Effects of marine heatwaves on temperate sponge physiology and reproduction

Climate change is causing not only a gradual rise in global temperatures but also an increase in the frequency and severity of climate extremes. Marine Heat Waves (MHWs) are events of extreme sea surface temperature that can range from a few days to years, affecting small areas to entire oceans, and their frequency and intensity are expected to increase in the future. Sponges are considered the most ancient metazoans populating global oceans and make an important contribution to biodiversity and ecosystem functioning in shallow, mesophotic and deep ecosystems due to the multiple ecological functions they provide. The occurrence of MHWs has often been associated with sponge mass mortality and bleaching events in tropical, temperate and polar benthic ecosystems, but the biological effects and adaptive responses of sponges to these conditions are largely unknown. Crella incrustans is a poecilosclerid sponge commonly distributed in subtidal habitats of temperate Pacific regions and it is increasingly being considered a model organism for ecophysiological studies. The main goal of this thesis is to assess the biological impacts of MHWs on temperate sponge physiology and reproduction and to assess possible adaptive strategies to overcome these extreme climate events with these specific aims: 1) characterise reproductive period and modality of C. incrustans, including the description of larval ontogeny, release, settlement and metamorphosis; 2) investigate the impacts of near-future extreme temperatures on C. incrustans morphology, physiology and reproductive success; 3) investigate the occurrence of carryover effects of MHW conditions on the microbiome and developmental performance in C. incrustans; 4) to investigate the function of symbiotic bacteria that are vertically transmitted to larvae under MHW conditions in C. incrustans.

In my first chapter, I characterised the reproductive modality and the early life stages of C. incrustans. Through histological analyses, I described the gametogenesis and larval ontogeny and using in vivo observations, I characterised larval release, settlement and metamorphosis. C. incrustans presented spermatocytes, oocytes and embryos in the sponge mesohyl during the Austral summer. Non-tufted parenchymella larvae were also released in laboratory conditions during the Austral summer. I found that 94.3% of larvae settled within two days and metamorphosed within a week. My main finding was that C. incrustans is a simultaneous hermaphrodite with asynchronous gametogenesis that releases larvae during the Austral summer, which is consistent with the majority of other poecilosclerid sponges. Importantly, in this chapter, I showed that C. incrustans is a suitable species for ecophysiological studies involving sponge reproduction and early life stages.

In my second chapter, I investigated the impact of mean and extreme temperatures predicted to occur within the next 40 years, on the physiology, morphology and recruitment success of C. incrustans. In particular, I experimentally exposed adult sponges to future predicted average sea temperature (+ 2.5° C, according to the SSP3-7.0 scenario) and then simulated a MHW (16 days duration and a thermal peak at 22°C). The main finding was that under predicted average temperatures, C. incrustans did not show any morphological or physiological modifications. However, when exposed to the MHW conditions there was a significant increase in sponge respiration rate, significant weight loss with the occurrence of tissue regression and around 50% mortality. MHW conditions also caused a significantly shorter period of recruitment, a lower recruitment rate and a higher mortality rate of sponge settlers. Interestingly, sponges that survived the simulated MHW showed respiration rates like controls two weeks after the thermal peak, indicating some resilience to MHWs. These results indicate that MHW conditions have a much stronger impact on C. incrustans physiology, morphology and recruitment than future mean temperatures but also that this sponge species is likely to persist under future climate scenarios.

In my third chapter, I investigated the occurrence of carryover effects of heatwave conditions on the sponge microbiome and post-settlement developmental features. I experimentally tested the effects of heatwave conditions (10 days at 21°C) on adult and larval microbiomes of C. incrustans. I also tested the effects of parental exposure to heatwave conditions on subsequent settler mortality, growth rate and metamorphosis duration under prolonged heatwave conditions (30 days at 21°C). My main finding was that the microbial community of adult sponges changed significantly after ten days at 21°C, with a significant decrease in symbiotic bacteria (i.e., Arenicellales) and a significant increase in stress-associated bacteria (e.g., Flavobacteriales and Clostridiales). Sponge larvae derived from control sponges were mainly characterised by a few bacterial taxa also abundant in adults (i.e., Arenicellales), confirming the occurrence of vertical transmission. The microbial community of sponge larvae derived from MHW-exposed sponges showed similar levels of Arenicellales to control larvae and a significant increase in endosymbiotic Verrucomicrobia. Settlers derived from heatwave-exposed sponges had better developmental performance, including a greater growth rate and longer metamorphosis period compared to settlers derived from control sponges and exposed to heatwave conditions. These results show the occurrence of MHW-induced carryover effects across life-stages in C. incrustans and suggest the occurrence of selective vertical transmission under MHW conditions as a possible adaptive strategy to overcome extreme thermal events.

In my fourth data chapter, I investigated the function of vertically transmitted bacteria, the Arenicellales, under heatwave conditions in C. incrustans. To characterise the genetic potential of vertically transferred bacteria under MHW conditions, I enriched bacterial cells and performed shotgun sequencing of genomic DNA in C. incrustans. My main finding was that the metagenomic assembled genome (MAG) classified as Arenicellales belongs to a new family, genus and species, herein named Candidatus Larvaebacter crellansis. Predicted metabolic pathways revealed that this MAG has the genetic capacity to utilise sponge-derived molecules such as amino acids, aromatic compounds and osmolytes as a source of nutrients. However, it also has the genetic potential to produce molecules required by the sponge host, such as essential vitamins, including cobalamin, which is an important cofactor of several metabolic reactions and to enhance thermal tolerance in unicellular algae. Ca. Larvaebacter crellansis has the genetic potential for the production of terpenes, known for being antifoulants and feeding deterrents in sponges. This bacterium also has the potential for the production of catechol, known to be important for substrate adhesion by marine invertebrates. Considering the great potential of metabolic interactions between this bacterial species and the sponge host, my study supports the occurrence of selective vertical transmission of biologically relevant symbionts under heatwave conditions in C. incrustans.

Overall, my thesis shows that MHWs can profoundly affect temperate sponge physiology and reproductive success. Despite that, temperate sponges may show great phenotypic plasticity in response to these extreme temperatures, such as morphological modifications, with the occurrence of tissue regression and selective vertical transmission of biologically relevant bacteria in C. incrustans. In conclusion, despite the detrimental effects of MHWs on temperate sponges, some species are likely to persist under future climate conditions.