Sediment bacterial diversity in the upwelling regions of Alappuzha, on the Southwest coast of India

Upwelling is an oceanographic phenomenon characterized by the upward movement of cold nutrient-rich water from the deep ocean to the surface. The Southwest coast of India experiences coastal upwelling during the southwest monsoon (SWM-June to August) due to the influence of strong south western winds, which, combined with the Coriolis effect, along with this wind featuring calm, muddy waters and suspended sediments close to the coast, especially the Mudbanks in Alappuzha. Understanding the spatiotemporal variance within the community is vital to comprehending the mechanisms underlying biogeochemical cycles. However, the bacterial community structure and function in the upwelling and Mudbank regions on the Alappuzha coast have received little attention. In this study, we analyzed the abundance and diversity of bacterial communities in sediment during pre-monsoon (PRM) and monsoon (MON) periods in three stations, M1, M2, and M3. During the MON, Mudbanks appeared only at station M2, while upwelling occurred across the study region. Comparative analysis of family to genus/species level showed that PRM and MON seasons have distinct microbial communities with variations in their relative abundance in the M2 (Mudbank and upwelling prevalent) compared to the other two stations such as M1 and M3 (upwelling alone). The total count (0.32-6.12×106) (TC) and total viable count (0.62-2.56×106) (TVC) indicating bacterial abundance were higher in the PRM  than in MON. However, the relative abundance of Proteobacteria was comparatively high in all the stations, with higher abundance during MON than PRM. Vibrio, Photobacterium, and Ferrimonas were detected and their relative abundance was comparatively higher in the upwelling period than PRM. The abundance of lower trophic levels communities such as the genus Bacillus, Tepidibacter, Vibrio, Photobacterium, etc. showed spatiotemporal variations. Our results indicate the variations in bacterial community structure which could be coupled with variations in organic substances, changes in currents, and oceanographic features.