Jellyfish blooms have increased in coastal areas around the world and the outbreaks have become longer and more frequent over the past few decades. The Mediterranean Sea is among the heavily affected regions and the common bloom - forming taxa are scyphozoans Aurelia aurita s.l., Pelagia noctiluca , and Rhizostoma pulmo . Jellyfish have few natural predators, therefore their carcasses at the termination of a bloom represent an organic-rich substrate that supports rapid bacterial growth, and may have a large impact on the surrounding environment. The focus of this study was to explore whether jellyfish substrate have an impact on bacterial community phylotype selection. We conducted in situ jellyfish - enrichment experiment with three different jellyfish species. Bacterial dynamic together with nutrients were monitored to assess decaying jellyfish-bacteria dynamics. Our results show that jellyfish biomass is characterized by protein rich organic matter, which is highly bioavailable to ‘jellyfish - associated’ and ‘free - living’ bacteria, and triggers rapid shifts in bacterial population dynamics and composition. Based on 16S rRNA clone libraries and denaturing gradient gel electrophoresis (DGGE) analysis, we observed a rapid shift in community composition from unculturable Alphaproteobacteria to culturable species of Gammaproteobacteria and Flavobacteria . The results of sequence analyses of bacterial isolates and of total bacterial community determined by culture independent genetic analysis showed the dominance of the Pseudoalteromonadaceae and the Vibrionaceae families. Elevated levels of dissolved proteins, dissolved organic and inorganic nutrient release, bacterial abundance and carbon production as well as ammonium concentrations characterized the degradation process. The biochemical composition of jellyfish species may influence changes in the amount of accumulated dissolved organic and inorganic nutrients. Our results can contribute insights into possible changes in bacterial population dynamics and nutrient pathways following jellyfish blooms which have important implications for ecology of coastal waters.

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Four meroplanktonic scyphomedusae, Aurelia aurita, Chrysaora hysoscella, Cotylorhiza tuberculata and Rhizostoma pulmo, and the holoplanktonic, non-resident Pelagia noctiluca have formed blooms in the northern Adriatic over the last 200 years. Published data about the historical occurrences of these five scyphomedusae, in combination with our data, were used to analyse their long-term fluctuations in this northernmost part of the Mediterranean Sea. Analysis of the most recent blooms was complemented with environmental descriptors (temperature, salinity, pH, chlorophyll a, zooplankton dry weight and major river discharges). Continuous wavelet transformation analysis of the historical time series of scyphomedusae occurrences and environmental parameters revealed that the five species have been present regularly in the northern Adriatic over the last 200 years, with two major periods of jellyfish proliferations. The first period in the years around 1910 was characterised by significant periodicity of 8-12 years for each species, while the second period from the 1960s onwards was characterised by a shortened significant periodicity of less than 8 years.Pelagia noctiluca fluctuations were analysed in greater detail for the last four decades, revealing significant periodicities of *10 years, 2.5 years, 8-14 months, and 8 months. The significantly marked periodicity of about 10 years in the P. noctiluca spectrum indicates a pattern similar to that observed in the western Mediterranean. Wavelet analysis showed that the periodicity of occurrence of five jellyfish species has shortened in recent decades and the recurrence of blooms has increased, particularly for A. aurita and R. pulmo.

Despite accumulating evidence of the importance of the jellyfish-associated microbiome to jellyfish, its potential relevance to blue biotechnology has only recently been recognized. In this review, we emphasize the biotechnological potential of host–microorganism systems and focus on gelatinous zooplankton as a host for the microbiome with biotechnological potential. The basic characteristics of jellyfish-associated microbial communities, the mechanisms underlying the jellyfish-microbe relationship, and the role/function of the jellyfish-associated microbiome and its biotechnological potential are reviewed. It appears that the jellyfish-associated microbiome is discrete from the microbial community in the ambient seawater, exhibiting a certain degree of specialization with some preferences for specific jellyfish taxa and for specific jellyfish populations, life stages, and body parts. In addition, different sampling approaches and methodologies to study the phylogenetic diversity of the jellyfish-associated microbiome are described and discussed. Finally, some general conclusions are drawn from the existing literature and future research directions are highlighted on the jellyfish-associated microbiome.

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