused an increase in bacterioplankton abundance (10-32% after 48 h) and amino acid uptake (20-300%), particularly in the gyre centre. Conversely, iron additions showed a negligible response. These results contribute towards understanding the effect of low nutrient concentrations on bacterioplankton. Reduced abundance or metabolic activity of bacterioplankton due to low nutrient concentrations may impact marine primary production and carbon fluxes, which is a particular concern due to the expansion of oligotrophic gyres as a result of climate change. Iron and cobalt are essential micronutrients that are susceptible to forming particulate/insoluble species (e.g. via adsorption, oxidation, precipitation). These species can be lost from the surface ocean, thus reducing nutrient bioavailability. Therefore, mechanisms affecting their formation were also investigated. Cell surface iron adsorption was measured in the South Atlantic, with highest levels occurring in the gyre (~180-300 zmol Fe/cell, verses ~3-155 zmol Fe/cell in productive waters). This was hypothesised to be due to low ambient iron in the gyre, resulting in a larger free cell surface area for iron binding. Particulate cobalt formation was enhanced (>150%) in the presence of Aurantimonas (a manganese-oxidising bacteria), was generally elevated under higher manganese concentrations (e.g. ~13-60% increase upon adding MnCl2 to manganese-poor cultures), but was slightly reduced in the presence of nickel (6.2±7.4% decrease) or copper (7.4±12.2% decrease). These results further understanding of factors influencing micronutrient speciation, which is especially important considering potential changes to ocean biogeochemistry as a result of anthropogenic activity.

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