ABSTRACT: In situ concentrations of low molecular weight carboxylic acids and carbonyl compounds were determined in the upper 100 m water column in the Eastern Mediterranean Sea in September 1997. Concentrations ranged from 1 to 13 nM for pyruvate, 1 to 11 nM for glyoxylate and 30 to 70 nM for glycolaldehyde. No distinct diurnal pattern in the depth distribution of carboxylic acids and carbonyl compounds was detectable. Exposure of surface water to solar radiation, however, resulted in photochemical production of acetaldehyde (10 nmol l^-1 h^-1), acetone/acrolein (6 nmol l^-1 h^-1) and pyruvate (3 nmol l^-1 h^-1). Depth profiles of the optical properties of the total organic carbon (TOC) exhibited low TOC-normalized fluorescence (excitation 350 nm, emission 450 nm) and absorbance (abs_365nm) in the top 25 m, both increasing with depth. This indicates an increasing contribution of refractory molecules to the TOC pool with depth. Uptake rates of pyruvate by heterotrophic bacterioplankton ranged from 0.022 to 0.185 nmol l^-1 h^-1 and contributed ~10% to the total bacterial carbon production as measured by [³H]leucine incorporation. Bio-assay experiments with surface water amended with either 1 µM NO₃, 0.4 µM PO₄ or 1 µM glucose or in combination decreased pyruvate uptake as compared to the unamended control. Our results reveal that pyruvate might be an important source of carbon for bacterioplankton, with turnover times of pyruvate of 0.7 to 24 d.

KEY WORDS: Mediterranean Sea · Bacterioplankton · Carboxylic acids · Carbonyl compounds · Ultraviolet radiation · Dissolved organic matter · Pyruvate · Bacterial utilization

Published in AME Vol. 20, No. 2 (1999) on December 15
ISSN: 0948-3055. Copyright © Inter-Research, Oldendorf/Luhe, 1999