Ecological stoichiometry of aquatic fungi: current knowledge and perspectives

Ecological stoichiometry investigates how the ratios of elements in organisms shape their ecology and nutrient and energy fluxes in ecosystems. Despite their global distribution and essential roles in nutrient cycling, microbial decomposers are among the least known organisms in terms of elemental concentrations and stoichiometric relationships. This review compiles information currently available on aquatic fungi and the role of stoichiometric constraints in fungal ecology. These data show that elemental ratios of fungal biomass vary widely, with ranges exceeding those found for bacteria. In part, this variability may be related to hyphal growth rates, according to the growth rate hypothesis, but results have been equivocal so far and could be partly attributed to limited fungal homeostasis. However, this issue requires further investigation before firm conclusions can be drawn. Much evidence indicates that aquatic fungi enhance the quality of leaf litter to consumers by lowering C:N or C:P ratios, thereby affecting the life history of consumers and promoting nutrient and energy transfer in aquatic ecosystems. In contrast, pertinent data to assess the importance of resource stoichiometry on aquatic fungal community structure appears to be lacking at present. Differences in the stoichiometric requirements of fungi vs bacteria could partly explain literature observations on stoichiometric determinants of fungal–bacterial interaction in aquatic ecosystems. Numerous perspectives for future research unfold when applying stoichiometric theory to aquatic fungi and their role in aquatic food webs and ecosystems.