Necrosis in a population ofPetrosia ficiformis(Porifera, Demospongiae) in relation with environmental stress

Mass mortality of the keratose sponge Sarcotragus foetidus in the Aegean Sea (Eastern Mediterranean) correlates with proliferation of Vibrio bacteria in the tissues

In the last two decades, episodes of mass mortality in benthic communities have often been associated with climatic anomalies, but the ultimate mechanisms through which they lead to death have rarely been identified. This study reports a mass mortality of wild sponges in the Aegean Sea (Turkey, Eastern Mediterranean), which affected the keratose demosponge Sarcotragus foetidus in September 2021. We examined the occurrence of thermo-dependent bacteria of the genus Vibrio in the sponges, identified through 16S rRNA of colonies isolated from sponge tissue in specific culturing media. Six Vibrio sequences were identified from the sponges, three of them being putatively pathogenic (V. fortis, V. owensii, V. gigantis). Importantly, those Vibrios were isolated from only tissues of diseased sponges. In contrast, healthy individuals sampled in both summer and winter led to no Vibrio growth in laboratory cultures. A 50 years record of sea surface temperature (SST) data for the study area reveals a progressive increase in temperature from 1970 to 2021, with values above 24°C from May to September 2021, reaching an absolute historical maximum of 28.9°C in August 2021. We hypothesize that such elevated SST values maintained for several months in 2021 promoted proliferation of pathogenic Vibrio species (thermo-dependent bacteria) in S. foetidus, triggering or aggravating the course of sponge disease. Thus, vibrioisis emerges as one of the putative mechanisms through which global water warming in the Mediterranean Sea translates into sponge mortality. The historical time course of temperature data for the studied area in the Aegean Sea predicts that recurrent waves of elevated SST are likely to occur in the coming summers. If so, recurrent disease may eventually eliminate this abundant sponge from the sublittoral in the midterm, altering the original bathymetric distribution of the species and compromising its ecological role.

The Porifera microeukaryome: Addressing the neglected associations between sponges and protists

While bacterial and archaeal communities of sponges are intensively studied, given their importance to the animal's physiology as well as sources of several new bioactive molecules, the potential and roles of associated protists remain poorly known. Historically, culture-dependent approaches dominated the investigations of sponge-protist interactions. With the advances in omics techniques, these associations could be visualized at other equally important scales. Of the few existing studies, there is a strong tendency to focus on interactions with photosynthesizing taxa such as dinoflagellates and diatoms, with fewer works dissecting the interactions with other less common groups. In addition, there are bottlenecks and inherent biases in using primer pairs and bioinformatics approaches in the most commonly used metabarcoding studies. Thus, this review addresses the issues underlying this association, using the term "microeukaryome" to refer exclusively to protists associated with an animal host. We aim to highlight the diversity and community composition of protists associated with sponges and place them on the same level as other microorganisms already well studied in this context. Among other shortcomings, it could be observed that the biotechnological potential of the microeukaryome is still largely unexplored, possibly being a valuable source of new pharmacological compounds, enzymes and metabolic processes.

Response diversity in Mediterranean coralligenous assemblages facing climate change: Insights from a multispecific thermotolerance experiment

Climate change threatens coastal benthic communities on a global scale. However, the potential effects of ongoing warming on mesophotic temperate reefs at the community level remain poorly understood. Investigating how different members of these communities will respond to the future expected environmental conditions is, therefore, key to anticipating their future trajectories and developing specific management and conservation strategies. Here, we examined the responses of some of the main components of the highly diverse Mediterranean coralligenous assemblages to thermal stress. We performed thermotolerance experiments with different temperature treatments (from 26 to 29°C) with 10 species from different phyla (three anthozoans, six sponges and one ascidian) and different structural roles. Overall, we observed species-specific contrasting responses to warming regardless of phyla or growth form. Moreover, the responses ranged from highly resistant species to sensitive species and were mostly in agreement with previous field observations from mass mortality events (MMEs) linked to Mediterranean marine heat waves. Our results unravel the diversity of responses to warming in coralligenous outcrops and suggest the presence of potential winners and losers in the face of climate change. Finally, this study highlights the importance of accounting for species-specific vulnerabilities and response diversity when forecasting the future trajectories of temperate benthic communities in a warming ocean.

Genetic diversity, connectivity and gene flow along the distribution of the emblematic Atlanto-Mediterranean sponge Petrosia ficiformis (Haplosclerida, Demospongiae)

BACKGROUND

Knowledge about the distribution of the genetic variation of marine species is fundamental to address species conservation and management strategies, especially in scenarios with mass mortalities. In the Mediterranean Sea, Petrosia ficiformis is one of the species most affected by temperature-related diseases. Our study aimed to assess its genetic structure, connectivity, and bottleneck signatures to understand its evolutionary history and to provide information to help design conservation strategies of sessile marine invertebrates.

RESULTS

We genotyped 280 individuals from 19 locations across the entire distribution range of P. ficiformis in the Atlanto-Mediterranean region at 10 microsatellite loci. High levels of inbreeding were detected in most locations (especially in the Macaronesia and the Western Mediterranean) and bottleneck signatures were only detected in Mediterranean populations, although not coinciding entirely with those with reported die-offs. We detected strong significant population differentiation, with the Atlantic populations being the most genetically isolated, and show that six clusters explained the genetic structure along the distribution range of this sponge. Although we detected a pattern of isolation by distance in P. ficiformis when all locations were analyzed together, stratified Mantel tests revealed that other factors could be playing a more prominent role than isolation by distance. Indeed, we detected a strong effect of oceanographic barriers impeding the gene flow among certain areas, the strongest one being the Almeria-Oran front, hampering gene flow between the Atlantic Ocean and the Mediterranean Sea. Finally, migration and genetic diversity distribution analyses suggest a Mediterranean origin for the species.

CONCLUSIONS

In our study Petrosia ficiformis showed extreme levels of inbreeding and population differentiation, which could all be linked to the poor swimming abilities of the larva. However, the observed moderate migration patterns are highly difficult to reconcile with such poor larval dispersal, and suggest that, although unlikely, dispersal may also be achieved in the gamete phase. Overall, because of the high genetic diversity in the Eastern Mediterranean and frequent mass mortalities in the Western Mediterranean, we suggest that conservation efforts should be carried out specifically in those areas of the Mediterranean to safeguard the genetic diversity of the species.

The response of a boreal deep-sea sponge holobiont to acute thermal stress

Effects of elevated seawater temperatures on deep-water benthos has been poorly studied, despite reports of increased seawater temperature (up to 4 °C over 24 hrs) coinciding with mass mortality events of the sponge Geodia barretti at Tisler Reef, Norway. While the mechanisms driving these mortality events are unclear, manipulative laboratory experiments were conducted to quantify the effects of elevated temperature (up to 5 °C, above ambient levels) on the ecophysiology (respiration rate, nutrient uptake, cellular integrity and sponge microbiome) of G. barretti. No visible signs of stress (tissue necrosis or discolouration) were evident across experimental treatments; however, significant interactive effects of time and treatment on respiration, nutrient production and cellular stress were detected. Respiration rates and nitrogen effluxes doubled in responses to elevated temperatures (11 °C & 12 °C) compared to control temperatures (7 °C). Cellular stress, as measured through lysosomal destabilisation, was 2-5 times higher at elevated temperatures than for control temperatures. However, the microbiome of G. barretti remained stable throughout the experiment, irrespective of temperature treatment. Mortality was not evident and respiration rates returned to pre-experimental levels during recovery. These results suggest other environmental processes, either alone or in combination with elevated temperature, contributed to the mortality of G. barretti at Tisler reef.

Till death do us part: stable sponge-bacteria associations under thermal and food shortage stresses

Sporadic mass mortality events of Mediterranean sponges following periods of anomalously high temperatures or longer than usual stratification of the seawater column (i.e. low food availability) suggest that these animals are sensitive to environmental stresses. The Mediterranean sponges Ircinia fasciculata and I. oros harbor distinct, species-specific bacterial communities that are highly stable over time and space but little is known about how anomalous environmental conditions affect the structure of the resident bacterial communities. Here, we monitored the bacterial communities in I. fasciculata (largely affected by mass mortalities) and I. oros (overall unaffected) maintained in aquaria during 3 weeks under 4 treatments that mimicked realistic stress pressures: control conditions (13°C, unfiltered seawater), low food availability (13°C, 0.1 µm-filtered seawater), elevated temperatures (25°C, unfiltered seawater), and a combination of the 2 stressors (25°C, 0.1 µm-filtered seawater). Bacterial community structure was assessed using terminal restriction fragment length polymorphism (T-RFLP) analysis of 16S rRNA gene sequences and transmission electron microscopy (TEM). As I. fasciculata harbors cyanobacteria, we also measured chlorophyll a (chl a) levels in this species. Multivariate analysis revealed no significant differences in bacterial T-RFLP profiles among treatments for either host sponge species, indicating no effect of high temperatures and food shortage on symbiont community structure. In I. fasciculata, chl a content did not significantly differ among treatments although TEM micrographs revealed some cyanobacteria cells undergoing degradation when exposed to both elevated temperature and food shortage conditions. Arguably, longer-term treatments (months) could have eventually affected bacterial community structure. However, we evidenced no appreciable decay of the symbiotic community in response to medium-term (3 weeks) environmental anomalies purported to cause the recurrent sponge mortality episodes. Thus, changes in symbiont structure are not likely the proximate cause for these reported mortality events.

Bacterial consortium of Millepora dichotoma exhibiting unusual multifocal lesion event in the Gulf of Eilat, Red Sea

Colonies of the hydrocoral Millepora dichotoma along the Gulf of Eilat are exhibiting unusual tissue lesions in the form of white spots. The emergence and rapid establishment of these multifocal tissue lesions was the first of its kind reported in this region. A characterization of this morphological anomaly revealed bleached tissues with a significant presence of bacteria in the tissue lesion area. To ascertain possible differences in microbial biota between the lesion area and non-affected tissues, we characterized the bacterial diversity in the two areas of these hydrocorals. Both culture-independent (molecular) and culture-dependent assays showed a shift in bacterial community structure between the healthy and affected tissues. Several 16S rRNA gene sequences retrieved from the affected tissues matched sequences of bacterial clones belonging to Alphaproteobacteria and Bacteroidetes members previously associated with various diseases in scleractinian corals.

Epidemic mortality of the sponge Ircinia variabilis (Schmidt, 1862) associated to proliferation of a Vibrio bacterium

In recent years, several episodes of mass mortality of sessile epibenthic invertebrates, including sponges, have been recorded worldwide. In the present study, we report a disease event on Ircinia variabilis recorded in September 2009 along the southern Adriatic and Ionian seas (Apulian coast), with the aim to quantify the mortality incidence on the sponge population, to investigate the effect of the disease on the sponge tissues and to assess whether the disease is associated with vibrios proliferation. The injured sponges showed wide necrotic areas on the surface or disruption of the body in several portions. Necrotic areas were whitish and often were covered with a thin mucous coat formed by bacteria. In the most affected specimens, sponge organisation resulted partial or complete loss, with the final exposure of the dense skeletal network of spongine fibres to the environment. The results of microbiological cultural analysis using in parallel Marine Agar 2216 and thiosulphate/citrate/bile salts/sucrose agar demonstrated that, in affected specimens, vibrios represented 15.8 % of the total I. variabilis surface culturable bacteria. Moreover, all the isolated vibrios, grown from the wide whitish areas that characterize the surface of the diseased sponges, were identified, and their assignment to the Vibrio rotiferianus was consistent with phylogenetic analysis and data of morphological, cultural and biochemical tests. Studies on V. rotiferianus have shown that its pathogenicity, with respect to various aquatic organisms, is higher than that of Vibrio harveyi. The factors triggering the disease outbreak in Ircinia variabilis populations remain unclear. At present, we can hypothesize the involvement in the disease of a synergetic mechanism that, under stressful physiological conditions (high temperature, elevated nutrients and reduced water flow), induces sponge pathogens, in our case V. rotiferanius, to become virulent, making sponges unable to control their proliferation. Additional studies are needed to understand the etiological processes as well as the factors involved in sponges recovering from this epidemic event allowing them to face mass mortality. A drastic reduction of sponge-specific representatives could have marked a negative impact on the environmental health on account of their role in the sea remediation processes as filter-feeding organisms.

Sponge mass mortalities in a warming Mediterranean Sea: are cyanobacteria-harboring species worse off?

Mass mortality events are increasing dramatically in all coastal marine environments. Determining the underlying causes of mass mortality events has proven difficult in the past because of the lack of prior quantitative data on populations and environmental variables. Four-year surveys of two shallow-water sponge species, Ircinia fasciculata and Sarcotragus spinosulum, were carried out in the western Mediterranean Sea. These surveys provided evidence of two severe sponge die-offs (total mortality ranging from 80 to 95% of specimens) occurring in the summers of 2008 and 2009. These events primarily affected I. fasciculata, which hosts both phototrophic and heterotrophic microsymbionts, while they did not affect S. spinosulum, which harbors only heterotrophic bacteria. We observed a significant positive correlation between the percentage of injured I. fasciculata specimens and exposure time to elevated temperature conditions in all populations, suggesting a key role of temperature in triggering mortality events. A comparative ultrastructural study of injured and healthy I. fasciculata specimens showed that cyanobacteria disappeared from injured specimens, which suggests that cyanobacterial decay could be involved in I. fasciculata mortality. A laboratory experiment confirmed that the cyanobacteria harbored by I. fasciculata displayed a significant reduction in photosynthetic efficiency in the highest temperature treatment. The sponge disease reported here led to a severe decrease in the abundance of the surveyed populations. It represents one of the most dramatic mass mortality events to date in the Mediterranean Sea.

Exploring the role of microorganisms in the disease-like syndrome affecting the sponge Ianthella basta

A disease-like syndrome is currently affecting a large percentage of the Ianthella basta populations from the Great Barrier Reef and central Torres Strait. Symptoms of the syndrome include discolored, necrotic spots leading to tissue degradation, exposure of the skeletal fibers, and disruption of the choanocyte chambers. To ascertain the role of microbes in the disease process, a comprehensive comparison of bacteria, viruses, fungi, and other eukaryotes was performed in healthy and diseased sponges using multiple techniques. A low diversity of microbes was observed in both healthy and diseased sponge communities, with all sponges dominated by an Alphaproteobacteria, a Gammaproteobacteria, and a group I crenarchaeota. Bacterial cultivation, community analysis by denaturing gradient gel electrophoresis (Bacteria and Eukarya), sequencing of 16S rRNA clone libraries (Bacteria and Archaea), and direct visual assessment by electron microscopy failed to reveal any putative pathogens. In addition, infection assays could not establish the syndrome in healthy sponges even after direct physical contact with affected tissue. These results suggest that microbes are not responsible for the formation of brown spot lesions and necrosis in I. basta.

Re-plumbing in a Mediterranean sponge

Observations are reported for Dysidea avara sponges where once functioning oscula (outlets) are converted through internal re-plumbing into functioning oversized ostia (OSO; inlets). Flow tank studies employed high-speed photography and particle tracking of laser-illuminated 0.5-6.0 microm diameter glass beads to trace particles streaming into OSO. A fluorescein dye/glass bead uptake experiment showed that an oversized ostium was connected through internal structures to the lone osculum. Beginning 30 s after uptake and continuing over a 20 min period, dye streamed from the osculum, but no beads emerged. Scanning electron microscopy revealed that beads were deposited only on the inhalant side of particle filtering choanocyte chambers and not on the exhalant side, suggesting that internal re-plumbing had occurred. Functioning OSO were also found on freshly collected specimens in the field, making it highly unlikely that formation of OSO was only an artefact of sponges being held in a laboratory tank.

Growth and photosynthesis of two Mediterranean corals, Cladocora caespitosa and Oculina patagonica, under normal and elevated temperatures

The Ligurian Sea (NW Mediterranean) experienced warm summers in 1998, 1999 and from 2003 to 2005. The temperature was 1-3°C higher than the mean summer value (24°C) and remained high over a long period. During these summers, mass-mortality events, affecting several sessile benthic species,were reported. In the present study, we tested the long-term (3-7 weeks)effect of different temperatures (20°C measured in spring and autumn,24°C observed in summer, and 26°C and 28°C abnormal summer values)on two Mediterranean corals, Cladocora caespitosa and Oculina patagonica. Growth rate, photosynthetic efficiency(Fv/Fm), relative electron transport rate (ETR), zooxanthellae and chlorophyll (chl) contents were measured during 48 days incubation. At 20°C, all parameters remained constant during the whole experiment for both species. At higher temperatures, most physiological parameters were affected by only 2-5 weeks at 24°C, and were severely depressed at higher temperatures. Small replicate samples (nubbins) of O. patagonica significantly decreased their zooxanthellae and chl concentrations at all temperatures, after 2 weeks of incubation. Their Fv/Fm values, as well as their growth rates, were also gradually reduced during the incubation at all temperatures. However, only a few nubbins maintained at 28°C showed signs of tissue necrosis after 34 days, and these gradually recovered tissue when temperature was returned to normal. In nubbins of C. caespitosa, chl and zooxanthellae concentrations decreased only after 34 days of incubation at 26°C and 28°C. At the same time, tissue necrosis was observed,explaining the loss of the symbionts. Fv/Fm was reduced only after 34 days of incubation at the different temperatures, and growth rate was first enhanced, before collapsing by 30% at 24°C and by 90-100% at 26°C and 28°C. All samples maintained at 26°C and 28°C had died, due to tissue necrosis, by the end of the experiment. Results obtained suggest that O. patagonica is more able than C. caespitosa to resist high temperature conditions because of its rapid bleaching capacity. In contrast,it seems that C. caespitosa is living close to its thermal limit during the summer period; therefore, a long-term increase at 24°C or above could be lethal for this coral, just as was observed in situ during the recent warm summers.