Dissolved carbohydrates and its influence on bacterioplankton diversity in the euphotic zone of Equatorial Indian Ocean during southwest monsoon

The Equatorial Indian Ocean (EIO) is oligotrophic in nature, low in productivity and characterised by relatively high bacterial biomass and heterotrophic productivity. The microbial biomass is fuelled by dissolved organic carbon (DOC) whose lability depends upon its chemistry. During an expedition in the EIO, samples for dissolved carbohydrates (DCHO), dissolved monosaccharide (MCHO), dissolved polysaccharide (PCHO), total bacterial abundance, total organic carbon (TOC), nutrients, chlorophyll a (chl a), bacterial diversity, and phytoplankton composition were estimated along two transects (TS-1 & TS-2) in the euphotic depth (200 m). DCHO and MCHO samples were analysed using 3-methyl-2-benzothiozolinone hydrazone hydrochloride (MBTH) method. The DCHO, MCHO, and PCHO concentrations in TS-1 ranged from 2.4 to 5.8 μM; 0.7 to 2.7 μM; 0.7 to 3.4 μM, while in TS-2 it ranged from 2.2 to 4.8 μM; 0.3 to 2.8 μM; 0.2 to 3.8 μM, respectively. Vertically, DCHO concentrations decreased within the photic depths in most stations. PCHO concentrations were relatively greater in TS-2 than TS-1, which may be due to preferential removal of MCHO in TS-2. In the top 200 m of the water column, Proteobacteria was the most abundant bacterial phylum followed by Cyanobacteria, Actinobacteriota, Bacteroidota, and SAR-324 while dinoflagellates dominated the phytoplankton population. The distribution of DCHO and PCHO were inversely correlated to salinity, temperature, and chl a. The bacterial heterotrophs like alpha-Proteobacteria, Chloroflexii, and Verrumicrobiota along with SAR324 and Actinobacteriota were clustered with nutrients, while Bdellovibriota and Dadabacteria clustered with MCHO, PCHO, and DCHO. These results highlight the impact of DCHO & PCHO lability on the distribution of bacterial community and delineate oceanographic factors that regulate the distribution of DCHO and its constituents in EIO waters.