Macro and micro nutrient limitation of microbial productivity in oligotrophic subtropical Atlantic waters

The subtropical oceans are characteristically low-nutrient low-chlorophyll regions, but owing to their geographical dominance and rapid nutrient cycling may contribute > 30% of the total marine primary production. The present study investigates the addition of P, Fe, Co and Zn on rates of primary production and heterotrophic bacterial production, through a combination of mesoscale in situ (P, and P+ Fe) and in vitro (Co or Zn) bioassay incubation experiments. Results from the bioassay incubation experiments suggest that primary production and chlorophyll a biomass are limited by N and P in this oligotrophic region. However, both were increased further after addition of trace metal micronutrients in the order Fe + Zn >= Fe + Co > Fe approximate to Co. In contrast, rates of heterotrophic bacterial production did not appear to be P, or significantly, P + Fe limited, although in situ rates did increase during the first 12 h of mesoscale P fertilisation (which were not mirrored in the mesoscale P + Fe addition). The addition of Co to unfertilised waters increased heterotrophic bacterial production and the numbers of heterotrophic bacteria, Prochlorococcus spp. and Synechococcus spp., suggesting Co limitation. Prochlorococcus spp. were the most abundant autotrophs. The highest increases in both heterotrophic and autotrophic carbon assimilation were shown after in vitro addition of either Co or Zn to mesoscale enriched P+ Fe waters, suggesting multiple limitation of microbial growth rates in the subtropical oligotrophic north-east Atlantic.