The role of biominerals in the sinking flux and preservation of amino acids in the Southern Ocean along 170°W

In a study of the transport and transformation of particulate amino acids in the Southern Ocean, we found that silicate and calcium carbonate biominerals play an important role in the preservation of amino acids throughout the water column and surface sediments. Plankton, sinking particle and sediment samples were collected during the USJGOFS AESOPS transect across the Antarctic Polar Front (APF) along 170°W. Total hydrolyzable amino acids (THAA) made up of 17–27% of total organic carbon (Corg) in sinking particles and 6–23% of Corg in surface sediments. In addition to THAA, we measured amino acids bound in silicate (SiTHAA) and calcium carbonate (CaTHAA) biominerals. Although the fraction of biomineral bound to total amino acids in plankton was small, <1%, the ratio of mineral-bound to non-mineral-bound amino acids increased with depth in the water column and sediments. Mineral-bound amino acids often dominated the total amino acid pool in biomineral-rich sediments beneath the Antarctic Circumpolar Current. SiTHAA were enriched in glycine and threonine relative to THAA and were similar in composition to SiTHAA in diatom frustules isolated from APF sediments. CaTHAA were enriched in aspartic acid relative to THAA. The difference in composition between mineral-bound amino acids and non-mineral-bound amino acids increased with depth. acid composition has been used to develop a Degradation Index (sensu Dauwe and Middelburg, 1998). The unusual amino acid composition of Southern Ocean plankton, i.e., dominated by diatom cell walls, resulted in an apparent mismatch between the absolute value of the Degradation Index and the presumed extent of degradation. However, changes in amino acid composition that accompanied degradation were similar to those found in previous studies. Principal components analysis suggests that the greatest change in THAA composition occurred between the sediment surface floc layer and deeper sediments where particles had the longest residence time. Compositional changes observed in the water column suggested that degradation processes resulted in complete removal of amino acids, whereas changes in sediments were consistent both with selective degradation of plankton amino acids with depth and with the conversion of primarily phytoplankton biomass to that of bacterial biomass.