Culturable yeasts associated to different life stages of the farmed red seaweed Porphyra dioica

Abstract

Macroalgal-associated microbiomes are increasingly recognized as key determinants of host fitness, biogeochemical cycling, and cultivation stability in industrial aquaculture. While bacterial assemblages of red macroalgae have received considerable attention, the associated mycobiome, and in particular the yeast fraction, remains poorly characterized, despite mounting evidence that fungal taxa contribute to phytohormone signaling, surface colonization dynamics, and host stress tolerance. In this study, we characterized the culturable yeast fraction associated with five life stages of the red macroalga Porphyra dioica cultivated in an Integrated Multi-Trophic Aquaculture (IMTA) system in Ílhavo, Portugal. Yeast isolates were recovered from conchocelis, conchosporangia, young blade, and adult blade stages by direct plating and enrichment on selective media and taxonomically assigned by rRNA gene sequencing. Isolates were further screened for functional traits of ecological and biotechnological relevance, including auxin biosynthesis, biosurfactant and bioemulsifier production, and intracellular lipid accumulation. A total of 21 isolates were recovered and assigned to Rhodotorula mucilaginosa, Naganishia diffluens, Williopsis vartiovaarae, Taphrina sp., and Sporobolomyces roseus, spanning both Ascomycota and Basidiomycota. Yeast recovery was constrained by chloramphenicol-resistant bacterial overgrowth in algae samples cultivated with filtered, non-sterilized seawater, and therefore the dataset represents the recoverable culturable fraction. Assemblages were structured by host developmental stage: N. diffluens was recovered from conchocelis, R. mucilaginosa from conchosporangia, and a shift from ascomycetous taxa (W. vartiovaarae, Taphrina sp.) to pigmented basidiomycetous yeasts (N. diffluens, S. roseus) was observed in adult blades approaching harvest. Several isolates produced auxin, biosurfactants, and bioemulsifiers, and accumulated intracellular lipids, supporting potential roles as probiotics in industrial macroalgal cultivation and as candidate strains for value-added bioproducts. This study provides a first insight into the culturable yeast community associated with industrially cultivated P. dioica and establishes a basis for future studies addressing the role of yeasts in macroalgal performance cultivated under IMTA systems.