Microbial keystone players and biogeochemical impacts of Noctiluca scintillans bloom in Mandovi estuary on west coast of India
Abstract
A sporadic outbreak of the bioluminescent dinoflagellate Noctiluca scintillans (NS) occurred in the Mandovi estuary along the Goa coast of the Central Eastern Arabian Sea on 24th May 2023, prompted by pre-monsoon upwelling, eutrophic conditions, runoff and environmental changes. The algal bloom in the Mandovi region presented a unique and novel observation, characterized by an NS density of 8.4?�?10?� cells/L (19?%) and large cell diameters ranging from 400 to 600?�m. This bloom co-occurred with dominant diatom species, including Melosira (17?%), Coscinodiscus (12.5?%), and Leptocylindrus (7?%), contributing to a total phytoplankton density of 43.1?�?10?� cells/L. This distinctive assemblage marks the first recorded occurrence of such a combination in the Mandovi region. Environmental conditions included wind speed of 5?m/s, atmospheric temperature of 30?�?1.5 �C, sea surface temperature of 29.13?�?0.5 �C, and salinity of 32 PSU. Hydrochemical analysis revealed high ammonia levels (13.49?�M), low dissolved oxygen (DO) with associated hypoxic conditions. Metagenomic sequencing of the V3-V4 regions of the 16S rRNA gene identified Vibrio as the dominant genus of associated bacteria and Methanobrevibacter d__Archaea, opportunistically thrived at the peak of the bloom, suggesting that they could be important players influencing the dynamics and biogeochemical cycling of the blooms. Free-living organisms in surrounding waters dominated with g_Methanobrevibacter of d__Archaea followed by f_ Rhodobacteraceae, g_ Prevotella and f_ Flavobacteriaceae of d_Bacteria. The study highlights the role of local hydrodynamics and pre-monsoon coastal upwelling in driving NS blooms. It emphasizes the influence of keystone bacterial groups, such as Methanobrevibacter and Vibrio, on bloom dynamics and biogeochemical processes. Distinct bacterial populations associated with NS reflect its unique ecological interactions and mixotrophic behavior, offering insights into bloom ecology, host-associated microbial dynamics, and environmental health implications.
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