Mycelial internal retranslocation and shifted nutrient uptake explain stoichiometric flexibility
Strict homeostasis is typically assumed for heterotrophs, for example in metazoans with a determined body shape, whereas in addition to other mechanisms discussed below the modular indeterminate growth form of autotrophic plants allows more flexible adjustments (Güsewell 2004; Persson et al. 2010). In soil fungi, an indeterminate mycelial lifestyle may also permit stronger stoichiometric flexibility than expected. Fungi cannot only dynamically translocate elements within their mycelium depending on element demands (Watkinson et al.2006), but also internally recycle elements and cytoplasm. Mycelial tip growth is sustained by cytoplasm transport towards active hyphal tips (Moore et al. 2011), and recycling mechanisms of hyphal autolysis by intracellular degrading enzymes allow efficient mycelial expansion (Reyes et al. 1990; Lilly et al. 1991; Pusztahelyiet al. 2006). Given that N and P are more abundant in active cytoplasm, while C is also mainly bound in the hyphal wall structure, variable C:nutrient ratios in contrast to more stable N:P appear to be the logical outcome of this mycelial growth (only about 2 % of N is bound in cell walls compared to 60-70 % in proteins (Paustian & Schnürer 1987a; Peter 2005)) (Fig. 6). Not only the parallel shifts in fungal N and P, but also the spatial and temporal variability of C:N and C:P ratios in fungal mycelia observed here lend strong support to this idea, since N and P accumulated in the outer “active growth” zone and proportionately decreased with progressing mycelial expansion. Previous reports of more active outer mycelia (Zheng 2015) and temporal relative reductions in RNA and N and P concentrations (Levi & Cowling 1969; Newell & Statzell-Tallman 1982; Grimmett et al. 2013) also support this result. Whether the inner mycelium is dead or simply less active (and still involved in transport; Fricker et al. 2017) is currently unclear.
As a mechanism for non-homeostasis in microbial individuals, mainly P storage has been discussed so far (Fricker et al. 2008; Scottet al. 2012). In plants, by contrast, diverse additional processes are known, including shifted element uptake, nutrient resorption and investment in different tissues (Frost et al.2005). Our data as well as previous stoichiometric assessments in fungi also provide evidence for more complex mechanisms in fungi (Fig. 6; Levi & Cowling 1969). Resorption and efficient use of N and P is supported by the spatial stoichiometric flexibility (here retranslocation). However, the response to low N conditions (after only 12 days) was not driven by a simple reduction of the outer active zone, but may rather relate to small-scale spatial processes throughout the young growing mycelium (Fig. 6; see also Klein and Paschke (2004)). Beside a spatial reduction in active cytoplasm, the parallel shift in fungal N and P concentrations may also relate to reduced uptake (as described for plants) and a decline of both elements throughout the mycelium. Here, homeostatic N:P ratios simply relate to a tight coupling of the synthesis of N-rich proteins with P-demanding ribosomal activity (Loladze & Elser 2011).
Regarding element storage, the parallel reduction in N and P concentrations lend little support to N or P storage as the major mechanism of non-homeostasis. Only along the applied P gradient may polyphosphate stores have been depleted before reducing internal P (and N) concentrations below optima (Scott et al. 2012), which may also explain the lack of enzymatic responses to P limitation (Fig. S2). On the other hand, C storage may enhance wide C:element ratios, for example in glucose supplemented SEA media (Wilson et al. 2010). Future studies need to apply detailed chemical and microscopic analyses to reveal the contribution of these physiological mechanisms to mycelial non-homeostasis (Hall et al. 2011b).