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

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

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

<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

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

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

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

Seed- versus transplant-based eelgrass (Zostera marina L.) restoration success in a temperate marine lake

Despite active seagrass restoration gaining traction as a tool to halt and reverse worldwide seagrass losses, overall success remains limited. Restoration strategies, through seeding or transplantation, face different environmental bottlenecks that limit success. Choosing the most appropriate strategy of the two for a specific location, however, is hampered by lack of direct practical comparisons between strategies within a single system. To investigate potential life stage dependent bottlenecks, we compared seed-based and transplant-based restoration of Zostera marina in the subtidal saltwater Lake Grevelingen. Our results demonstrate that seedling recruitment was negatively impacted by bioturbation from the lugworm Arenicola marina and sediment movement due to hydrodynamic exposure. Transplant-based restoration was clearly more successful butsurprisingly best predicted by leaf gluing by the ragworm Platynereis dumerilii. This previously undescribed interaction caused seagrass leaves to clump and reduce effective photosynthetic surface and leaf movement. We suggest that the observed behavior of these worms may result from a lack of foodweb interactions, illustrating the importance of trophic control for seagrass restoration. Thus, in addition to recognizing life stage dependent environmental bottlenecks for restoration strategy selection, seagrass restoration may also require the active recovery of their associated food webs.</p