Production and export of particulate fatty acids, carbohydrates and combined amino acids in the euphotic zone

The dynamics of particulate organic matter (POM) in the euphotic zone were evaluated at the molecular level by the combination of two methods, 13C-labeling of photosynthetic products and sediment trap collection of sinking particles, in Sagami Bay, Japan. Determinations of the fatty acids, carbohydrates and combined amino acids of photosynthetic products and POM sinking out of the euphotic zone enabled us to calculate the biochemical compound-based export efficiency (specific-compound e ratio). Carbohydrates and combined amino acids were separated into two fractions, methanol/water-soluble and -insoluble (residual) fractions after organic solvent extraction for fatty acid determinations. The specific-compound e ratios of carbohydrates (0.25) and combined amino acids (0.27) in the residual fraction were similar to the ratio of bulk particulate organic carbon (bulk POC) (0.26), while the ratios of fatty acids, and carbohydrates and combined amino acids in the methanol/water fraction were lower at 0.080, 0.078 and 0.064, respectively. Based on the specific production rates and the specific sinking rates of biochemical compounds, it was concluded that the dynamics of biochemical constituents of POM in the euphotic zone differed as follows: (i) the dynamics of combined amino acids in the residual fraction was similar to that of bulk POC; (ii) carbohydrates in the residual fraction were actively produced and actively degraded in the euphotic zone and actively removed by sinking; and (iii) fatty acids, and carbohydrates and combined amino acids in the methanol/water fraction, were actively produced and actively degraded in the euphotic zone and less removed by sinking. Major fatty acids, and carbohydrates and combined amino acids in the methanol/water fraction, occur primarily as organic constituents of intact phytoplankton. The export efficiency of intact phytoplankton was less than 10% and the carbon biomass of intact phytoplankton was less than one-third of the POC sinking out of the euphotic zone.