The study area is one of the most ecologically heterogeneous stretches of the Mediterranean. It spans rocky shores, sandy beaches, seagrass meadows, busy commercial and fishing ports, and the Mar Menor, one of Europe’s largest hypersaline lagoons, protected under the Ramsar Convention, Natura 2000, and Special Protection Area frameworks. It also includes the Salt Ponds of San Pedro del Pinatar, where active salt exploitation pushes salinity to an extraordinary 70.4 grams per liter — more than double typical seawater. Sampling ran from autumn 2024 through summer 2025, with triplicate water and sediment collections at each site in every season, accompanied by in situ measurements of temperature, pH, dissolved oxygen, salinity, conductivity, and nutrients, plus laboratory analyses of chlorophyll-a, sediment granulometry, and elemental composition.

Recovering yeasts from marine environments is technically demanding because their densities in seawater are typically very low. The team used a membrane filtration approach, concentrating 1-liter water samples and suspended sediment through sterile nitrocellulose membranes, then plating the concentrates onto five different culture media prepared with filtered seawater or supplemented with sodium chloride. An enrichment protocol was added for sediments to boost recovery of rare viable cells. Colonies were grouped into morphotypes based on color, size, shape, and texture, and a stratified selection strategy preserved the relative representation of each morphotype while reducing redundancy. Species identification relied on sequencing the internal transcribed spacer (ITS) region and the D1/D2 domain of the large subunit rRNA gene, with pragmatic similarity thresholds of at least 98.4 percent for ITS and 99.5 percent for LSU used to assign species proxies. Isolates falling below these thresholds were conservatively treated as potentially novel species pending formal description.

The taxonomic picture that emerged is dominated by the phylum Ascomycota, which accounted for 62 percent of identified species, distributed mainly among the classes Pichiomycetes, Dothideomycetes, Saccharomycetes, and Dipodascomycetes. Basidiomycota made up the remaining 38 percent, chiefly Tremellomycetes and Microbotryomycetes. The genera Rhodotorula and Candida were the most represented in both seawater and sediment. Rhodotorula mucilaginosa was the single most abundant species overall — 25 percent of water isolates and 10 percent of sediment isolates — and the only taxon detected at all ten sampling sites. Candida parapsilosis, a species better known as a human commensal, ranked second, a pattern the authors link to substantial anthropogenic influence along the coast.

Perhaps the most eye-catching component of the dataset is the prevalence of pigmented and melanized fungi. Roughly 10 percent of isolates were stress-tolerant black yeast-like fungi, including Aureobasidium pullulans, Aureobasidium melanogenum, Hortaea werneckii, Exophiala oligosperma, and Zalaria alba. Hortaea werneckii, one of the most halotolerant eukaryotes known, dominated both water and sediment at the hypersaline Salt Ponds site, where it co-occurred with various Debaryomyces species. Red-pigmented carotenoid-producing taxa such as Rhodotorula, Cystobasidium, Sporobolomyces, Symmetrospora, and Rhodosporidiobolus were also abundant. The researchers argue that intense solar radiation, hypersalinity, and seasonal thermal stress characteristic of the semi-arid Murcian coast select for these melanized and carotenoid-rich organisms, whose pigments protect against ultraviolet exposure and oxidative damage.

Diversity was not evenly distributed. Gamma diversity — total species richness pooled across all sites and seasons — was significantly higher in seawater than in sediment, with 77 species detected in the water column versus 44 in the benthos. The authors suggest the water column acts as a dynamic reservoir of yeast diversity, favored by greater dispersal potential, while sediments impose selective pressures that limit which species persist. Spatially, the Port of Cartagena, one of the most important harbors in the western Mediterranean, showed the highest richness of all: 26 species from the water column and 17 from sediments, with the top diversity indices across every metric measured. In contrast, sandy sediments at Portmán, Calblanque, and La Manga supported the lowest diversity, consistent with the constant reworking of unconsolidated substrates by waves, which leaves little organic matter and few stable surfaces for microbial colonization.

Beta diversity analyses revealed just how different these communities are from place to place. Jaccard distances between sites ranged from 0.69 to 0.96, indicating pronounced taxonomic differentiation. Sørensen-based partitioning showed that species turnover accounted for nearly 95 percent of total beta diversity, meaning the coastal gradient hosts largely distinct yeast assemblages rather than nested subsets of a single community. Seasonally, richness stayed remarkably stable — 37 species in autumn, 36 in winter, 40 in spring, and 30 in summer — but community composition shifted dramatically, with only six species, including Candida parapsilosis, Filobasidium magnum, Hortaea werneckii, and Rhodotorula mucilaginosa, present across all four seasons. PERMANOVA confirmed significant seasonal differences in assemblage structure, and among the environmental variables tested, temperature was the only significant individual factor, explaining 13.6 percent of the variance.

Beyond their ecological roles in organic matter recycling and nutrient cycling, the yeasts displayed impressive functional versatility. Screening of 90 representative strains showed that about 40 percent produced at least one extracellular lytic enzyme, with many producing several. Six activities were tested on agar plates: amylase, cellulase, pectinase, xylanase, chitinase, and protease. Aureobasidium pullulans was positive for all six, while Symmetrospora marina and Sampaiozyma vanillica showed notable amylase and chitinase activity. The standardized API-ZYM micromethod, which probes 19 enzymatic activities, confirmed broad repertoires in the black yeast-like fungi, with several species expressing more than half of the activities tested. Stress tolerance assays added to the picture: all isolates grew at 5 percent sodium chloride, roughly 40 percent tolerated 10 to 15 percent, and about 90 percent resisted oxidative stress induced by 1 millimolar hydrogen peroxide, with the melanized species showing the highest resistance.

These traits matter for biotechnology. Marine yeasts are known to produce robust enzymes, ferment in seawater, accumulate lipids, synthesize antimicrobials and anticancer compounds, and degrade environmental pollutants. Their resilience stems from complex physiological machinery, including mitogen-activated protein kinase signaling pathways that orchestrate adaptive transcriptional responses to salinity, temperature, pH, and oxidative stress. The enzymatic repertoire documented in the Murcian isolates also signals a central ecological function: degrading complex polymers such as cellulose, chitin, and pectin within coastal ecosystems, thereby feeding carbon and nutrients back into the microbial food web. Strains with this level of enzymatic breadth and stress tolerance are promising candidates for industrial bioprospecting, bioremediation of contaminated coastal sites, and bioconversion processes.

The survey carries a clinical warning as well. Opportunistic pathogens of potential human concern — Candida parapsilosis, Exophiala oligosperma, Nakaseomyces glabratus, and Diutina catenulata — were predominantly detected at anthropogenically impacted sites, supporting their proposed value as bioindicators of marine contamination. Candida parapsilosis, which peaked at 25 percent relative abundance in summer, has been designated a high-priority fungal pathogen by the World Health Organization, underscoring the need to connect environmental monitoring with public health surveillance. The authors conclude that marine yeasts emerge from this study as both sensitive gauges of ecosystem health and reservoirs of robust strains with significant biotechnological potential, and they call for future work combining cultivation-based and culture-independent approaches to fully capture the diversity and metabolic capacities of this underexplored microbial frontier.

Subject of Research: Microbiology

Article Title: Unveiling the diversity, ecology, and biotechnological potential of culturable marine yeasts in Western Mediterranean coastal ecosystems

Article References: Martínez-Rodrigo, A., Núñez, A., Franco, A., Madrid, M., Vicente-Soler, J., Cansado, J., & Soto, T. (2026). Unveiling the diversity, ecology, and biotechnological potential of culturable marine yeasts in Western Mediterranean coastal ecosystems. IMA Fungus, 17, Article e182209. https://doi.org/10.3897/imafungus.17.182209

Image Credits: AI Generated

DOI: 10.3897/imafungus.17.182209

Keywords: marine yeasts, Mediterranean Sea, mycology, biotechnology, microbiology, biodiversity, Mar Menor, black yeast fungi, enzymes, bioprospecting, Unveiling, diversity