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.