Potential of epiphytic macroalgal vegetation as indicator of conservation status of Mediterranean algal forests

Macroalgal forests are habitats of critical importance in coastal areas because of the ecosystem services they provide, since these communities represent major biodiversity repositories, nursery areas, and bioindicators of high environmental quality1. In the Mediterranean, most macroalgal forests are formed by habitat-forming brown seaweeds (Cystoseira, Ericaria, Gongolaria). These ecosystems are seriously threatened by multiple stressors, both natural and anthropogenic, and regressed in the last decades with consequent habitat fragmentation1, shifting from complex tridimensional ecosystems to algal communities with smaller size and simpler structure. The interactions between macroalgal forests and their associated biota have a crucial ecological relevance, resulting in a key-interplay which should be considered to plan conservation programs in coastal areas2. Epiphytes respond faster to anthropogenic stressors than canopy-forming algae. Therefore, the assessment of common patterns in epiphytic communities may allow the use of these assemblages as tools for the evaluation of changes in the status of macroalgal forests3. Epiphytic assemblages associated with forests of the brown seaweed Gongolaria barbata in different conservation conditions (continuous and fragmented) were investigated in late spring-early summer 2023 at 5 sites along the Conero Riviera (Ancona). Sampling was carried out following a nested design; a branch was collected from six thalli of G. barbata at each each site. Morphometric parameters of the branches were measured (length, width, number of secondary branches per branch and surface). Epiphytes were manually removed from each branch and identified by microscopic observation. The most common epiphytes were also identified based on a DNA barcoding approach, using suitable markers for species identification (tufA, rbcL, psbA, COI, ITS). Overall, 21 macroalgal epiphytes were recorded, 5 of which were primary epiphytes (directly attached to Gongolaria) and 16 were also found as secondary and tertiary epiphytes. Vertebrata fruticulosa was present in all samples as primary epiphyte. Ceramium siliquosum was the species most frequently found as secondary epiphyte, mostly growing on Vertebrata, and Didymosporangium repens was the species most frequently found as tertiary epiphyte, exclusively growing on C. siliquosum. The results did not show clear differences between continuous and fragmented populations, although the epiphytic community at the most urbanized site differed significantly from the others both in terms of abundance and species number. The DNA sequence data unraveled some cases of cryptic diversity in some genera of green and red algae (Ulva, Corallina, Pachymeniopsis, Vertebrata). Further analyses are currently ongoing and the study is being extended to populations of Gongolaria barbata of other Mediterranean regions. Moreover, the work will be complemented by DNA metabarcoding data based on sequencing of the 23S marker (flank Domain V of the 23S plastid rRNA gene). For this purpose, 23S sequences are being produced and processed to build a sequence library to be used for species assignment of eDNA results.