Similarity in microbial amino acid uptake in surface waters of the North and South Atlantic (sub-)tropical gyres

The Earth’s most extensive biomes – the oceanic subtropical gyres – are considered to be expanding with current surface ocean warming. Although it is well established that microbial communities control gyre biogeochemistry, comparisons of their metabolic activities between gyres are limited. In order to estimate metabolic activities including production of microbial communities, the uptake rates of amino acids leucine, methionine and tyrosine at ambient concentrations were estimated in surface waters of the Atlantic Ocean using radioisotopically labelled tracers. Data were acquired during six research cruises covering main oceanic provinces herein termed: North and South Atlantic Gyres, Bermuda Atlantic Time-series Study site (BATS), Equatorial region, and Mauritanian Upwelling (off Cape Blanc). Data were divided between provinces, the extents of which were identified by ocean colour data, in order to achieve provincial mean uptake rates. Leucine and methionine uptake rates did not differ between sampling periods, and were comparable between the North and South subtropical gyres. Furthermore, variation in uptake rates measured throughout the two oligotrophic gyres, where sampling covered ∼4 × 106 km2, was considerably lower than that measured within the Mauritanian Upwelling and Equatorial regions, and even at the BATS site. Tyrosine was generally the slowest of the amino acids to be taken up, however, it was assimilated faster than methionine within the Mauritanian Upwelling region. Thus, we propose that one value for leucine (12.6 ± 3.2 pmol L−1 h−1) and methionine (10.0 ± 3.3 pmol L−1 h−1) uptake could be applied to the oligotrophic subtropical gyres of the Atlantic Ocean. However, with the significantly lower uptake rates observed at the BATS site, we would not advise extrapolation to the Sargasso Sea.