Aureobasidium species are ascomycetous fungi belonging to the family Saccotheciaceae within the order Dothideales. They are dimorphic organisms, capable of growing both as yeasts and as filaments, and they produce dark, slimy colonies with micronematous conidiogenous cells and aseptate ellipsoidal conidia that bud in a yeast-like fashion. The genus is famously cosmopolitan: its members have been isolated from air, human skin, water, mangrove sediments, house dust, rocks, soil, wood, and food. Some species are polyextremotolerant, surviving in hypersaline water, glacial ice, deserts, frozen food, and radiation-polluted sites. Others are prized in biotechnology for producing enzymes and natural products, or deployed as biocontrol agents against plant pathogens such as Botrytis cinerea, Penicillium expansum, and Monilinia laxa. Aureobasidium pullulans even degrades aromatic pollutants and plastics. Yet despite this ecological and industrial prominence, the true species-level diversity of the genus in tropical regions, particularly on flowers, remained poorly explored.

Flowers are far from simple substrates. Their carpels, stamens, pollen, and nectaries create microenvironments rich in sugars and transient moisture, subject to intense biotic interactions with pollinators and other microbes. Previous studies had reported Aureobasidium pullulans from tiger lilies and monkshood in South Korea, from pink node flowers in India, from nectar of kidneyvetch and snapdragons in Spain, and from hummingbird flowers in the United States. In Brazil, A. pullulans and A. thailandense had been isolated from bromeliad inflorescences. But Thailand, one of the world’s most biodiverse countries with an estimated fifteen thousand plant species, had yielded only a handful of Aureobasidium reports before this survey. The research team, led by P. Kodchasee, C. Senwanna, J. Kumla, and N. Suwannarach of Chiang Mai University, set out to change that.

Between July and September 2024, the researchers randomly collected healthy flowers from thirty-one plant species across Mueang Chiang Mai, Phrao, and Mueang Phayao districts. The blossoms were placed in sterile bags, refrigerated, and processed within twenty-four hours. Using sterile forceps, the team excised internal floral parts including carpels, stamens, pollen, and nectaries, immersed them in sterile saline, vortexed the suspension, and spread it onto potato dextrose agar supplemented with chloramphenicol to suppress bacteria. After seventy-two hours of incubation at twenty-five degrees Celsius in darkness, colonies with the characteristic shiny appearance of Aureobasidium were selected and purified. In total, fifty-four strains were isolated, with the highest numbers recovered from Catharanthus roseus and Ocimum tenuiflorum, four strains each, followed by Jatropha integerrima, Pentas lanceolata, Plumeria obtusa, Senna spectabilis, and Tectona grandis with three strains each.

What followed was a polyphasic taxonomic analysis of unusual rigor. The researchers extracted genomic DNA from two-week-old cultures and amplified five loci: the internal transcribed spacer region, the twenty-eight S large ribosomal subunit, and the protein-coding genes rpb2, tef1, and tub. These sequences were compiled into a concatenated alignment of three thousand one hundred sixty-four characters spanning one hundred fifty-five taxa. Maximum likelihood analysis in RAxML and Bayesian inference in MrBayes produced congruent tree topologies. The resulting phylogeny placed the fifty-four strains into sixteen novel species and five previously known ones: A. albui, A. castaneae, A. melanogenum, A. musti, and A. tremulum. Four of those known species, A. albui, A. castaneae, A. musti, and A. tremulum, are recorded from Thailand for the first time. Most of the new species clustered within the A. melanogenum and A. thailandense species groups, while A. bougainvilleae, A. catharanthi, A. oroxyli, and A. tremulum formed a distinct well-supported clade of their own.

Delimiting species within morphologically plastic groups like Aureobasidium is notoriously difficult, and the team deployed multiple species-recognition frameworks to test their boundaries. Coalescent-based methods including Poisson tree processes in maximum-likelihood, Bayesian, and multi-rate implementations, together with the general mixed Yule-coalescent model, were applied to the phylogeny. The bPTP method delimited the highest number of putative species at forty-nine, followed by mlbPTP with thirty-nine and mPTP with twenty-three, while GMYC identified ninety-six species hypotheses. These methods were inconsistent with one another and with the multilocus phylogeny in several lineages, either over-lumping distinct taxa or over-splitting closely related strains. The researchers therefore turned to genealogical concordance phylogenetic species recognition, or GCPSR, using the pairwise homoplasy index test in SplitsTree4. The PHI results showed no evidence of significant recombination among the new species and their closest relatives in either the A. melanogenum group, with a value of 0.9965, or the A. thailandense group, with a value of 0.9612. Because values at or above 0.05 indicate genetic distinctness, these results provided strong independent confirmation that the newly delimited species represent evolutionarily isolated lineages rather than arbitrary clusters.

Morphological and physiological characterization added further discriminating power. The researchers examined colonies on potato dextrose, malt extract, and oatmeal agars, recording colors according to the Methuen Handbook of Colour and measuring microscopic structures including hyphae, conidiogenous cells, conidia, chlamydospores, and endoconidia. Cardinal growth temperatures were determined on malt extract agar at four, ten, twenty-five, thirty, and thirty-seven degrees Celsius. Carbon and nitrogen assimilation profiles, assessed using standardized yeast methodology, proved especially valuable. Glucose fermentation was negative for all sixteen novel species. All assimilated ammonium sulfate, ethylamine hydrochloride, L-lysine, cadaverine, and creatine as nitrogen sources, but none assimilated potassium nitrate or sodium nitrite. Distinctive carbon assimilation patterns separated closely related species: A. chiangmaiense was the only species able to assimilate inulin, A. oroxyli was the only one unable to assimilate N-acetyl-D-glucosamine and galactitol, and A. savitreeae produced light pink pigmentation and dark brown sediment when grown on D-glucose, D-xylose, L-arabinose, sucrose, raffinose, melezitose, and erythritol. Pigmentation and sediment formation during substrate utilization emerged as useful phenotypic fingerprints for distinguishing sister taxa.

The sixteen novel species carry names that reflect their hosts, habitats, and the people who inspired them. A. anthicola, A. florale, A. floricola, and A. florigenum all refer to their flower-dwelling lifestyle. A. bougainvilleae, A. catharanthi, A. oroxyli, and A. plumeriae honor the plant genera from which they were isolated. A. chiangmaiense and A. phayaoense reference the provinces where the fungi were found, while A. kwanphayaoense is named for Phayao Lake. A. lannaense invokes the historic Kingdom of Lanna, the old name for northern Thailand. A. pollinicola refers to its association with pollen, and A. siamense invokes Siam, the former name of Thailand. Two species, A. saisamorniae and A. savitreeae, are named in honor of Thai mycologists Saisamorn Lumyong and Savitree Limtong, celebrating their contributions to Thai mycology. Holotype specimens have been deposited in the Chiang Mai University Biology Department Herbarium, and ex-type living cultures are maintained at the Sustainable Development of Biological Resources Culture Collection, the Thailand Bioresource Research Center Culture Collection, and the Guizhou Medical University Culture Collection.

The ecological story behind this diversity is as compelling as the taxonomy. Flowers represent nutrient-rich yet selective microhabitats where sugar-rich nectar, fluctuating conditions, and short life spans favor fast-growing, stress-tolerant microorganisms. Aureobasidium possesses a suite of adaptive traits suited to this environment: osmotolerance in high-sugar nectar, resistance to ultraviolet radiation through melanin production, and secretion of extracellular polysaccharides that facilitate adhesion and biofilm formation. Floral structures also serve as sites of microbial exchange, where pollinators transfer microorganisms among plant hosts, promoting continual dispersal of Aureobasidium from diverse reservoirs into blossoms. This combination of ecological suitability, physiological adaptability, and dispersal dynamics likely explains why the genus thrives and diversifies in floral habitats. The researchers note that A. pullulans effectively colonizes stigmas and suppresses fire blight through competitive exclusion of the pathogen, suggesting that the newly discovered flower-associated species may play similar protective roles in tropical ecosystems.

The study demonstrates that flowers in northern Thailand harbor unexpectedly high Aureobasidium diversity, with twenty-one species recovered from thirty-one plant species. The application of a polyphasic approach integrating multilocus phylogenetic analyses, morphological and physiological characteristics, growth temperature profiles, and GCPSR supported by PHI tests provides robust species delimitation within this complex genus. The findings substantially expand knowledge of Aureobasidium taxonomy, diversity, and geographic distribution in tropical regions and underscore the ecological significance of flowers as key fungal niches. The researchers recommend that future taxonomic studies employ a standardized set of molecular markers including ITS, rpb2, and tef1, alongside broader geographic sampling, culture-independent methods, seasonal surveys, and genome-based approaches to fully characterize the diversity, distribution, and functional roles of flower-associated Aureobasidium. As tropical floral habitats continue to yield surprises, this discovery signals that the fungal kingdom’s species count, already estimated in the millions, remains far from complete.

Subject of Research: Mycology

Article Title: Unexpected diversity of flower-associated Aureobasidium reveals sixteen novel species from northern Thailand

Article References: Senwanna, C., Kodchasee, P., Kathongthung, C., Samarakoon, M. C., Khunnamwong, P., Kumla, J., & Suwannarach, N. (2026). Unexpected diversity of flower-associated Aureobasidium reveals sixteen novel species from northern Thailand. IMA Fungus, 17, Article e188009. https://doi.org/10.3897/imafungus.17.188009

Image Credits: AI Generated

DOI: 10.3897/imafungus.17.188009

Keywords: Aureobasidium, fungi, Thailand, taxonomy, flowers, biodiversity, phylogenetics, IMA Fungus, Unexpected, diversity, flower-associated, reveals