Sponges from Zanzibar host diverse prokaryotic communities with potential for natural product synthesis

Sponges are one of the most dominant organisms in marine ecosystems. One reason for their success is their association with microorganisms that are besides the host itself responsible for the chemical defence. Sponge abundances have been increasing on coral reefs in the Western Indian Ocean (WIO) and are predicted to increase further with rising anthropogenic impacts on coral reefs. However, there is a paucity of information on chemical ecology of sponges from the WIO and their prokaryotic community composition. We used a combination of Illumina sequencing and a predictive metagenomic analysis to (i) assess the prokaryotic community composition of sponges from Zanzibar, (ii) predict the presence of KEGG metabolic pathways responsible for bioactive compound production and (iii) relate their presence to the degree of observed chemical defence in their respective sponge host. We found that sponges from Zanzibar host diverse prokaryotic communities that are host species-specific. Sponge-species and respective specimens that showed strong chemical defences in previous studies were also predicted to be highly enriched in various pathways responsible for secondary metabolite production. Hence, the combined sequencing and predictive metagenomic approach proved to be a useful indicator for the metabolic potential of sponge holobiont

Sponge chemical defenses are a possible mechanism for increasing sponge abundance on reefs in Zanzibar

<div><p>Coral reefs are experiencing increasing anthropogenic impacts that result in substantial declines of reef-building corals and a change of community structure towards other benthic invertebrates or macroalgae. Reefs around Zanzibar are exposed to untreated sewage and runoff from the main city Stonetown. At many of these sites, sponge cover has increased over the last years. Sponges are one of the top spatial competitors on reefs worldwide. Their success is, in part, dependent on their strong chemical defenses against predators, microbial attacks and other sessile benthic competitors. This is the first study that investigates the bioactive properties of sponge species in the Western Indian Ocean region. Crude extracts of the ten most dominant sponge species were assessed for their chemical defenses against 35 bacterial strains (nine known as marine pathogens) using disc diffusion assays and general cytotoxic activities were assessed with brine shrimp lethality assays. The three chemically most active sponge species were additionally tested for their allelopathic properties against the scleractinian coral competitor <i>Porites</i> sp.. The antimicrobial assays revealed that all tested sponge extracts had strong antimicrobial properties and that the majority (80%) of the tested sponges were equally defended against pathogenic and environmental bacterial strains. Additionally, seven out of ten sponge species exhibited cytotoxic activities in the brine shrimp assay. Moreover, we could also show that the three most bioactive sponge species were able to decrease the photosynthetic performance of the coral symbionts and thus were likely to impair the coral physiology.</p></div

Degree of antimicrobial activity by crude extracts of ten sponge species.

<p>Degree of antimicrobial activity by crude extracts of ten sponge species.</p

Length of inhibition zones (mean radius, mm + SE) for environmental and pathogenic bacterial strains.

<p>Bacterial inhibition by sponge crude extracts for environmental and pathogenic bacterial strains were compared. * indicates a significant difference between the inhibition of environmental vs. pathogenic bacterial strains (PERMANOVA, p < 0.05).</p

Description of the 35 bacterial strains used in the antimicrobial assay, including nine known pathogens for marine diseases percent similarity indicates how close the bacterial sequence of the isolate is to the closest strain in the NCBI BLAST databank.

<p>Description of the 35 bacterial strains used in the antimicrobial assay, including nine known pathogens for marine diseases percent similarity indicates how close the bacterial sequence of the isolate is to the closest strain in the NCBI BLAST databank.</p

The most abundant sponge species at Bawe Island, Zanzibar, their percent coverage and natural extract yield.

<p>The data for the benthic cover of the different sponge species were obtained by a previous study [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0197617#pone.0197617.ref076" target="_blank">76</a>].</p

<i>In situ</i> allelopathic effects of sponge extracts on the photosynthetic yield of a branching <i>Porites</i> coral.

<p>Phytagel strips containing natural concentrations of sponge secondary metabolites reduced the maximum photosynthetic quantum yield (bars) of the symbiotic algae (zooxanthellae) in a branching <i>Porites</i> coral after 16–18 h of exposure (mean + SE, n = 6, except <i>Callyspongia</i> sp., n = 4). Letters indicate significant differences between treatment, control strips and unexposed coral tissue (control coral).</p

Mortality rates (+ SE) of the brine shrimp larvae in the lethality assay.

<p>The mortality rates of the brine shrimp larvae are displayed in response to exposure to the different sponge crude extract concentrations at 1000μg ml^-1 and 100μg ml^-1 after 48 hours (mean + SE, n = 6). * indicates a significant mortality rate compared to control (PERMANOVA, p < 0.05).</p