Population genomics, polyploidy, climate resilience and the microbiota of two habitat-building coralline algae

Maerl bed habitats support an array of marine biodiversity and are recognised as important blue carbon ecosystems. However, little is currently known about the population structure, spatial distribution of genetic diversity, or microbiota of the coralline algae that build these habitats, or how resilient they are to climate change. Here, we investigate these knowledge gaps in two key maerl-forming species, Phymatolithon calcareum and Lithothamnion corallioides . In southern England, we identified genetically distinct populations of maerl of the two species in Dorset, while there was evidence of admixture between maerl beds in Cornwall, the extent of which differed among sites and species. A genetically distinct clonal lineage, however, was revealed in the Helford River for L. corallioides . Additionally, we distinguished the genetically distinct coarse growth form of P. calcareum in Falmouth as a unique, apparently triploid, phenotype of P. calcareum . In south-west Wales, only L. corallioides was found in Milford Haven, a population that is genetically diverse and isolated from populations in England. Analysis of climate change resilience indicated that P. calcareum maerl in Cornwall may have slightly higher risks of being maladapted to temperatures and salinities predicted in 2050 compared to other P. calcareum maerl sampled. Lastly, metagenomics analysis revealed differences in microbiota between dense healthy maerl beds compared to scattered dead maerl. As a habitat of conservation priority, our findings will be central to ensuring maerl beds receive appropriate levels of protection to prevent further loss or degradation by human activity and climate change.