Antibacterial and antibiotic potentiating activities of tropical marine sponge extracts

Increasing prevalence of antibiotic resistance has led research to focus on discovering new antimicrobial agents derived from the marine biome. Although ample studies have investigated sponges for their bioactive metabolites with promising prospects in drug discovery, the potentiating effects of sponge extracts on antibiotics still remains to be expounded. The present study aimed to investigate the antibacterial capacity of seven tropical sponges collected from Mauritian waters and their modulatory effect in association with three conventional antibiotics namely chloramphenicol, ampicillin and tetracycline. Disc diffusion assay was used to determine the inhibition zone diameter (IZD) of the sponge total crude extracts (CE), hexane (HF), ethyl acetate (EAF) and aqueous (AF) fractions against nine standard bacterial isolates whereas broth microdilution method was used to determine their minimum inhibitory concentrations (MICs), minimum bactericidal concentrations (MBCs) and antibiotic potentiating activity of the most active sponge extract. MIC values of the sponge extracts ranged from 0.039 to 1.25 mg/mL. Extracts from Neopetrosia exigua rich in beta-sitosterol and cholesterol displayed the widest activity spectrum against the 9 tested bacterial isolates whilst the best antibacterial profile was observed by its EAF particularly against Staphylococcus aureus and Bacillus cereus with MIC and MBC values of 0.039 mg/mL and 0.078 mg/mL, respectively. The greatest antibiotic potentiating effect was obtained with the EAF of N. exigua (MIC/2) and ampicillin combination against S. aureus. These findings suggest that the antibacterial properties of the tested marine sponge extracts may provide an alternative and complementary strategy to manage bacterial infections

Prophylactic antioxidants and phenolics of seagrass and seaweed species: A seasonal variation study in a Southern Indian Ocean Island, Mauritius

The seasonal variations in the polyphenolic contents and potential antioxidant activities of seven seaweed species (Padina gymnospora, Gracilaria salicornia, Palisada papillosa, Galaxaura rugosa, Enteromorpha intestinalis, Codium arabicum and Dictyosphaeria cavernosa) and five seagrass species (Syringodium isoetifolium, Halodule uninervis, Thalassodendron ciliatum, Halophila ovalis and Halophila stipulacea) were assessed. In summer, the highest total phenolic content was recorded in the seaweed P. gymnospora and the lowest in G. rugosa. The total phenolic contents in the seagrass species were significantly higher than those observed in the seaweed species during both seasons. The highest flavonoid concentrations (FC) were observed in the seaweed species E. intestinalis in winter and in the seaweed P. gymnospora in summer. All tested species had higher FC in winter. The highest antioxidant activity (assessed using the Ferric ion reducing antioxidant power) was in the seaweed P. gymnospora during summer (FRAP: 9.7 ± 0.3 X10¯³ Fe²+mM/g DW). However, the seaweed P. gymnospora extract and the extracts from all 5 seagrass species had significantly different (p<0.01) antioxidant activities (assessed using the Trolox equivalent antioxidant capacity assay) in winter compared to the summer season. The collective data are indicative of the potential of Mauritian seaweeds and seagrasses as possible sources of secondary metabolites for pharmaceuticals. Further analysis using bio-efficacy models are warranted to justify the phytoceutical capacity of the seaweeds and seagrasses.Keywords: Antioxidant; Total Phenols; Total Flavonoids; FRAP; TEAC; Seagrass; Seaweed; Seasonal variation; Mauritiu

Effects of territorial damselfish on corallivorous fish assemblage composition and coral predation in the Mauritian lagoon

Predation is a significant stressor for many coral species. Understanding how predation interacts with factors such as climate change is key to coral conservation. Territorial damselfish of the genus Stegastes form territories on branching corals (genus Acropora) on which they cultivate algae, and while defending these territories, provide corals with indirect protection from predation. However, it is not known how the protection afforded by Stegastes spp. varies with corallivorous fish assemblage composition, nor whether corallivore assemblages themselves may be affected by the presence of Stegastes spp. and their associated Acropora spp. habitats. This study examined relationships among predation protection by Stegastes spp., branching Acropora coral cover, and the number and species richness of corallivorous fish present within a given area, namely the Mauritian lagoon in the western Indian Ocean. Predation on bleaching-resilient massive Porites lutea corals within and outside Stegastes territories was surveyed at sites around Mauritius island. Corallivorous fish assemblages, branching coral cover, and Stegastes spp. density were also surveyed at each site visit. Results show that high predation was correlated with the presence of high numbers of corallivores, but predation was lower within Stegastes spp. territories irrespective of all observed corallivore densities. Greater numbers of Stegastes spp. were correlated with increasing density of obligate corallivores. Non-Acroporid coral cover was positively correlated with species richness of obligate corallivores, while branching Acropora coral cover was negatively correlated with overall corallivore density. This study shows for the first time that predation on bleaching-resilient massive corals is lower within Stegastes spp. territories regardless of the number of corallivores present, adding to the growing body of knowledge on the complex relationships between Stegastes spp. and their environment. The findings also indicate possible effects of the presence of Stegastes spp. and their branching Acropora habitats on corallivorous fish density in areas with low coral diversity, which may warrant further study

The Roles and Interactions of Symbiont, Host and Environment in Defining Coral Fitness

Background: Reef-building corals live in symbiosis with a diverse range of dinoflagellate algae (genus Symbiodinium) that differentially influence the fitness of the coral holobiont. The comparative role of symbiont type in holobiont fitness in relation to host genotype or the environment, however, is largely unknown. We addressed this knowledge gap by manipulating host-symbiont combinations and comparing growth, survival and thermal tolerance among the resultant holobionts in different environments.\ud Methodology/Principal Findings: Offspring of the coral, Acropora millepora, from two thermally contrasting locations, were experimentally infected with one of six Symbiodinium types, which spanned three phylogenetic clades (A, C and D), and then outplanted to the two parental field locations (central and southern inshore Great Barrier Reef, Australia). Growth and survival of juvenile corals were monitored for 31–35 weeks, after which their thermo-tolerance was experimentally assessed. Our results showed that: (1) Symbiodinium type was the most important predictor of holobiont fitness, as measured by growth, survival, and thermo-tolerance; (2) growth and survival, but not heat-tolerance, were also affected by local environmental conditions; and (3) host population had little to no effect on holobiont fitness. Furthermore, coral-algal associations were established with symbiont types belonging to clades A, C and D, but three out of four symbiont types belonging to clade C failed to establish a symbiosis. Associations with clade A had the lowest fitness and were unstable in the field. Lastly, Symbiodinium types C1 and D were found to be relatively thermo-tolerant, with type D conferring the highest tolerance in A. millepora.\ud Conclusions/Significance: These results highlight the complex interactions that occur between the coral host, the algal symbiont, and the environment to shape the fitness of the coral holobiont. An improved understanding of the factors affecting coral holobiont fitness will assist in predicting the responses of corals to global climate change

Effects of Light, Food Availability and Temperature Stress on the Function of Photosystem II and Photosystem I of Coral Symbionts

Background: Reef corals are heterotrophic coelenterates that achieve high productivity through their photosynthetic dinoflagellate symbionts. Excessive seawater temperature destabilises this symbiosis and causes corals to "bleach," lowering their photosynthetic capacity. Bleaching poses a serious threat to the persistence of coral reefs on a global scale. Despite expanding research on the causes of bleaching, the mechanisms remain a subject of debate.\ud \ud Methodology/Principal Findings: This study determined how light and food availability modulate the effects of temperature stress on photosynthesis in two reef coral species. We quantified the activities of Photosystem II, Photosystem I and whole chain electron transport under combinations of normal and stressful growth temperatures, moderate and high light levels and the presence or absence of feeding of the coral hosts. Our results show that PS1 function is comparatively robust against temperature stress in both species, whereas PS2 and whole chain electron transport are susceptible to temperature stress. In the symbiotic dinoflagellates of Stylophora pistillata the contents of chlorophyll and major photosynthetic complexes were primarily affected by food availability. In Turbinaria reniformis growth temperature was the dominant influence on the contents of the photosynthetic complexes. In both species feeding the host significantly protected photosynthetic function from high temperature stress.\ud \ud Conclusions/Significance: Our findings support the photoinhibition model of coral bleaching and demonstrate that PS1 is not a major site for thermal damage during bleaching events. Feeding mitigates bleaching in two scleractinian corals, so that reef responses to temperature stresses will likely be influenced by the coinciding availabilities of prey for the host

Development of Gene Expression Markers of Acute Heat-Light Stress in Reef-Building Corals of the Genus Porites

Coral reefs are declining worldwide due to increased incidence of climate-induced coral bleaching, which will have widespread biodiversity and economic impacts. A simple method to measure the sub-bleaching level of heat-light stress experienced by corals would greatly inform reef management practices by making it possible to assess the distribution of bleaching risks among individual reef sites. Gene expression analysis based on quantitative PCR (qPCR) can be used as a diagnostic tool to determine coral condition in situ. We evaluated the expression of 13 candidate genes during heat-light stress in a common Caribbean coral Porites astreoides, and observed strong and consistent changes in gene expression in two independent experiments. Furthermore, we found that the apparent return to baseline expression levels during a recovery phase was rapid, despite visible signs of colony bleaching. We show that the response to acute heat-light stress in P. astreoides can be monitored by measuring the difference in expression of only two genes: Hsp16 and actin. We demonstrate that this assay discriminates between corals sampled from two field sites experiencing different temperatures. We also show that the assay is applicable to an Indo-Pacific congener, P. lobata, and therefore could potentially be used to diagnose acute heat-light stress on coral reefs worldwide

Does the coral host protect its algal symbionts from heat\ud and light stresses?

This study investigated the potential protective role of the host by exposing coral larvae infected with two distinct clades of Symbiodinium (A and D) in two conditions, cultured and in hospite, to three combinations\ud of heat and light levels (control: 24 °C + 40 μmol photons m-2 s-1; high temperature and low light (HT+LL): 31 °C + 40 μmol photons m-2 s-1; high temperature and high light (HT+HL): 31 °C + 390 μmol photons m-2 s-1). HT+HL treatments decreased maximum quantum yield of photosystem II, Fv/Fm, in both cultured and in hospite zooxanthellae, however, the effect was more pronounced in cultured samples. HT+LL led to a decrease in maximum relative photosynthetic electron transport rate (rETRmax) only in cultured algae, while high light reduced rETRmax in both Symbiodinium growth conditions, with rETRmax approaching zero in cultured Symbiodinium. When Symbiodinium cells in hospite were exposed to HT+HL treatments, non-photochemical quenching increased; however for cultured Symbiodinium it declined drastically indicating that this particular form of photoprotection was depleted. These results suggest that in hospite Symbiodinium sustain less photophysiological\ud damage from HT+HL treatments in comparison to their respective free-living counterparts suggesting that the coral host provides some degree of protection to Symbiodinium from thermal and light stresses

Prediction of recurrences of mass coral bleaching / mortality and vulnerability of reef-building corals to climate change in Mauritian and Japanese waters

The bleaching / mortality response and prediction of recurrences of mass coral mortality in Japanese and Mauritian reef-building corals caused by climate change have yet to be explored thoroughly and incorporated into appropriate conservation and adaptive management strategies. We model likely “extinction dates” of live reefs from Okinawa and Ishigaki Islands of Japan, and one from Mauritius using historical and predicted surface seawater temperature (SST) data. Modelling of forecast SSTs at the two Japanese sites, where differential bleaching / mortalities occurred in 1998, revealed extinction dates of yrs 2000 - 2010 and 2070 - 2080 for Okinawan and Ishigaki reefs, respectively. On the other hand coral reefs of Mauritius escaped the severe warming in 1998, most probably due to the massive cloud cover during the critical month. Based on the warmest temperature being about 28.7 that year (Hadisst data) (which did not cause mass bleaching) similar recurrence would occur about 2025

Spatio-temporal variation in density of microphytoplankton genera in two tropical coral reefs of Mauritius

Spatial and temporal (seasonal and interannual) variation in microphytoplankton was investigated at two tropical coral reef sites off Mauritius (Belle Mare [BM] and Flic-en-Flac [FEF]), from 2010 to 2012. Each site was divided into three zones: coast, lagoon and reef. Microphytoplankton, nutrient and chlorophyll a samples were collected, and physico-chemical parameters were measured in situ. A total of 56 microphytoplankton genera were identified (43 diatoms, 8 dinoflagellates and 5 cyanobacteria). Total microphytoplankton density exhibited spatial and temporal variation, being higher at BM and near the coast, and mostly high in summer. Nutrient concentrations were highest near the coast as a result of run-off events and anthropogenic inputs, especially during heavy rainfall, and this resulted in elevated total microphytoplankton density. There were no clear diversity patterns at the two sites or within the different zones. At both sites, Navicula, Cocconeis and Fragilaria were the dominant diatoms, and Gymnodinium, Alexandrium and Protoperidinium were the dominant dinoflagellates. There was a shift in dominance from Navicula in summer to Fragilaria in winter. This seasonal shift may reflect how these species respond differently to environmental change, which could have implications for primary production at spatial and temporal scales.Keywords: chlorophyll a, cyanobacteria, diatoms, dinoflagellates, interannual variation, physico-chemical, seasonal variationAfrican Journal of Marine Science 2014, 36(4): 423–43

Differential effects of thermal and chemical stressors on tissue balls from scleractinian corals

Coral cell aggregates (tissue balls) from four species (Acropora muricata, Fungia repanda, Pavona cactus and Pocillopora damicornis) were used as an indicator to investigate the effects on the corals of thermal stress and of chemical extracts from three sponges (Adocia sp., Haliclona sp. and Lissodendoryx sp.) and one ascidian (Didemnum molle). The formation and disintegration of tissue balls were studied through exposure to a temperature range of 23–30 °C at time intervals of 0–90 min, and to sponge and ascidian crude extracts at concentrations of 50–200 μg ml–1 at temperatures of 23 and 30 °C and at time intervals of 10, 60 and 120 min. The negative effect of temperature on overall tissue ball density (number per cm2 of coral surface) was greatest at higher temperatures (28 and 30 °C) but varied among coral species. Tissue balls of P. damicornis were the most robust whereas those of A. muricata were the most sensitive. High concentrations of extracts of Adocia sp., Haliclona sp. and Lissodendoryx sp. generally inhibited the formation of tissue balls or caused their disintegration, or both, most markedly at 30 °C. Adocia sp. induced the least negative effects and Haliclona sp. the most. No tissue balls were formed in the presence of D. molle extracts (50 and 100 μg ml–1), indicating a high level of interference with tissue ball formation. The differential susceptibility to thermal and chemical stressors exhibited by the corals under study have possible implications for the interactions of the corals with other sedentary reef organisms under climate change-driven ocean warming.Keywords: Ascidia, chemical ecology, coral cell aggregates, sponges, thermal stress, zooxanthellaeAfrican Journal of Marine Science 2014, 36(4): 439–44