The Functional Role of Fungi and Bacteria in Sulfur Cycling During Kelp (Ecklonia Radiata) Degradation: Unconventional Use of PiCrust2

Macroalgae is a major source of detritus in coastal ecosystems, contributing approximately 1521 ± 732 Tg C year⁻¹ to global net primary production. Fungal remineralisation of Ecklonia radiata detritus produces substantial amounts of dimethylsulfoniopropionate, total alkalinity, and dissolved inorganic carbon, supporting coastal biogeochemical cycles. To expand on the role of fungi during E. radiata degradation, we examined changes in fungal and bacterial communities at the start and after 21 days in a mesocosm, comparing microbial functional roles between blades and stipes. We employed next-generation sequencing to evaluate the potential contributions of fungi and bacteria, and additionally utilized FUNGuild, FungalTraits, and PiCrust2 databases. We cross-referenced the metabolic pathways predicted by PiCrust2 with the literature to determine whether these pathways have been documented in fungi. Of the 423 metabolic pathways identified, 342 have also been reported in fungi, including 281 redox-related pathways, 220 associated with nicotinamide adenine dinucleotide, and 194 linked to sulfur metabolism. These overlaps suggest that bacteria and fungi could play complementary roles in kelp degradation, contributing distinct yet interconnected functions. Our results highlight that these metabolic pathways cannot be attributed to bacteria alone and fungi are essential to kelp remineralisation.