High environmental stress and productivity increase functional diversity along a deep-sea hydrothermal vent gradient

Alpha and beta diversities of hydrothermal vent macrofaunal communities along the southwestern Pacific back-arc basins

Ecosystems face various pressures, often leading to loss of biodiversity. Understanding how biodiversity is spatially structured, what are the driving factors, and the ecological and evolutionary processes involved is essential to assess communities' resilience to disturbances and guide efficient conservation measures. Hydrothermal vents from national waters of the West Pacific are targeted by mining industries for their mineral resources that include metals used in high-tech equipment. Although exploitation has not yet started, such activity could significantly affect ecosystem biodiversity and functioning. Here, we describe the distribution of hydrothermal biodiversity in the Southwest Pacific back-arc basins and the Futuna Volcanic Arc at different spatial scales in relation to environmental conditions and geography. We focused on three assemblages dominated by symbiotic megafauna: snails (Alviniconcha spp. and Ifremeria nautilei) and mussels (Bathymodiolus spp.). Faunal assemblages exhibit strong spatial structuring: between habitats along the dilution gradient of the hydrothermal fluid, and between geographic basins, with a faunal split between the Western and the Eastern basins of this region, and to a lesser extent, between fields in a basin. Species replacement along the chemical gradient drives faunal changes between Ifremeria and Bathymodiolus assemblages, while a drop in the number of species is noted when making this comparison with the Alviniconcha assemblage. While these local changes may result from environmental filtering along the diffuse flow gradient, geological settings and current geographic barriers, which drive colonization and speciation at larger scales, are likely shaping the vent community changes between the Eastern and Western basins. This result has significant implications for biodiversity conservation, especially in this mineral-rich setting. The Manus Basin is isolated and displays the highest proportion of endemism while the Woodlark Basin represents an important stepping-stone between the Eastern basins and Manus Basin, making them potentially highly vulnerable to mining with a risk of biodiversity loss.

The spatiotemporal variations of marine nematode populations may serve as indicators of changes in marine ecosystems

The volcanic island, Kueishan Island, harbors two unique shallow-water ecosystems: hydrothermal vents and coral communities. The unique geologic features render the island an ideal place as a spectrum for studying two different ecosystems and mimicking the impacts of climate change on coral reef biota in the future. However, little is known about the meiofauna community there. Hence, we investigated the diversity and composition of free-living marine nematodes over two years by collecting individuals from sediments sampled across a gradient of habitats, including hydrothermal vents, buffering sites, and coral reefs. During the first year, we also monitored abiotic factors, such as sediment and water properties, along with biotic factors, including bacterial diversity assessed through amplicon sequencing, to evaluate their influence on the nematode community. Our findings revealed markedly low nematode abundance and diversity at sulfide-rich vent sites (abundance < 5 ind./L; Shannon index < 1) throughout the study period, contrasting with the highest levels observed at the coral community site (<165.4 ind./L, Shannon index = 1.65). The food supply seemed to be the main factor that drove the difference, as nematode abundance and diversity increased with sedimentary total organic carbon and bacterial diversity. In addition, significant differences in nematode composition were observed between the different sampling sites. Combined with nematode and microbiome data, the buffering site that endured more stress from vent activities was recognized. Our results suggest that the dynamics of nematode communities could be incorporated into projects assessing environmental impacts on coral reef ecosystems.

Fluid chemistry alters faunal trophodynamics but not composition on the deep-sea Capelinhos hydrothermal edifice (Lucky Strike vent field, Mid-Atlantic Ridge)

The recently discovered deep-sea Capelinhos hydrothermal edifice, ~ 1.5 km of the main Lucky Strike (LS) vent field (northern Mid-Atlantic Ridge), contrasts with the other LS edifices in having poorly-altered end-member hydrothermal fluids with low pH and chlorine, and high metal concentrations. Capelinhos unique chemistry and location offer the opportunity to test the effects of local abiotic filters on faunal community structure while avoiding the often-correlated influence of dispersal limitation and depth. In this paper, we characterize for the first time the distribution patterns of the Capelinhos faunal communities, and analyze the benthic invertebrates (> 250 µm) inhabiting diffusive-flow areas and their trophic structures (δ13C, δ15N and δ34S). We hypothesized that faunal communities would differ from those of the nearest LS vent edifices, showing an impoverished species subset due to the potential toxicity of the chemical environment. Conversely, our results show that: (1) community distribution resembles that of other LS edifices, with assemblages visually dominated by shrimps (close to high-temperature focused-fluid areas) and mussels (at low-temperature diffuse flow areas); (2) most species from diffuse flow areas are well-known LS inhabitants, including the bed-forming and chemosymbiotic mussel Bathymodiolus azoricus and (3) communities are as diverse as those of the most diverse LS edifices. On the contrary, stable isotopes suggest different trophodynamics at Capelinhos. The high δ15N and, especially, δ13C and δ34S values suggest an important role of methane oxidation (i.e., methanotrophy), rather than the sulfide oxidation (i.e., thiotrophy) that predominates at most LS edifices. Our results indicate that Capelinhos shows unique environmental conditions, trophic structure and trophodynamics, yet similar fauna, compared to other LS edifices, which suggest a great environmental and trophic plasticity of the vent faunal communities at the LS.

Stressors of emerging concern in deep-sea environments: microplastics, pharmaceuticals, personal care products and deep-sea mining

Although most deep-sea areas are remote in comparison to coastal zones, a growing body of literature indicates that many sensitive ecosystems could be under increased stress from anthropogenic sources. Among the multiple potential stressors, microplastics (MPs), pharmaceuticals and personal care products (PPCPs/PCPs) and the imminent start of commercial deep-sea mining have received increased attention. Here we review recent literature on these emerging stressors in deep-sea environments and discuss cumulative effects with climate change associated variables. Importantly, MPs and PPCPs have been detected in deep-sea waters, organisms and sediments, in some locations in comparable levels to coastal areas. The Atlantic Ocean and the Mediterranean Sea are the most studied areas and where higher levels of MPs and PPCPs have been detected. The paucity of data for most other deep-sea ecosystems indicates that many more locations are likely to be contaminated by these emerging stressors, but the absence of studies hampers a better assessment of the potential risk. The main knowledge gaps in the field are identified and discussed, and future research priorities are highlighted to improve hazard and risk assessment.

Functional space expansion driven by transitions between energetically advantageous traits in the deep sea

Climate change is shifting community structure and biodiversity on a global scale, in part due to alterations of chemical and thermal energy availability. These changes may impact ecosystem functioning through their influence on functional diversity. We investigate patterns of functional diversity, functional niches, and functional traits in bivalve communities across the energetic gradient of the deep Atlantic Ocean. We use the functional traits feeding type, tiering, and motility level to define the axes of functional space and the unique combinations of these traits as functional niches. We find that increased energy affords new species, added into functional space through niche expansion rather than niche packing. Underlying this pattern are complex dynamics of gains and losses of individual functional niches, with few adapted to the low- and high-energy extremes, and most occurring at intermediate energy. Adaptive qualities of specific traits are evidenced by those functional niches occurring at energetic extremes. Tradeoffs between these traits within the intermediate energy zone underlie an increased coexistence of functional niches, which in turn drives a unimodal pattern of functional niches and expansion of used functional space. This work suggests that energy-limited communities may be especially vulnerable to continued shifts in food availability through the Anthropocene.

Autochthony and isotopic niches of benthic fauna at shallow-water hydrothermal vents

The food webs of shallow-water hydrothermal vents are supported by chemosynthetic and photosynthetic autotrophs. However, the relative importance of these two basal resources for benthic consumers and its changes along the physicochemical gradient caused by vent plumes are unknown. We used stable carbon and nitrogen isotopes (i.e., δ¹³C and δ¹⁵N) and Bayesian mixing models to quantify the dietary contribution of basal resources to the benthic fauna at the shallow-water vents around Kueishan Island, Taiwan. Our results indicated that the food chains and consumer production at the shallow-water vents were mainly driven by photoautotrophs (total algal contribution: 26–54%) and zooplankton (19–34%) rather than by chemosynthetic production (total contribution: 14–26%). Intraspecific differences in the trophic support and isotopic niche of the benthic consumers along the physicochemical gradient were also evident. For instance, sea anemone Anthopleura sp. exhibited the greatest reliance on chemosynthetic bacteria (26%) and photoautotrophs (66%) near the vent openings, but zooplankton was its main diet in regions 150–300 m (32–49%) and 300–700 m (32–78%) away from the vent mouths. The vent-induced physicochemical gradient structures not only the community but also the trophic support and isotopic niche of vent consumers.

Beta diversity differs among hydrothermal vent systems: Implications for conservation

Deep-sea hydrothermal vent habitats are small, rare and support unique species through chemosynthesis. As this vulnerable ecosystem is increasingly threatened by human activities, management approaches should address biodiversity conservation. Diversity distribution data provide a useful basis for management approaches as patterns of β-diversity (the change in diversity from site to site) can guide conservation decisions. Our question is whether such patterns are similar enough across vent systems to support a conservation strategy that can be deployed regardless of location. We compile macrofaunal species occurrence data for vent systems in three geological settings in the North Pacific: volcanic arc, back-arc and mid-ocean ridge. Recent discoveries in the Mariana region provide the opportunity to characterize diversity at many vent sites. We examine the extent to which diversity distribution patterns differ among the systems by comparing pairwise β-diversity, nestedness and their additive components. A null model approach that tests whether species compositions of each site pair are more or less similar than random provides insight into community assembly processes. We resolve several taxonomic uncertainties and find that the Mariana arc and back-arc share only 8% of species despite their proximity. Species overlap, species replacement and richness differences create different diversity distributions within the three vent systems; the arc system exhibits much greater β-diversity than both the back-arc and mid-ocean ridge systems which, instead, show greater nestedness. The influence of nestedness on β-diversity also increased from the arc to back-arc to ridge. Community assembly processes appear more deterministic in the arc and ridge systems while back-arc site pairs deviate little from the null expectation. These analyses reflect the need for a variety of management strategies that consider the character of diversity distribution to protect hydrothermal vents, especially in the context of mining hydrothermal deposits.

Relationships between biodiversity and ecosystem functioning proxies strengthen when approaching chemosynthetic deep-sea methane seeps

As biodiversity loss accelerates globally, understanding environmental influence over biodiversity-ecosystem functioning (BEF) relationships becomes crucial for ecosystem management. Theory suggests that resource supply affects the shape of BEF relationships, but this awaits detailed investigation in marine ecosystems. Here, we use deep-sea chemosynthetic methane seeps and surrounding sediments as natural laboratories in which to contrast relationships between BEF proxies along with a gradient of trophic resource availability (higher resource methane seep, to lower resource photosynthetically fuelled deep-sea habitats). We determined sediment fauna taxonomic and functional trait biodiversity, and quantified bioturbation potential (BPc), calcification degree, standing stock and density as ecosystem functioning proxies. Relationships were strongly unimodal in chemosynthetic seep habitats, but were undetectable in transitional 'chemotone' habitats and photosynthetically dependent deep-sea habitats. In seep habitats, ecosystem functioning proxies peaked below maximum biodiversity, perhaps suggesting that a small number of specialized species are important in shaping this relationship. This suggests that absolute biodiversity is not a good metric of ecosystem 'value' at methane seeps, and that these deep-sea environments may require special management to maintain ecosystem functioning under human disturbance. We promote further investigation of BEF relationships in non-traditional resource environments and emphasize that deep-sea conservation should consider 'functioning hotspots' alongside biodiversity hotspots.