Relation of stoichiometric shifts with fungal growth and activity
In a defined glucose medium characterized by N or P limitations alone, varying fungal C:N:P values were good linear predictors of fungal growth and activity (Fig. S2, Table S4 and S6). In parallel to shifts in element concentrations, biomass production, fungal density and enzymatic activity were positively affected by higher N supply, as well as biomass and density by increasing P supply (Fig. S2). Thus, models examining the explanatory power of fungal C:N and C:P ratios distinguished them as good predictors of fungal growth, activity and the ratio of phosphatase and leucine-aminopeptidase production (Table S4 and S5). By contrast, in complex media with more diverse C sources, observed strong shifts in C:N and C:P values did not allow to predict N or P limitations. Along varying N supply with cellulose as a C source, fungal traits did not respond positively to increasing N availability, showing no indication of N limitation despite high fungal C:N values (Fig. S3). Likewise, in soil extract agar the addition of N and P alone did not affect biomass production or fungal density, nor did it show a significant interaction with the addition of cellulose or glucose (Fig. 2b, c). By contrast, cellulose and glucose additions shifted fungal biomass from 7.8 mg (± 2.7) to 19.9 mg (± 2.1) and 64.4 mg (± 10.6), respectively. Only enzymatic activity responded positively to N additions, while glucose strongly suppressed fungal enzymatic activity also in the presence of additional N (Fig. 2d). Consequently, stoichiometric ratios did not correlate with fungal growth or activity in cellulose and SEA media.