Oceans and seas are the largest ecosystem on Earth and are constantly subjected to multiple natural and anthropogenic perturbations that are projected to increase in the future, with possible important implications for the global climate. The complex pool of marine dissolved organic matter (DOM) – one of the largest reservoirs of carbon in the biosphere – is almost exclusively accessible to diverse members of the marine microbial community carrying out different types of metabolisms to process the broad spectrum of compounds present in this oceanic DOM pool. Thus, microbes with their high metabolic activity and abundance have a major impact on the biogeochemical state of the ocean. Hence, to predict the response of the marine ecosystems to natural/anthropogenic perturbations, a mechanistic understanding on the relation between the organic matter field and the metabolic network operated by the microbial community is required.

One major perturbation to the marine ecosystem (and its services) is bloom-forming gelatinous marine zooplankton or ‘jellyfish’. Regardless the debate over the accuracy of their reported global increase and on the true cause of the observed fluctuations in abundance of jellyfish, the increase in their population size can have serious ecological and detrimental socio-economic consequences, especially in coastal marine ecosystems. The invasive ctenophore Mnemiopsis leiydi is one of those jellyfish species that is still spreading and increasing in population size, with large impact on the ecosystems it invades. Ever since its introduction to the commercially important northern Adriatic coastal zone in the year 2016, M. leiydi forms large-scale blooms. Yet, the implications of the introduction of this invasive ctenophore for this dynamic coastal marine ecosystem, remain unknown.

In particular, as its massive blooms start to decay, sinking ctenophore detritus represents a major perturbation and a largely overlooked, but significant source of DOM for ambient microbial communities. The link between jellyfish and microbes has been addressed by only a few studies thus far, demonstrating that jellyfish detritus is rapidly degraded by opportunistic, even potentially pathogenic, microbial phylotypes with possible implications for the marine carbon and nitrogen cycle, marine food web structure and human health and wellbeing. However, the exact processes and mechanisms of microbial degradation of the ctenophore detritus remain unclear and need to be investigated to understand the implications of invasive ctenophore blooms for the biogeochemical state of the invaded ecosystem.

We will apply an integrated interdisciplinary approach to tackle this problem – from the molecular to the ecosystem scale. We will characterize the ctenophore OM using state-of-the-art analytical tools and link it to the metabolic processes operated by the ctenophore-degrading microbial community using cutting edge -omics techniques combining the emerging fields of marine meta- and exo-proteomics with the metagenomic approach. The remineralization rates of specific ctenophore-OM compounds will be determined via biochemical characterization of key microbial enzymes. The implications of the microbially-mediated degradation of different bloom-forming gelatinous zooplankton detritus for the surrounding ecosystem will be evaluated. The relationships between all involved processes and players will be established using physical model and extended to the physical-biogeochemical model to perform case studies in realistic spatial and temporal context. Altogether, this knowledge will enable us understanding the implications of jellyfish blooms on marine biogeochemical cycles, to predict the response of marine ecosystems to this perturbation and allow us to search for mitigation measures for jellyfish blooms and its effects on coastal seas’ ecosystem services including human health and wellbeing.