High taxonomic diversity and miniaturization in benthic communities under persistent natural CO2 disturbances

Rapid Evolution in Action: Environmental Filtering Supports Coral Adaptation to a Hot, Acidic, and Deoxygenated Extreme Habitat

The semienclosed Bouraké lagoon in New Caledonia is a natural system that enables observation of evolution in action with respect to stress tolerance in marine organisms, a topic directly relevant to understanding the consequences of global climate change. Corals inhabiting the Bouraké lagoon endure extreme conditions of elevated temperature (> 33°C), acidification (7.2 pH units), and deoxygenation (2.28 mg O2 L-1), which fluctuate with the tide due to the lagoon's geomorphology. To investigate the underlying bases of the apparent stress tolerance of these corals, we combined whole genome resequencing of the coral host and ITS2 metabarcoding of the photosymbionts from 90 Acropora tenuis colonies from three localities along the steep environmental gradient from Bouraké to two nearby control reefs. Our results highlight the importance of coral flexibility to associate with different photosymbionts in facilitating stress tolerance of the holobiont; but, perhaps more significantly, strong selective effects were detected at specific loci in the host genome. Fifty-seven genes contained SNPs highly associated with the extreme environment of Bouraké and were enriched in functions related to sphingolipid metabolism. Within these genes, the conserved sensor of noxious stimuli TRPA1 and the ABCC4 transporter stood out due to the high number of environmentally selected SNPs that they contained. Protein 3D structure predictions suggest that a single-point mutation causes the rotation of the main regulatory domain of TRPA1, which may be behind this case of natural selection through environmental filtering. While the corals of the Bouraké lagoon provide a striking example of rapid adaptation to extreme conditions, overall, our results highlight the need to preserve the current standing genetic variation of coral populations to safeguard their adaptive potential to ongoing rapid environmental change.

Genomic signals of adaptation to a natural CO2 gradient over a striking microgeographic scale

Our study explores genomic signs of adaptation in A. lixula to different water pH conditions. To achieve this, we analysed the genomics variation of A. lixula individuals living across a natural pH gradient in Canary Islands, Spain. We use a 2b-RADseq protocol with 74 samples from sites with varying pH levels (from 7.3 to 7.9 during low tide) and included a control site. We identified 14,883 SNPs, with 432 identified as candidate SNPs under selection to pH variations through redundancy analysis. While all SNPs indicated genomic homogeneity, the 432 candidate SNPs under selection displayed genomic differences among sites and along the pH gradient. Out of these 432 loci, 17 were annotated using published A. lixula transcriptomes, involved in biological functions such as growth. Therefore, our findings suggest local adaptation in A. lixula populations to acidification in CO2 vents, even over short distances of 75 m, underscoring their potential resistance to future Ocean Acidification.

High taxonomic diversity and miniaturization in benthic communities under persistent natural CO2 disturbances

Metabarcoding techniques have revolutionized ecological research in recent years, facilitating the differentiation of cryptic species and revealing previously hidden diversity. In the current scenario of climate change and ocean acidification, biodiversity loss is one of the main threats to marine ecosystems. Here, we explored the effects of ocean acidification on marine benthic communities using DNA metabarcoding to assess the diversity of algae and metazoans. Specifically, we examined the natural pH gradient generated by the Fuencaliente CO2 vent system, located near La Palma Island (Canary Islands). High-resolution COI metabarcoding analyses revealed high levels of taxonomic diversity in an acidified natural area for the first time. This high number of species arises from the detection of small and cryptic species that were previously undetectable by other techniques. Such species are apparently tolerant to the acidification levels expected in future oceans. Hence and following our results, future subtropical communities are expected to keep high biodiversity values under an acidification scenario, although they will tend toward overall miniaturization due to the dominance of small algal and invertebrate species, leading to changes in ecosystem functions.