Jellyfish modulate bacterial dynamic and community structure

Abstract

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.

Figure 1. Response of bacterial abundance (A) and production (B) to different jellyfish substrate additions during the enrichment experiment.

A - Aurelia, P - Pelagia, R - Rhizostoma, C - control.

Figure 2. Changes in dissolved protein and ammonium (NH₄ ⁺) concentrations during the enrichment experiment.

(A) treatment with Aurelia, (B) treatment with Pelagia, (C) treatment with Rhizostoma; ▪ dissolved protein concentration in jellyfish treatment □ dissolved protein concentration in control; • NH₄ ⁺ concentration in jellyfish treatment ○ NH₄ ⁺ concentration in control.

Figure 3. DGGE similarity dendrogram from all sampling time points during the enrichment experiment.

Samples collected at the beginning of the experiment (T0), on days 1, 2, 3, 6 and 9 in the control (C1, C2, C6, C9), on days 3, 6 and 9 from the treatment with Aurelia (A3, A6, A9), treatment with Pelagia (P1, P3, P6, P9) and treatment with Rhizostoma (R2, R3, R6). The similarity dendrogram was constructed from DGGE banding patterns using the Bray - Curtis coefficient; clustering was performed using the UPGMA method. I - cluster I, II - cluster II, III - cluster III.

Figure 4. The distribution of phyla in each 16S rRNA library (% of clones) from different treatments.

DNA extraction from the sample at the beginning of the experiment (T0), on day 9 from the control (C9), on day 9 from the treatment with Aurelia (A9) and treatment with Pelagia (P9), and on day 6 from the treatment with Rhizostoma (R6).