Stoichiometric traits of Microbial species and communities for predicting freshwater ecosystem responses to global changes – StoichioMic

After conceptually considering the causes and consequences of elemental plasticity in microbial biomass, the elemental composition of several strains of aquatic hyphomycetes (aquatic fungi) was experimentally assessed by manipulating mineral element inputs to culture media, with various sources of carbon (dissolved or particulate). Secondly, in order to understand the impact of the elemental composition of microorganisms on detritus mineralization processes, microbial assemblages and natural communities were incubated under controlled conditions along gradients of nitrogen or inorganic phosphorus. These incubations were carried out under more or less stressful conditions (ambient or heated temperatures, presence or absence of antifungal contaminants - copper), enabling us to estimate the effect of these stressors on recycling processes. Finally, experiments were carried out in artificial rivers in which nutrients and temperature were manipulated, in the presence or absence of competitors (benthic microalgae) and predators (detritivorous invertebrates).

The results of this project show that microorganisms are plastic in terms of elemental composition - more so for phosphorus than for nitrogen - but that the microorganisms have an optimal elemental ratio that maximizes their activity. These optimal ratios are significantly impacted by the environmental stressors tested (see Figure showing cellulose decomposition in the presence of Copper), predictably modifying the needs of the microorganisms. These results will make it possible to parameterize carbon cycle prediction models more reliably, as well as ecosystem functioning models more generally.

The results of the StoichioMic project provide a new vision of the determinants and consequences of the stoichiometric plasticity of decomposing microorganisms in the functioning of ecosystems. These results will undoubtedly be used to parameterize models of recycling and storage of organic matter, both in soils and in aquatic environments. This project has also opened up many new questions, including the question of the development of bio-ecological trait approaches for aquatic fungi and questions relating to the forms in which detrital matter is recycled by decomposers.

The results of this Young Researcher project have led to the publication of 5 scientific articles and the training of several students (2 Master 2 and a doctoral thesis). This project has also enabled the initiation of international collaborations, including the construction of a database of aquatic fungi traits currently underway on a European scale.

In ecosystems, plants and microbial decomposers are generally considered as the two most important biological components of ecosystems, plants furnishing organic carbon and energy to all food webs components, and decomposers permitting to recycle, through their mineralization activity, dead organic matter. Despite its fundamental importance for ecosystem functioning, microbial mineralization of detritus remains hardly predictable, most probably because microbial decomposers are also able, depending on environmental conditions, to immobilize inorganic nutrients from their environment to fulfill their nutrient stoichiometric demand. To date, most models aimed at understanding or predicting nutrient recycling ensured by microbial decomposers have considered fixed decomposers’ elemental ratios. Yet, several studies showed that many decomposer species (including both bacteria and fungi) are largely variable in their elemental contents. In this context, determining the balance between nutrient immobilization and mineralization of microbial communities appears far from being trivial. The development of approaches based upon stoichiometric traits of microbial species and communities might represent a promising way to solve this question.
Focusing on stream microbial decomposers of leaf litter (firstly aquatic hyphomycetes, then more complex, natural communities), the StoichioMic project first aims at understanding in further details the elemental plasticity of microbial decomposers, at the specific and at the community level, and its consequences on the balance between nutrient immobilization and nutrient recycling (Task 1). In a second step, StoichioMic will furnish important data for understanding and predicting the response of microbial decomposers elemental plasticity to a selection of global change parameters and, in turn, consequences on nutrient immobilization and recycling (Task 2). Among these parameters, temperature increase, changes in carbon quality, and occurrence of contaminants will be specifically investigated, these stressors being certainly among the most important currently impacting microbial processes. Finally, StoichioMic will permit to investigate how these responses are modulated by the presence of competitors (plants) and predators (detritivores), rendering the results of the project more transposable to field situations (Task 3).
StoichioMic will bring new insights into the understanding of microbial ecology and, in particular, on the determinism of microbial community structure and on ecological processes ensured by microorganisms. In particular, StoichioMic should give experimental and theoretical evidences of the importance of microorganisms’ elemental plasticity on the intensity of nutrient mineralization and immobilization. These parameters should be of particular interest for predicting ecosystem services ensured by microorganisms in ecosystems, in particular nutrient retention and recycling. StoichioMic should also give some practical information for ecosystem managers, such as a new potential bioindicator of nitrogen and phosphorus bioavailability in streams, and information concerning the nature of the leaf litter species maximizing nutrient retention in streams. StoichioMic will also give fundamental information for predicting the consequences of diverse global change parameters, including climate change, eutrophication, and contaminations, on microbial communities and C, N and P biogeochemical cycles in ecosystems. This ambitious and original project will be included in the well-established Ecological Stoichiometry conceptual framework that permits to relate organism physiology to community structures and explicitly considers organisms’ elemental contents and requirements for predicting consequences on C, N and P biogeochemical cycles.