Heatwave hit phase shifted coral reefs: Zoantharian mass mortality record.

Phase shift is characterized by an abrupt change in the structure of a community in response to a disturbance that can break its resistance, displacing it from its natural variation. This phenomenon has been recognized in several ecosystems and often points to human activities as the main cause. However, reactions of shifted communities to anthropogenic impacts have been less studied. In recent decades, heatwaves resulting from climate change have strongly affected coral reefs. Mass coral bleaching events are recognized as the main cause of coral reef phase shifts on a global scale. In 2019, an unprecedented heatwave hit the southwest Atlantic Ocean causing mass coral bleaching in non-degraded and phase-shifted reefs of Todos os Santos Bay, at an intensity never recorded in a 34-year historical series. We analyzed the effects of this event on the resistance of phase-shifted reefs, dominated by the zoantharian Palythoa cf. variabilis. Using benthic coverage data from 2003, 2007, 2011, 2017, and 2019, we analyzed three non-degraded reefs and three phase-shifted reefs. We estimated the coverage and bleaching of corals and P. cf. variabilis on each reef. There was a reduction in coral coverage in non-degraded reefs before the 2019 mass bleaching event (i.e., heatwave). However, there was no significant coral coverage variation after the event and the structure of non-degraded reef communities did not change. In phase-shifted reefs the coverage of zoantharians did not change significantly before the 2019 event, however, after the mass bleaching, there was a significant reduction in the coverage of these organisms. Here we revealed that the resistance of the shifted community was broken, and its structure was altered, indicating that reefs in this condition were more susceptible to bleaching disturbance than non-degraded reefs.

Mangrove microbial community recovery and their role in early stages of forest recolonization within shrimp ponds.

Shrimp farming is blooming worldwide, posing a severe threat to mangroves and its multiple goods and ecosystem services. Several studies reported the impacts of aquaculture on mangrove biotic communities, including microbiomes. However, little is known about how mangrove soil microbiomes would change in response to mangrove forest recolonization. Using genome-resolved metagenomics, we compared the soil microbiome of mangrove forests (both with and without the direct influence of shrimp farming effluents) with active shrimp farms and mangroves under a recolonization process. We found that the structure and composition of active shrimp farms microbial communities differ from the control mangrove forests, mangroves under the impact of the shrimp farming effluents, and mangroves under recolonization. Shrimp farming ponds microbiomes have lower microbial diversity and are dominated by halophilic microorganisms, presenting high abundance of multiple antibiotic resistance genes. On the other hand, control mangrove forests, impacted mangroves (exposed to the shrimp farming effluents), and recolonization ponds were more diverse, with a higher abundance of genes related to carbon mobilization. Our data also indicated that the microbiome is recovering in the mangrove recolonization ponds, performing vital metabolic functions and functionally resembling microbiomes found in those soils of neighboring control mangrove forests. Despite highlighting the damage caused by the habitat changes in mangrove soil microbiome community and functioning, our study sheds light on these systems incredible recovery capacity. Our study shows the importance of natural mangrove forest recovery, enhancing ecosystem services by the soil microbial communities even in a very early development stage of mangrove forest, thus encouraging mangrove conservation and restoration efforts worldwide.

The flourishing and vulnerabilities of zoantharians on Southwestern Atlantic reefs.

In the Southwestern Atlantic reefs (SWA), some species of massive scleractinians and zoantharians are adapted to turbid waters, periodic desiccation, and sediment resuspension events. Moreover, phase shifts in this region have mostly been characterized by the emergence of algae and, less typically, zoantharians. However, nutrient excess and organic pollution are key drivers of the hard coral habitat degradation and may, thus, favor the emergence of novel zoantharian-dominated habitats. Many zoantharian species, particularly those from the genera Palythoa and Zoanthus, have traits that could help them thrive under conditions detrimental to reef-building corals, including rapid growth, several asexual reproduction strategies, high morphological plasticity, and generalist nutrient acquisition strategies. Thus, in a near future, stress-tolerant zoantharians may thrive in nutrient-enriched subtidal SWA locations under low heat stress, such as, upwelling. Overall, coral-zoantharian phase shifts in the SWA may decrease the species richness of reef communities, ultimately influencing ecosystem functioning and services, such as the provision of nursery habitats, fish biomass production, and coastline protection. However, zoantharians will also be threatened at intertidal zones, which are expected to experience higher heat stress, solar radiation, and sea-level rise. Although zoantharians appear to cope well with some local stressors (e.g., decreasing water quality), they are vulnerable to climate change (e.g., heatwaves), invasive species (Tubastraea spp.), microplastics, diseases, and mostly restricted to a narrow depth range (0-15 m depth) in SWA reefs. This shallow zone is particularly affected by climate change, compressing the three-dimensional habitat and limiting depth refugia in deeper SWA reefs. As mesophotic ecosystems have been hypothesized as short-term refuges to disturbances for some species, the narrow depth limit of zoantharians seems to be a potential factor that might increase their vulnerability to growing climate change pressures in SWA shallow-water reefs. Together, these could lead to both range expansions in some locations and loss of suitable reef habitats in other sites. Additional research is needed to better understand the systemic responses of these novel SWA reefs to the concert of increasing and interactive local and global stressors, and their implications for ecosystem functioning and service provisions.

The role of competition in the phase shift to dominance of the zoanthid Palythoa cf. variabilis on coral reefs.

Phase shift phenomena are becoming increasingly common. However, they are also opportunities to better understand how communities are structured. In Southwest Atlantic coral reefs, a shift to the zoanthid Palythoa cf. variabilis dominance has been described. To test if competition drove this process, we carried out a manipulative experiment with three coral species. To estimate the natural frequency of encounters we assess the relationship between the proportion of encounters and this zoanthids coverage. The contact causes necrosis in 78% of coral colonies (6.47 ± SD 7.92 cm(2)) in 118 days. We found a logarithmic relationship between the proportion of these encounters and the cover of P. cf. variabilis, where 5.5% coverage of this zoanthid is enough to put 50% of coral colonies in contact, increasing their partial mortality. We demonstrate that zoanthid coverage increase followed by coral mortality increase will reduce coral cover and that competition drives the phase shift process.