Lentic and lotic habitats as templets for fungal communities: traits, adaptations, and their significance to litter decomposition within freshwater ecosystems

Abstract

Decomposition of plant matter is a key ecosystem process and considerable research has examined plant litter decay processes in freshwater habitats. Fungi are common inhabitants of the decomposer microbial community and representatives of all major fungal phyla have been identified within freshwater systems. Development and application of quantitative methods over the last several decades have firmly established that fungi are central players in the decomposition of plant litter in freshwaters and are important mediators of energy and nutrient transfer to higher trophic levels. Despite the critical roles that fungi play in carbon and nutrient cycling in freshwater ecosystems, there are notable differences in the types and adaptations of fungal communities between lotic and lentic habitats. These differences can be explained by the wide range of hydrologic, physical, chemical and biological conditions within freshwater systems, all of which can influence the presence, type, and activity of fungal decomposers and their impact on litter decomposition. This following seeks to provide a brief overview of the types, adaptations, and role of fungi within lotic and lentic freshwater ecosystems, with a particular emphasis on their importance to litter decomposition and the key environmental conditions that impact their growth and decay activities. This discussion will specifically focus on fungal dynamics occurring on plant litter in forested headwater streams and emergent freshwater marshes, since published data concerning their role in these systems is considerably more abundant in comparison to other freshwater habitats. watershed (headwaters), where water begins its journey down slope in response to gravity. These headwater streams eventually connect with other streams that flow into catchment basins forming lentic freshwater ecosystems, such as ponds, lakes, and inland wetlands, or eventually coalesce further to form larger rivers that flow into coastal regions forming lakes (e.g., oxbows), floodplain habitats, and tidal marshes as freshwater transitions into the marine environment. Along this freshwater continuum, there are marked changes in hydrologic, physical, chemical, and biologic conditions, all of which forms a habitat templet (e.g., Townsend and Hildrew, 1994) that influences the presence, types, adaptations, and decay activities of fungal decomposers. Despite the critical roles that fungi play in carbon and nutrient cycling, there are notable differences in fungal communities between lotic and lentic freshwater habitats, which can be explained by the spatial and temporal heterogeneity in environmental conditions encountered within these systems. In streams and rivers, aquatic hyphomycetes are among the most wellrecognized and extensively studied fungal group. These fungi comprise an ecological assemblage of ~300-320 species (Shearer et al., 2007; Duarte et al., 2013b) that typically dominate the fungal communities associated with decaying plant litter (Nikolcheva et al., 2005; Seena et al., 2008; Duarte et ...

Periphytic algae decouple fungal activity from leaf litter decomposition via negative priming

“…Our experiment provides empirical evidence of negative priming because algae increased fungal production but suppressed leaf litter dry mass loss rates—a notable decoupling, since aquatic fungi (i.e., hyphomycetes which dominate in flowing environments) are considered major drivers of plant litter decomposition in stream ecosystems (Gessner et al, ; Kuehn, ; Romaní et al, ; Suberkropp & Chauvet, ). At a mechanistic level, algae may suppress litter decomposition through two effects, one apparent and one actual: (a) accrual of new algal biomass could counterbalance mass lost due to heterotrophic degradation of litter C, thereby reducing apparent decomposition, and (b) preferential substrate use of algal‐derived labile C substrates by heterotrophs could reduce actual heterotrophic decomposition of litter (Guenet et al, ; Halvorson et al, ).…”

“…Our experiment provides empirical evidence of negative priming because algae increased fungal production but suppressed leaf litter dry mass loss rates-a notable decoupling, since aquatic fungi (i.e., hyphomycetes which dominate in flowing environments) are considered major drivers of plant litter decomposition in stream ecosystems (Gessner et al, 2010;Kuehn, 2016;Romaní et al, 2006;Suberkropp & Chauvet, 1995). At a mechanistic level, algae may suppress litter decomposition through two effects, one apparent and one actual:…”

Changes in nutrient stoichiometry, elemental homeostasis and growth rate of aquatic litter-associated fungi in response to inorganic nutrient supply

“…Unfortunately, data on elemental stoichiometry and the degree of homeostasis in aquatic litter-associated microorganisms are scarce (Newell and Statzell-Tallman, 1982;Sanzone et al, 2001;Findlay et al, 2002). Virtually nothing is known about the stoichiometry, elemental homeostasis and responses to nutrients in aquatic fungi, despite their key role in litter processing (Gessner et al, 2007;Danger et al, 2016;Kuehn, 2016). Fungal biomass accrual and associated immobilization of N and P lead to strong control by fungi of C:N and C:P stoichiometry of decomposing plant litter (Tant et al, 2013;Cornut et al, 2015;Manning et al, 2015).…”

“…Thus, from the perspective of stoichiometric imbalance, and because carbon use efficiency of leaf litter-associated fungi is often quite high (averaging around 33%, but reaching 60% at high N and P availability; Suberkropp, 1991;Gulis and Suberkropp, 2003c), these microorganisms must either retain N and P from organic substrates more efficiently than C, or immobilize inorganic nutrients from stream water. Fungi, such as aquatic hyphomycetes, that dominate microbial communities on decaying plant litter in streams (Gessner et al, 2007;Kuehn, 2016) are known to obtain N and P from both their substrate and the water column (Suberkropp, 1995;Cheever et al, 2013). Mining N and P from organic substrates requires considerable energy and resource expenditure to produce extracellular enzymes to attack recalcitrant molecules and cleave amino or phosphate groups (Sinsabaugh et al, 2014).…”

High interspecific variation in nutrient excretion within a guild of closely related caddisfly species

“…This positive relationship is stoichiometrically intuitive because the nutritional value of detritus is attributed to microbial colonization, including epidetrital algae (Bärlocher , Moore et al. , Kuehn ). For instance, biofilms on submerged sedge contained more P (87.8 μg/mg) and twice as much N (21.3 μg/mg) than the sedge itself (51.1 μg P/mg and 9.52 μg N/mg).…”