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).