Tolerance of sponge assemblages to temperature anomalies: resilience and proliferation of sponges following the 1997-8 El-Niño southern oscillation.

Coral reefs across the world are under threat from a range of stressors, and while there has been considerable focus on the impacts of these stressors on corals, far less is known about their effect on other reef organisms. The 1997-8 El-Niño Southern Oscillation (ENSO) had notable and severe impacts on coral reefs worldwide, but not all reef organisms were negatively impacted by this large-scale event. Here we describe how the sponge fauna at Bahia, Brazil was influenced by the 1997-8 ENSO event. Sponge assemblages from three contrasting reef habitats (reef tops, walls and shallow banks) at four sites were assessed annually from 1995 to 2011. The within-habitat sponge diversity did not vary significantly across the study period; however, there was a significant increase in density in all habitats. Multivariate analyses revealed no significant difference in sponge assemblage composition (ANOSIM) between pre- and post-ENSO years for any of the habitats, suggesting that neither the 1997-8 nor any subsequent smaller ENSO events have had any measurable impact on the reef sponge assemblage. Importantly, this is in marked contrast to the results previously reported for a suite of other taxa (including corals, echinoderms, bryozoans, and ascidians), which all suffered mass mortalities as a result of the ENSO event. Our results suggest that of all reef taxa, sponges have the potential to be resilient to large-scale thermal stress events and we hypothesize that sponges might be less affected by projected increases in sea surface temperature compared to other major groups of reef organisms

Natural photosynthetic microboring communities produce alkalinity in seawater whereas aragonite saturation state rises up to five

Bioerosion, resulting from microbioerosion or biogenic dissolution, macrobioerosion and grazing, is one the main processes involved in reef carbonate budget and functioning. On healthy reefs, most of the produced carbonates are preserved and accumulate. But in the context of global change, reefs are increasingly degraded as environmental factors such as ocean warming and acidification affect negatively reef accretion and positively bioerosion processes. The recent 2019 SROCC report suggests that if CO2 emissions in the atmosphere are not drastically reduced rapidly, 70%-99% of coral reefs will disappear by 2,100. However, to improve projections of coral reef evolution, it is important to better understand dynamics of bioerosion processes. Among those processes, it was shown recently that bioeroding microflora which actively colonize and dissolve experimental coral blocks, release significant amount of alkalinity in seawater both by day and at night under controlled conditions. It was also shown that this alkalinity production is enhanced under ocean acidification conditions (saturation state of aragonite comprised between 2 and 3.5) suggesting that reef carbonate accumulation will be even more limited in the future. To better understand the conditions of production of alkalinity in seawater by boring microflora and its possible consequences on reef resilience, we conducted a series of experiments with natural rubble maintained under natural or artificial light, and various saturation states of aragonite. We show here that biogenic dissolution of natural reef rubble colonized by microboring communities dominated by the chlorophyte Ostreobium sp., and thus the production of alkalinity in seawater, can occur under a large range of saturation states of aragonite, from 2 to 6.4 under daylight and that this production is directly correlated to the photosynthetic activity of microboring communities. We then discuss the possible implications of such paradoxical activities on reef resilience

Carbonate dissolution by reef microbial borers : a biogeological process producing alkalinity under different pCO(2) conditions

Rising atmospheric CO2 is acidifying the world's oceans, affecting both calcification and dissolution processes in coral reefs. Among processes, carbonate dissolution by bioeroding microflora has been overlooked, and especially its impact on seawater alkalinity. To date, this biogeological process has only been studied using microscopy or buoyant weight techniques. To better understand its possible effect on seawater alkalinity, and thus on reef carbonate budget, an experiment was conducted under various seawater chemistry conditions (2(arag)3.5 corresponding to 440pCO(2) (mu atm)940) at 25 degrees C under night and daylight (200 mu mol photons m(-2)s(-1)) with natural microboring communities colonizing dead coral blocks (New Caledonia). Both the alkalinity anomaly technique and microscopy methods were used to study the activity of those communities dominated by the chlorophyte Ostreobium sp. Results show that (1) the amount of alkalinity released in seawater by such communities is significant and varies between 12.8 +/- 0.7 at (Arag)similar to 2 and 5.6 +/- 0.4mmol CaCO3 m(-2) day(-1) at (Arag)similar to 3-3.5 considering a 12:12 photoperiod; (2) although dissolution is higher at night (similar to 80 vs. 20% during daylight), the process can occur under significant photosynthetic activity; and (3) the process is greatly stimulated when an acidity threshold is reached (pCO(2)920 mu atm vs. current conditions at constant light intensity). We show that carbonate dissolution by microborers is a major biogeochemical process that could dissolve a large part of the carbonates deposited by calcifying organisms under ocean acidification

The Seychelles

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