Concentrations and uptake of neutral monosaccharides along 14°W in the equatorial Pacific: Contribution of glucose to heterotrophic bacterial activity and the DOM flux

Abstract

We examined concentrations and uptake of dissolved neutral monosaccharides (DNMS) in order to determine the contribution of DNMS to heterotrophic bacterial production and to the flux of dissolved organic matler (DOM) in the equatorial Pacific. DNMS concentrations were greater during El Nifio-affected months of February-April 1992 than during August-October 1992; in contrast, glucose turnover was the oppositeturnover was faster in August-October than in February-April. The variation in sugar concentrations and turnover probably resulted from El Nifio-induced changes in primary production; as El Nifio waned primary production increased, which appeared to stimulate bacterial activity, especially glucose turnover, that in turn forced down DNMS concentrations. In all months, however, DNMS concentrations were low, especially com:pared with total dissolved organic carbon concentrations (< 1%). Glucose was the dominant neutral monosaccharide and alone supported 15-47% of bacterial production. Other monosaccharides apparently did not support much bacterial growth; concentrations of other sugars were low, as probably was turnover. Respiration of glucose (30-60% of uptake) and mannose (60-90%) was relatively high, suggesting that DNMS supported a large fraction of bacterial respiration as well as biomass production. These results point to the importance of DNMS and glucose in particular in supporting bacterial growth and in contributing to the flux #of labile DOM.The concentration of labile dissolved organic matter (DOM) in aquatic ecosystems is usually low, but rates of bacterial activity and biomass production (Cole et al. 1988;Ducklow and Carlson 1992) indicate that DOM fluxes can be quite high. In spite of the recognized importance of DOM, we have few direct measures of DOM fluxes and know little about the composition and uptake of specific components of&e labile DOM pool. Nearly all previous studies on labile DOM have focused on dissolved free amino acids (DFAA) and on the support of bacterial production by DFAA.Free amino acids can provide much C and N for bacterial growth, which in turn suggests that the DFAA is a large component of the labile DOM flux in at least some marine environments. In estuaries, DFAA seems to support a large fraction (if not all) of bacterial production (Billen and Fontigny 1987; Jorgensen et al. 1993;Hoch and Kirchman 1995). Dissolved combined amino acids ' Corresponding author. AcknowledgmentsWe thank R. Penner for his comments about the manuscript.

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