Marine heatwaves (MHWs) are increasing in intensity and frequency globally due to anthropogenic climate change. Temperate mesophotic ecosystems (TMEs) are light-dependent habitats found at depths of approximately 30–150m, between shallow euphotic zones and the deep sea. TMEs typically experience lower, more stable temperatures than adjacent shallow ecosystems, which may result in intraspecific differences in thermal tolerance between populations. Sponges, often the dominant benthic organisms in TMEs, display varied responses to thermal stress. We hypothesised that shallow-water sponges may be more resilient to heat stress than those from adjacent upper-mesophotic zones. Here we have deposited all data and code used in the analysis for this study.

Marine heatwaves are increasingly subjecting organisms to unprecedented stressful conditions, but the biological consequences of these events are still poorly understood. Here we experimentally tested the presence of carryover effects of heatwave conditions on the larval microbiome, settlers' growth rate and metamorphosis duration of the temperate sponge Crella incrustans.

Additional file 2. Supplementary tables.

Additional file 2. Supplementary tables.

Climate change is driving global declines of marine habitat-forming species through physiological effects and through changes to ecological interactions, with projected trajectories for oceanwarming and acidification likely to exacerbate such impacts in coming decades. Interactions between habitat-formers and their microbiomes are fundamental for host functioning and resilience, but how such relationships will change in future conditions is largely unknown. We investigated independent and interactive effects of warming and acidification on a large brown seaweed, the kelp Ecklonia radiata, and its associated microbiome in experimental mesocosms. Microbial communities were affected by warming and, during the first week, by acidification. During the second week, kelp developed disease-like symptoms previously observed in the field. The tissue of some kelp blistered, bleached and eventually degraded, particularly under the acidification treatments, affecting photosynthetic efficiency. Microbial communities differed between blistered and healthy kelp for all treatments, except for those under future conditions of warming and acidification, which after two weeks resembled assemblages associated with healthy hosts. This indicates that changes in the microbiome were not easily predictable as the severity of future climate scenarios increased. Future ocean conditions can change kelp microbiomes and may lead to host disease, with potentially cascading impacts on associated ecosystems.

1. Whilst the successful establishment and spread of invasive species can be determined by above ground processes, results are often equivocal. Emergent research, mostly from terrestrial ecosystems, demonstrates that below-ground processes (nutrient cycling, chemical properties) under microbial control can mediate interactions between native and invasive plants. Because microbes can control similar sediment properties in marine ecosystem that influence plant fitness, we argue that below-ground properties should also exert strong control interactions between native and invasive marine macrophytes. 2. We coupled surveys of microbial communities and chemistry of sediments collected from an invasive alga (Caulerpa cylindracea), a native competitor (the seagrass Posidonia oceanica) and unvegetated sediments with a large field experiment, in which we manipulated the presence/absence of the canopies of both species to determine the effects of above- and below-ground processes on the success of C. cylindracea. 3. P. oceanica and C. cylindracea sediments have microbial communities and predicted metabolic process that reflect aerobic and anaerobic conditions, respectively. Moreover, the nutritional quantity of organic matter was higher, but quality was lower in C. cylindracea sediments compared to the two native habitats. The growth of C. cylindracea fragments was equally low in the presence or absence of a P. oceanica canopy, whereas the growth of C. cylindracea was higher in the canopy removed vs. present treatment, possibly because, in the absence of a C. cylindracea canopy, fragments are released from intra-specific competition for resources. 4. Synthesis: Sediment/soil processes are increasingly recognised as important drivers of the success and hence impacts of invasive plants. We extended this theory to marine ecosystems and suggest biotic resistance to invasion may not always be attributable to intact canopies, but may also result from indirect effects of native macrophytes on sediment quality and microbial processes. This information may, in part, resolve why above-ground interactions don’t always explain invasive plant success and thus can be used to develop better informed management strategies.

Eukaryotic-like proteins (ELPs) are classes of proteins that are found in prokaryotes, but have a likely evolutionary origin in eukaryotes. ELPs have been postulated to mediate host-microbiome interactions. Recent work has discovered that prokaryotic symbionts of sponges contain abundant and diverse genes for ELPs, which could modulate interactions with their filter-feeding and phagocytic host. However, the extent to which these ELP genes are actually used and expressed by the symbionts is poorly understood. Here we use metatranscriptomics to investigate ELP expression in the microbiomes of three different sponges (Cymbastella concentrica, Scopalina sp. and Tedania anhelens). We developed a workflow with optimized rRNA removal and in silico subtraction of host sequences to obtain a reliable symbiont metatranscriptome. This showed that between 1.3 and 2.3% of all symbiont transcripts contain genes for ELPs. Two classes of ELPs (cadherin and tetratricopetide repeats) were abundantly expressed by in the C. concentrica and Scopalina sp. microbiomes, while ankyrin repeat ELPs were predominant in the T. anhelens metatranscriptome. Comparison to non-ELP containing transcripts indicated a constitutive expression of ELPs across a range of bacterial and archaeal symbionts. Expressed ELPs also contained domains involved in protein secretion and/or were co-expressed with proteins involved in extra-cellular transport. This suggests these ELPs are likely exported, which could allow for direct interaction with the sponge. Our study shows that ELP genes in sponge symbionts represent actively expressed functions that could mediate molecular interaction between symbiosis partners.