Microplastics and Their Role in Carbon Dynamics in Sundarbans, Northeast Coastal Bay of Bengal

Saini, Nirupama (2026) Microplastics and Their Role in Carbon Dynamics in Sundarbans, Northeast Coastal Bay of Bengal. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Nirupama Saini (20RS011)) 20RS011.pdf - Submitted Version Restricted to Repository staff only Download (23MB)

Abstract

Mangroves are highly efficient carbon sinks, despite occupying a limited fraction of global coastal zones. Their presence at the land–ocean interface makes them vulnerable to accumulation of pollutants, including microplastics (MPs). MPs are increasingly recognized as a novel pool of anthropogenic carbon ("plastic-carbon"), which can alter microbial community structure and function. Yet, their implications for carbon processes within mangrove ecosystems remain poorly understood. Sundarbans, the world’s largest contiguous mangrove forest and a UNESCO World Heritage Site as well as a Ramsar Site, lies on the Ganga-Brahmaputra-Meghna Delta, a well-recognized hotspot of plastic pollution. Despite this, assessment of MPs load and their impact on the regional carbon dynamics is lacking. This thesis aims to provide a comprehensive assessment of MPs and their influence on surface-water carbon dynamics in the Indian Sundarbans, combining field observations, molecular analyses, and experimental approaches. Sampling was conducted at Sagar Island, largest island of Indian Sundarbans, as a part of Sundarbans Biological Observatory Time Series (SBOTS), and Boguran Jalpai (without mangrove vegetation) as part of Coastal Bay of Bengal Time Series (CBoBTS). Carbon dynamics of surface water was first investigated using SBOTS dataset (2014 to 2022), focusing on carbonate system parameters (pH, total alkalinity, and calculated pCO₂), to characterize underlying key drivers before investigating the role of MPs. Multiple linear regression (MLR) and generalized additive (GAM) modelling revealed the significant influence of seasonality and nutrient stoichiometry in shaping surface-water carbon dynamics. Bimonthly sampling (October 2021–October 2022) at Stn1, Stn2, and Stn3 of SBOTS recorded a total of 4038 MPs, with surface-water mean abundance ranging from 5 to 58 MPs/L. A significantly higher abundance of MPs, particularly colourless fragments, was observed during the monsoon season (one-way ANOVA, p<0.05), correlating with high precipitation rates. Twelve distinct polymer types were identified using ATR-FTIR and micro-Raman spectroscopy. The mean plastic-particulate organic carbon (pPOC) associated with MPs was 41.63 ± 72.22 μg/L. MPs were extensively weathered and colonized by microbes, indicating their contribution to plastic-derived dissolved organic carbon (pDOC) and biogenic organic carbon (pBOC) pool. Monthly sampling at Boguran Jalpai recorded MP abundance ranging from 1 to 47 MPs/L (surface water), highlighting variability between mangrove and non-mangrove estuarine systems. Molecular-based analyses showed that the microbial community inhabiting plastics (the “Plastisphere”) was distinct from those in ambient water and sediment. The Linear discriminant analysis effect size (LEfSe) showed Planctomycetes, Planctomicrobium, Rhodopirellula, and Sphingomicrobium as significantly enriched on plastic surface (LDA score > 3). Bacterial strains were cultured and isolated from plastisphere, including Oceanimonas sp. strain NSJ1 and Fictibacillus sp. strain NSJ2 with identified genomic features for degrading polyaromatic hydrocarbons and plastic polymers. NSJ2 was identified as a novel species (Fictibacillus plastispherium sp. nov.) based on phenotypic, chemotaxonomic, and genomic analyses. The metagenomic analysis of Sundarbans bacterioplankton community further revealed the 748.2 hits per billion nucleotides associated with plastic-degrading enzymes targeting 17 polymer types (72.9% synthetic and 27.1% natural), suggesting the interaction of plastic-carbon with the microbial carbon turnover. Laboratory-scale mesocosm experiment showed that the addition of MPs affected dissolved nutrient concentrations of surrounding water, while also supporting distinct microbial assemblages. These findings demonstrate that MPs do not behave simply as passive pollutants in mangrove estuaries but actively interact with microbial communities and biogeochemical processes. This highlights MPs as an overlooked yet potential driver of carbon dynamics, with the capacity to shape regional carbon budget of the Bay of Bengal, in the Northern Indian Ocean.

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