Dissolved carbohydrate and microbial ATP in the North Atlantic: concentrations and interactions

Abstract

A selective and sensitive spectrophotometric assay for monosaccharide (MCHO) before and after a hydrolysis step has permitted the estimation of total carbohydrate (TCHO) and of polysaccharide (PCHO) by difference. The concentrations and diel variations of MCHO, TCHO, PCHO, and dissolved organic carbon (DOC) were estimated on some 90 samples obtained at 15 stations between Rhode Island and Spain. DOC ranged from 570 to 1330 μg C 1 −1 with a mean of 940 μg C 1 −1. MCHO, calculated as hexose, ranged from 65 to 356 μg 1 −1 (mean 163 μg 1 −1), accounting for 3.5 to 13.2% of the DOC. TCHO varied from 175 to 583 μg 1 −1 (mean 348 μg 1 −1), 8.0 to 24.5% of the DOC. PCHO ranged from 0 to 379 μg 1 −1 (mean 184 μg 1 −1), up to 16.0% of the DOC. MCHO, TCHO, and PCHO were positively correlated with DOC at the 0.01 level. PCHO as a % of TCHO decreased significantlyfrom 69% in coastal to 37% in mid-ocean samples. The vertical distribution of the microbial plankton smaller and larger than 3 μm was determined at 12 of the stations by ATP assay on samples sequentially filtered through Nuclepore membranes. Living biomass for the bacterioplankton (0.2 to 3.0 μm) calculated from the ATP concentration ranged from 1 to 55 μg C 1 −1 (approx 10 4 to 10 6 bacterial cells ml −1) and accounted for 3 to 80% of the total living biomass in the microbial plankton, averaging 30% in the photic and 40% in the aphotic zone. Vertical profiles of bacterial and protist ATP at six stations are compared with those for dissolved carbohydrates, DOC, chlorophyll a, phaeopigments, dissolved oxygen, and temperature. These include four diel drift stations in which a daylight sampling was followed by a predawn resampling on the following day. Carbohydrate peaks were often associated with accumulations of organisms in the > 3- μm size fraction, which were low in chlorophyll a, possibly indicative of collections of protozooplankton. During the day there was evidence of net release of carbohydrate at the depth of the chlorophyll a maxima at oceanic stations but not at neritic stations. Appreciable bacterioplankton maxima (0.2 to 3.0 μm ATP) occurred most often with carbohydrate minima.