Dissolved and particulate amino acids and carbohydrates in the sea surface microlayer

Abstract

Eleven pairs of sea-surface microlayer (⩽250 μm) and subsurface water (5–15 cm depth) samples were collected near the coast of Baja California. Particulate matter was removed by filtration through polycarbonate membrane filters (8, 1, 0.2, 0.1, and 0.05 μm pore sizes) and the filtrates analyzed for hydrolyzable amino acids and total carbohydrates. Dissolved free amino acids were measured in some samples on board ship. Hydrolyzable amino acids, free amino acids, and total carbohydrates were more concentrated in nearly all microlayer samples than in subsurface water samples taken at the same location. For hydrolyzable amino acids and total carbohydrates, the enrichment was observed for both particulate and dissolved material. Averaging both microlayer and subsurface water samples, about 20% of the dissolved organic carbon and about 60% of particulate organic carbon was identified as carbohydrate or amino acid carbon. Particulate hydrolyzable amino acids were mainly in the 1–8 and 0.2–1 μm size ranges, but particulate total carbohydrates did not usually have pronounced maxima in particle-size distributions. Relative to their proportions in subsurface-water dissolved organic matter, hydrolyzable amino acids were increased over total carbohydrates in the microlayer and in particulate material. Nonpolar hydrolyzable amino acids also had higher concentrations relative to other amino acids in microlayer and particulate organic matter. Differences in microlayer and subsurface water compositions are probably related both to differences in the surface activity of polymers containing amino acids and carbohydrates, and to interactions between surface-active molecules and particles.