Quantification and correlation of fungal C, N and P contents in nutrient manipulation experiments
In homeostatic fungal growth, not only would the ratio of elements but also the relative concentration of each element in mycelia have to remain constant. In this scenario, fungal size would be the only determinant of respective total element masses in mycelia, irrespective of nutrient supply, i.e. the larger a fungus grows the more elements it accumulates. This was the case for the total mass of fungal C, which showed a strong linear correlation with biomass (Fig. 3a). Comparing model sums of squares (see pie charts Fig. 3), biomass was the primary determinant for total C mass (Fig. 3d), except for a weak treatment effect of glucose fertilization in SEA (Fig. 3a, Fig. S7l). Total N and P masses, however, were less strongly coupled to fungal biomass production. The correlation with fungal biomass was also positive, but R² and slope values were much lower (Fig. 3b,c). Mostly, nutrient manipulations exerted stronger impacts on total fungal N and P masses than biomass production (see pie charts Fig. 3; Fig. 3e,f)). Thus, contrary to the homeostatic assumption that C, N and P concentrations remain constant within fungal mycelia independent of environmental conditions, fungal mycelia must have physiological mechanisms to reduce or increase P and N concentrations, while C remains relatively constant (Fig. S7).
Since fungal N:P ratios were more stable than C:nutrient ratios in all media types, fungal N and P must have responded to nutrient manipulations in a concerted way, even though only the availability of one element changed (Fig. 1, 2). Correlating the relative reductions of N and P concentrations in fungal mycelia within the different applied nutrient gradients supported this conclusion (Fig. 4). For example, along a gradient of C:N manipulated in defined glucose medium, under conditions of low N supply the proportion of N in fungal mycelia decreased on average to 31 % of its maximum values. In parallel, even though the P supply in the medium was not experimentally manipulated, the mycelium P concentrations was reduced to an average of 41 % of its highest value, resulting in a correlation with an R² of 0.66 and a slope as high as 0.67 between fungal N and P concentrations (Fig. 4a). In case of N manipulations, the parallel shift in P concentration was comparably strong for all media tested (Fig. 4a, c, e, Fig. S7), whereas in P manipulated media the parallel shift in N was slightly weaker, indicated by lower slope values (Fig. 4b, d). Still, correlations were also very clear. None of the elements positively correlated with C concentrations, which slightly increased in N and P limited conditions in some isolates in glucose media (Fig. S7c,f) and raised from 35.5 ± 1.9 % (mean ± SE) in SEA to 44.3 ± 2.2 % in SEA + Glu (P < 0.001) (Fig. S7l).