Microbial degradation of jellyfish detritus promotes phytoplankton growth in coastal marine ecosystems

Gelatinous zooplankton, hereinafter cnidarian subphylum Medusozoa and phylum Ctenophora or ‘jellyfish’, are widespread in marine ecosystems, and some species can form massive blooms under favorable environmental conditions. As these blooms decay, they release large amounts of labile, protein-rich organic matter (jelly-OM) into the water column, providing readily available substrate for microbial communities. To investigate the microbial response to jelly-OM, we conducted a two-stage microcosm experiment simulating conditions during a bloom of the invasive ctenophore Mnemiopsis leidyi in a coastal pelagic environment. In the first stage of our experiment, jelly-OM stimulated rapid growth of opportunistic bacteria, particularly Pseudoalteromonadaceae , known key degraders of jelly-OM, consistently dominating the microbial communities driving jelly-OM degradation across experiments, regardless of jellyfish species. These bacteria both respired and assimilated jelly-derived carbon. The bacterial degradation of jelly-OM was linked to elevated activities of leucine aminopeptidase, lipase, chitinase, and alkaline phosphatase, leading to substantial ammonium accumulation. Metagenomic analysis revealed enhanced microbial metabolism of amino acids, lipids, and carbohydrates in jelly-OM treatments. Compared to decaying phytoplankton, jelly-OM triggers a distinct microbial response, reflecting its unique biochemical composition and highlighting its potential to alter coastal ecosystem structure and function. In the second stage of our microcosm experiment, a fresh microbial assemblage was exposed to 0.2 µm pre-filtered residues from bacterial jelly-OM degradation. After three days, primary production and phytoplankton biomass increased significantly, reaching levels comparable to seasonal phytoplankton peaks in the region. Growth of phytoplankton community, dominated by diatoms, was likely fueled by accumulated ammonium. Concurrently, the bacterial community shifted towards taxa typically associated with phytoplankton blooms, without significant enrichment in metabolic pathways compared to controls. Our in situ data suggests a potential coupling between jellyfish and phytoplankton via rapid bacterial degradation of jelly-OM. Thus, jellyfish blooms appear to be significant sources of both organic and inorganic nutrients, capable of triggering ecosystem shifts, such as enhanced primary production. As jellyfish are projected to thrive under future ocean conditions, our findings underscore the need to re-evaluate their role in biogeochemical cycles—particularly as overlooked drivers of phytoplankton dynamics.