Distribution, origin and transformation of amino sugars and bacterial contribution to estuarine particulate organic matter

•Particulate amino sugars were investigated in an estuary.•Amino sugar concentrations and yields were greater during the monsoon.•Particulate organic matter was extensively degraded during the monsoon.•Bacteria influenced particulate organic matter biochemical composition.•Terrestrial material with bacterial signature was transported to coastal waters.

Amino sugars including bacterial biomarker muramic acid (Mur) were investigated in suspended particulate matter (SPM) to understand their distribution, origin, and biogeochemical cycling and the contribution of bacteria to particulate organic matter (POM) of the Mandovi estuary. SPM was collected from 9 sampling stations in the Mandovi estuary during the pre-monsoon (March) and monsoon (August). Total particulate amino sugar (TPAS) concentrations and yields varied spatially and were 2 to 5 times higher during the monsoon than the pre-monsoon. Negative correlation between salinity and TPAS-C yields [TPAS-C/particulate organic carbon (POC)×100] indicates the influence of terrestrial organic matter on the transport of TPAS-carbon. Glucosamine (GlcN), galactosamine (GalN), and mannosamine (ManN) were abundant during the monsoon. Low GlcN/GalN ratios (<3) indicate bacteria as the major source of amino sugars. Higher amino sugar yields and lower GlcN/GalN ratios during the monsoon than the pre-monsoon indicate enhanced transformation and greater bacterial contribution to POM during the former season. Degradation trends observed with TPAS were well supported by those obtained with carbohydrates and amino acids. Based on Mur concentrations, bacteria accounted for 24% to 35% of the POC and 24% to 62% of the total particulate nitrogen (TPN). Intact bacterial cells, however accounted for a small proportion of POC (2.5% to 4%) and TPN (9% to 11%). Our study suggests that POM was subjected to extensive diagenetic transformation, and its composition was influenced by bacteria, especially during the monsoon.