Carbohydrates in size fractionated dissolved organic matter in a station of the Bay of Bengal

Abstract

Seawater samples were collected from 4 depths (2, 100, 500 and 1000 m) at one station in the Bay of Bengal. Very high molecular weight (VHMW) (>30 kDa to 0.2 µm) and high molecular weight (HMW) (10 to 30 kDa) fractions of the dissolved organic matter (DOM) were collected using Amicon stirred Ultrafiltration Cell, and analysed for dissolved organic carbon (UDOC), total carbohydrates (UTCHO) and neutral sugars (UNS). UDOC concentrations were relatively higher in HMW fraction whereas, UTCHO and UNS were relatively enriched in VHMW fraction. Concentrations of UDOC, UTCHO and UNS generally decreased with increasing water depth. The UNS-C accounted for 2.2 to 6.2% of UDOC and generally decreased with depth. The observed decrease suggests utilization by in situ heterotrophic organisms. Monosaccharide composition data suggest that rhamnose, fucose and galactose were relatively more abundant in VHMW fraction, as well as glucose in HMW fraction. The weight percentage fractions of most of the monosaccharides of both VHMW and HMW did not show consistent decrease with depth. The yields and concentrations of UNS indicated a size-related diagentic sequence where VHMW represents the most reactive fraction.