Extracellular Enzymatic Activities of Oceanic Pelagic Fungal Strains and the Influence of Temperature

Abstract

Although terrestrial and aquatic fungi are well-known decomposers of organic matter, the role of marine fungi remains largely unknown. Recent studies based on omics suggest that marine fungi potentially play a major role in elemental cycles. However, there is very limited information on the diversity of extracellular enzymatic activities performed by pelagic fungi in the ocean and how these might be affected by community composition and/or critical environmental parameters such as temperature. In order to obtain information on the potential metabolic activity of marine fungi, extracellular enzymatic activities (EEA) were investigated. Five marine fungal species belonging to the most abundant pelagic phyla (Ascomycota and Basidiomycota) were grown at 5 °C and 20 °C, and fluorogenic enzymatic assays were performed using six substrate analogues for the hydrolysis of carbohydrates (β-glucosidase, β-xylosidase, and N-acetyl-β-D-glucosaminidase), amino acids (leucine aminopeptidase), and of organic phosphorus (alkaline phosphatase) and sulfur compounds (sulfatase). Remarkably, all fungal strains were capable of hydrolyzing all the offered substrates. However, the hydrolysis rate (Vmax) and half-saturation constant (Km) varied among the fungal strains depending on the enzyme type. Temperature had a strong impact on the EEAs, resulting in Q10 values of up to 6.1 and was species and substrate dependent. The observed impact of temperature on fungal EEA suggests that warming of the global ocean might alter the contribution of pelagic fungi in marine biogeochemical cycles.

Planktonic Marine Fungi: A Review

“…Spores, yeasts (in suspension) and fragmented mycelia in suspension or as an agar plug/block are inoculated into a liquid assay medium in flasks. For yeast species, optical density of subsamples is measured periodically at 660 nm, which reflects growth (Heitger & Baltar, 2023; Salazar Alekseyeva et al., 2022). Filamentous fungi do not produce homogenous growth in liquid media.…”

“…Another approach involves measuring the concentration of ergosterol as a proxy for fungal biomass (Gessner, 2020), with a focus on Ascomycota and Basidiomycota (Dikarya). The ergosterol extraction method was recently adapted to marine fungi to allow for the quantification of fungal biomass in the oligotrophic (low productive) regions (Salazar Alekseyeva et al., 2022). However, the ergosterol‐based method falls short when dealing with Chytridiomycota taxa (Gutiérrez et al., 2020), which is important to consider during phytoplankton blooms.…”

“…Marine fungi participate in sulfur (S) cycling primarily via sulfate assimilation to S‐bearing amino acids (Morales et al., 2019; Sen et al., 2021). Nonetheless, extracellular mineralization of organic S (e.g., algal sulfate esters) by pelagic fungi (Ascomycota and Basidiomycota), such as the hydrolysis of sulfate esters present in algal cell walls, was recently reported in studies of fungal isolates from the Atlantic Ocean and the Antarctic Peninsula (Salazar Alekseyeva et al., 2022). The activity (hydrolysis rate and half‐saturation constant) of observed enzymes was temperature‐ and species‐dependent, and agrees with previous studies that described fungal sulfatases as thermo‐sensitive enzymes with an optimal temperature of 25°C (Korban et al., 2017).…”

Influence of Salinity on the Extracellular Enzymatic Activities of Marine Pelagic Fungi

“…For the enzymatic assays, sterile 96-well microplates with F bottom and low protein binding (XT64.1, Carl Roth Karlsruhe, Baden-Wurtemberg, Germany) were used. The protocol was based on Hoppe [45] and Salazar Alekseyeva, Herndl, and Baltar [67]. The MCA fluorophore was dissolved in 2-methoxyethanol to a final concentration of 100 µM, 50 µM, 10 µM, and 1 µM, whereas MUF was diluted to a final concentration of 2000 µM, 1000 µM, 100 µM, and 50 µM.…”

“…Several microorganisms, specially fungi, perform cellulolytic activities with three major classes of enzymes such as endoglucanases, exoglucanases, and β-glucosidases [72][73][74][75][76]. The hydrolysis of cellulose has been reported by a wide variety of marine fungi [67,75,[77][78][79][80][81][82][83][84], but their capability to decompose this complex compound depends on their enzymatic machinery [85]. Vaz, Rosa [86] and Gonçalves, Paço [87] showed that 76% and 68%, respectively, of the marine fungi they studied were able to release enzymes related to cellulolytic activity.…”