Brown Meets Green: Light and Nutrients Alter Detritivore Assimilation of Microbial Nutrients From Leaf Litter

In aquatic detrital-based food webs, research suggests that autotroph-heterotroph microbial interactions exert bottom-up controls on energy and nutrient transfer. To address this emerging topic, we investigated microbial responses to nutrient and light treatments during Liriodendron tulipifera litter decomposition and fed litter to the caddisfly larvae Pycnopsyche sp. We measured litter-associated algal, fungal, and bacterial biomass and production. Microbes were also labeled with 14C and 33P to trace distinct microbial carbon (C) and phosphorus (P) supporting Pycnopsyche assimilation and incorporation (growth). Litter-associated algal and fungal production rates additively increased with higher nutrient and light availability. Incorporation of microbial P did not differ across diets, except for higher incorporation efficiency of slower-turnover P on low-nutrient, shaded litter. On average, Pycnopsyche assimilated fungal C more efficiently than bacterial or algal C, and Pycnopsyche incorporated bacterial C more efficiently than algal or fungal C. Due to high litter fungal biomass, fungi supported 89.6–93.1% of Pycnopsyche C growth, compared to 0.2% to 3.6% supported by bacteria or algae. Overall, Pycnopsyche incorporated the most C in high nutrient and shaded litter. Our findings affirm others\u27 regarding autotroph-heterotroph microbial interactions and extend into the trophic transfer of microbial energy and nutrients through detrital food webs

Detrital Nutrient Content and Leaf Species Differenitally Affect Growth and Nutritional Regulation of Detritivores

© 2018 The Authors Resource nutrient content and identity are common bottom–up controls on organismal growth and nutritional regulation. One framework to study these factors, ecological stoichiometry theory, predicts that elevated resource nitrogen (N) and phosphorus (P) contents enhance organism growth by alleviating constraints on N and P acquisition. However, the regulatory mechanisms underlying this response – including whether responses depend on resource identity – remain poorly understood. In this study, we tested roles of detrital N and P contents and identity (leaf species) in constraining growth of aquatic invertebrate detritivores. We synthesized results from seven detritivore species fed wide nutrient gradients of oak and maple detritus in the laboratory. Across detritivore taxa, we used a meta-analytic approach quantifying effects of detrital leaf species and N and P contents on growth, consumption, and N- and P-specific assimilation and growth efficiencies. Detritivore growth rates increased on higher-N and P detritus and on oak compared to maple detritus. Notably, the mechanisms of improved growth differed between the responses to detrital nutrients versus leaf species, with the former driven by greater consumption rates despite lower assimilation efficiencies on higher-nutrient detritus, and the latter driven by improved N and P assimilation and N growth efficiencies on oak detritus. These findings suggest animal nutrient acquisition changes flexibly in response to resource changes, altering the fate of detrital N and P throughout regulation. We affirm resource identity and nutrients as important bottom–up controls, but suggest these factors act through separate pathways to affect organism growth and thereby change detrital ecosystems under anthropogenic forest compositional change and nutrient enrichment

Global Patterns and Controls of Nutrient Immobilization On Decomposing Cellulose In Riverine Ecosystems

Microbes play a critical role in plant litter decomposition and influence the fate of carbon in rivers and riparian zones. When decomposing low-nutrient plant litter, microbes acquire nitrogen (N) and phosphorus (P) from the environment (i.e., nutrient immobilization), and this process is potentially sensitive to nutrient loading and changing climate. Nonetheless, environmental controls on immobilization are poorly understood because rates are also influenced by plant litter chemistry, which is coupled to the same environmental factors. Here we used a standardized, low-nutrient organic matter substrate (cotton strips) to quantify nutrient immobilization at 100 paired stream and riparian sites representing 11 biomes worldwide. Immobilization rates varied by three orders of magnitude, were greater in rivers than riparian zones, and were strongly correlated to decomposition rates. In rivers, P immobilization rates were controlled by surface water phosphate concentrations, but N immobilization rates were not related to inorganic N. The N:P of immobilized nutrients was tightly constrained to a molar ratio of 10:1 despite wide variation in surface water N:P. Immobilization rates were temperature-dependent in riparian zones but not related to temperature in rivers. However, in rivers nutrient supply ultimately controlled whether microbes could achieve the maximum expected decomposition rate at a given temperature

Algal-Driven Priming of Cellulose Decomposition Along a Phosphorus Gradient In Stream Mesocosms

Primary producer-based and detrital-based energy pathways in streams are intertwined. Interactions between algae and heterotrophic microorganisms may accelerate or inhibit recalcitrant organic detritus decomposition depending on the environmental conditions. The magnitude and direction of this priming effect have been attributed to nutrient availability, but the influence of nutrients can be difficult to disentangle. To date, no studies have examined priming across a single-nutrient gradient where every other variable is tightly controlled. The purpose of this study was to investigate the role of algal priming on decomposition of standardized cellulose substrates across an extensive phosphorus (P) gradient in laboratory mesocosms. We placed unbleached cotton strips into 11 recirculating mesocosms, each divided into a light section and a dark section. Each mesocosm was subjected to a different P concentration ranging from 1 to 1024 µg P/L, with all other nutrients maintained at saturating concentrations. P stimulated algal biomass in the light, but fungal biomass responded more to P in the dark. Decomposition, measured as loss of tensile strength (an indicator of cellulose catabolism), and extracellular cellulase enzyme activities were lower in the light treatment than the dark treatment above ∼16 μg P/L, indicating a suppression of decomposition by algae. This negative priming was likely due to preferential use of algal exudates by fungi in the light treatments under P-replete conditions, which may be caused by maximum algal accrual and possibly bulk labile organic matter exudation above these concentrations. A lack of positive priming in this study may have been due to negligible nutrient concentrations in the cotton and saturating nitrogen (N) concentrations in the water, suggesting that positive priming is primarily associated with N mining and, to a lesser extent, P mining. This study highlights how complex interactions between algae and microbial heterotrophs coupled with altered light and nutrient regimes may have consequences for stream food webs

Brown meets green: Light and nutrients alter detritivore assimilation of microbial nutrients from leaf litter

In aquatic detrital-based food webs, research suggests that autotroph-heterotroph microbial interactions exert bottom-up controls on energy and nutrient transfer. To address this emerging topic, we investigated microbial responses to nutrient and light treatments during Liriodendron tulipifera litter decomposition and fed litter to the caddisfly larvae Pycnopsyche sp. We measured litter-associated algal, fungal, and bacterial biomass and production. Microbes were also labeled with C-14 and P-33 to trace distinct microbial carbon (C) and phosphorus (P) supporting Pycnopsyche assimilation and incorporation (growth). Litter-associated algal and fungal production rates additively increased with higher nutrient and light availability. Incorporation of microbial P did not differ across diets, except for higher incorporation efficiency of slower-turnover P on low-nutrient, shaded litter. On average, Pycnopsyche assimilated fungal C more efficiently than bacterial or algal C, and Pycnopsyche incorporated bacterial C more efficiently than algal or fungal C. Due to high litter fungal biomass, fungi supported 89.6-93.1% of Pycnopsyche C growth, compared to 0.2% to 3.6% supported by bacteria or algae. Overall, Pycnopsyche incorporated the most C in high nutrient and shaded litter. Our findings affirm others\u27 regarding autotroph-heterotroph microbial interactions and extend into the trophic transfer of microbial energy and nutrients through detrital food webs