Abstract
When jellyfish blooms decay, sinking jellyfish detrital organic matter (jelly-OM), rich in proteins and characterized by a low C:N ratio, becomes a significant source of OM for marine microorganisms. Yet, the key players and the process of microbial jelly-OM degradation and the consequences for marine ecosystems remain unclear. We simulated the scenario potentially experienced by the coastal pelagic microbiome after the decay of a bloom of the cosmopolitan Aurelia aurita s.l. We show that about half of the jelly-OM is instantly available as dissolved organic matter and thus, exclusively and readily accessible to microbes. During a typical decay of an A. aurita bloom in the northern Adriatic Sea about 100 mg of jelly-OM L–1 becomes available, about 44 μmol L–1 as dissolved organic carbon (DOC), 13 μmol L–1 as total dissolved nitrogen, 11 μmol L–1 of total hydrolyzable dissolved amino acids (THDAA) and 0.6 μmol L–1 PO43–. The labile jelly-OM was degraded within 1.5 days (>98% of proteins, ∼70% of THDAA, 97% of dissolved free amino acids and the entire jelly-DOC pool) by a consortium of Pseudoalteromonas, Alteromonas, and Vibrio. These bacteria accounted for >90% of all metabolically active jelly-OM degraders, exhibiting high bacterial growth efficiencies. This implies that a major fraction of the detrital jelly-OM is rapidly incorporated into biomass by opportunistic bacteria. Microbial processing of jelly-OM resulted in the accumulation of tryptophan, dissolved combined amino acids and inorganic nutrients, with possible implications for biogeochemical cycles.
Highlights
The complex pool of dissolved organic matter (DOM) in the oceans is one of the largest global reservoirs of carbon in the biosphere (662 Pg, Hansell et al, 2009)
Our approach ensured that we obtained a representative subsample of a decaying jellyfish biomass, which potentially becomes available to the ambient microbial community in the northern Adriatic Sea, where A. aurita regularly forms blooms in the spring – summer period (Kogovšek et al, 2010)
By mapping the reads back to the metagenomic assembled genomes (MAGs), we found that the relative abundance of some of the 84 MAGs increased in the jelly-OM treatment, mainly due to MAGs affiliated with Gammaproteobacteria (Supplementary Figure S5)
Summary
The complex pool of dissolved organic matter (DOM) in the oceans is one of the largest global reservoirs of carbon in the biosphere (662 Pg, Hansell et al, 2009). Jellyfish blooms are often shortlived and collapse abruptly due to various largely unexplored environmental factors (Pitt et al, 2014; Schnedler-Meyer et al, 2018; Goldstein and Steiner, 2020) With their “boom and bust” population dynamics, jellyfish can represent an important perturbation to the surrounding ecosystem. Despite of the debate over the reported global increase of their blooms and on the actual cause of the observed jellyfish fluctuations, the increase in their population size can have serious socio-economic and ecological consequences (Richardson et al, 2009; Condon et al, 2012; Purcell, 2012; Sanz-Martín et al, 2016) It is undisputed, that jellyfish represent a significant source of OM, especially in coastal ecosystems where large blooms are regularly reported (Kogovšek et al, 2010; Lucas et al, 2014)
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