Microbially Driven Organic Carbon Degradation and Nutrient Cycling during Macroalgal Decomposition

The release of labile organic carbon (OC) and nutrients
during
seasonal macroalgal blooms can undermine blue carbon sequestration
in coastal ecosystems. Although marine microorganisms mediate OC degradation
during macroalgal decay, the underlying mechanisms remain poorly defined.
This study employed an integrated multiomics approach (amplicon sequencing,
metagenomics, and metatranscriptomics) to investigate microbial regulation
of OC degradation and coupled nutrient cycling in coastal sediments
with and without decomposing Sargassaceae. Total carbon in sediments increased by over 33% in the Sargassaceae area. Microbial α-diversity in
the Sargassaceae area decreased significantly
(p < 0.05), while processes linked to OC degradation,
carbohydrate metabolism, nitrate (NO3–) reduction, inorganic phosphorus utilization, and sulfur metabolism
were significantly upregulated (p < 0.05). Accordingly,
gene expression and extracellular hydrolase activities targeting key
biopolymers (i.e., cellulose, hemicellulose, starch, and chitin) were
significantly upregulated (p < 0.05) in the area
with Sargassaceae. Metabolism reconstruction
of metagenome-assembled genomes identified Vibrio, Pseudoalteromonas, Alteromonas, and Exiguobacterium_A as primary OC degraders, with genomic capacities enriched in NO3– reduction and assimilatory sulfate reduction.
Key environmental driversincluding the C/N ratio, dissolved
organic carbon, total dissolved nitrogen (DON), and NO3–shaped microbial metabolic activities
during macroalgal decomposition. Our finding demonstrates that microbially
driven OC degradation is a pivotal process coupled with nutrients
cycling, advancing the mechanistic understanding of microbial carbon
processing and its biogeochemical linkages during macroalgal decomposition
in coastal ecosystems.