Daryl L. Moorhead

Functional diversity of microbial communities: A quantitative approach

Abstract Evaluating the biodiversity of microbial communities remains an elusive task because of taxonomic and methodological difficulties. An alternative approach is to examine components of biodiversity for which there exists a reasonable chance of detecting patterns that are biologically meaningful. One such alternative is functional diversity. We propose a procedure based on the Biolog identification system to quickly, effectively, and inexpensively assess aspects of the functional diversity of microbial communities. The numbers and types of substrates utilized by bacterial communities, as well as the levels of activities on various substrates and patterns of temporal development, constitute an information-rich data set from which to assess functional diversity. Data from six plant communities (black grama grassland. Sporobolus grassland, creosotebush bajada, herbaceous bajada, mesquite-playa fringe, and playa grassland) located along an elevational and moisture gradient at the Jornada Long-Term Ecological Research site in the northern Chihuahuan Desert, are analyzed to illustrate the procedure and its relevance to biodiversity. Our analyses demonstrate that the Biolog system can detect considerable variation in the ability of microbial communities to metabolize different carbon compounds. Variation in substrate use was compartmentalized differently along the moisture gradient. Differences in functional diversity were dependent upon the class of carbon sources (guild-specific results). A multifaceted approach to biodiversity that comprises both functional and taxonomic perspectives represents fertile ground for future research endeavors.

A THEORETICAL MODEL OF LITTER DECAY AND MICROBIAL INTERACTION

Despite the central role of microorganisms in the decomposition of dead organic matter, few models have integrated the dynamics of litter chemistry with microbial interactions. Here we propose a functional resolution of the microbial community that parallels the commonly used chemical characterization of plant litter, i.e., a guild of opportunist microorganisms that grows quickly and has high affinity for soluble substrates, a guild of decomposer specialists that grows more slowly and has high affinity for holocellulose substrates, and a guild of miners that grows very slowly and is specialized for degrading lignin. This guild-based decomposition model (GDM) includes the interactions of holocellulose and lignin, manifest as mutual feedback controls on microbial-based activities. It also includes N limitations on early stages of litter decay resulting from nutritional demands of microorganisms and N inhibition on late stages of litter decay resulting from reduced lignin degradation. Competitive interaction...

Antarctic climate cooling and terrestrial ecosystem response

The average air temperature at the Earths surface has increased by 0.06 °C per decade during the 20th century, and by 0.19 °C per decade from 1979 to 1998. Climate models generally predict amplified warming in polar regions, as observed in Antarcticas peninsula region over the second half of the 20th century. Although previous reports suggest slight recent continental warming, our spatial analysis of Antarctic meteorological data demonstrates a net cooling on the Antarctic continent between 1966 and 2000, particularly during summer and autumn. The McMurdo Dry Valleys have cooled by 0.7 °C per decade between 1986 and 2000, with similar pronounced seasonal trends. Summer cooling is particularly important to Antarctic terrestrial ecosystems that are poised at the interface of ice and water. Here we present data from the dry valleys representing evidence of rapid terrestrial ecosystem response to climate cooling in Antarctica, including decreased primary productivity of lakes (6–9% per year) and declining numbers of soil invertebrates (more than 10% per year). Continental Antarctic cooling, especially the seasonality of cooling, poses challenges to models of climate and ecosystem change.

Resource allocation to extracellular enzyme production : a model for nitrogen and phosphorus control of litter decomposition

Abstract Most models for plant litter decomposition link degradation rates to measures of climate or litter composition, rather than directly to microbial activity. We developed a model based on the premise that saprotrophic microbial communities maximize their productivity by optimizing their allocation of resources in the production of extracellular carbon, nitrogen and phosphorus-acquiring enzymes. In this model, enzyme activity indicators are used to estimate decomposition rates and to assess relative N and P availability. This approach may facilitate estimation of decomposition rates in the field and improve ecological forecasting.

Carbon use efficiency of microbial communities: stoichiometry, methodology and modelling.

Carbon use efficiency (CUE) is a fundamental parameter for ecological models based on the physiology of microorganisms. CUE determines energy and material flows to higher trophic levels, conversion of plant-produced carbon into microbial products and rates of ecosystem carbon storage. Thermodynamic calculations support a maximum CUE value of ~ 0.60 (CUE max). Kinetic and stoichiometric constraints on microbial growth suggest that CUE in multi-resource limited natural systems should approach ~ 0.3 (CUE max /2). However, the mean CUE values reported for aquatic and terrestrial ecosystems differ by twofold (~ 0.26 vs. ~ 0.55) because the methods used to estimate CUE in aquatic and terrestrial systems generally differ and soil estimates are less likely to capture the full maintenance costs of community metabolism given the difficulty of measurements in water-limited environments. Moreover, many simulation models lack adequate representation of energy spilling pathways and stoichiometric constraints on metabolism, which can also lead to overestimates of CUE. We recommend that broad-scale models use a CUE value of 0.30, unless there is evidence for lower values as a result of pervasive nutrient limitations. Ecosystem models operating at finer scales should consider resource composition, stoichiometric constraints and biomass composition, as well as environmental drivers, to predict the CUE of microbial communities.

Climate and litter quality controls on decomposition: An analysis of modeling approaches

Four mathematical models simulated decay of two litter types of contrasting quality over a 2-year period at four sites in North America. The litter types were Drypetes glauca and Triticum aestivum, representing litter with high and low nitrogen:lignin ratios, respectively. The field sites were an Arctic tussock tundra (Alaska, United States), a warm desert (New Mexico, United States), a temperate deciduous forest (New York, United States) and a tropical rain forest (Puerto Rico). Models captured the overall patterns of site and litter quality controls on decomposition; both simulated and observed mass losses were higher in warm, moist environments (both forests) than in cold (tundra) or dry sites (desert), and simulated and observed decay was more rapid for Drypetes than Triticum. However, predictions tended to underestimate litter mass loss in the tropical forest and overestimate decay in the desert and tundra, suggesting that site controls in model formulations require refinement for use under such a broad range of conditions. Also, predicted nitrogen content of litter residues was lower than observed in Drypetes litter and higher than observed for Triticum. Thus mechanisms describing loss of nitrogen from high-quality litter and nitrogen immobilization by low-quality litter were not captured by model structure. Individual model behaviors revealed different sensitivities to controlling factors that were related to differences in model formulation. As these models represent working hypotheses regarding the process of litter decay, results emphasize the need for greater resolution of climate and litter quality controls. Results also demonstrate the need for finer resolution of the relationships between carbon and nitrogen dynamics during decomposition.

Inorganic N and P dynamics of Antarctic glacial meltwater streams as controlled by hyporheic exchange and benthic autotrophic communities

Abstract The McMurdo Dry Valleys of South Victoria Land, Antarctica, contain numerous glacial meltwater streams that drain into lakes on the valley floors. Many of the streams have abundant perennial mats of filamentous cyanobacteria. The algal mats grow during streamflow in the austral summer and are in a dormant freeze-dried state during the rest of the year. NO3 and soluble reactive P (SRP) concentrations were lower in streams with abundant algal mats than in streams with sparse algal mats. NO3 and SRP concentrations were higher in the hyporheic zone of a stream with abundant algal mats than in the stream itself. An experimental injection of LiCl, NaNO3, and K3PO4 was conducted in Green Creek, which has abundant algal mats. Substantial hyporheic exchange occurred. The NO3 and PO4 concentrations at 50 m below the injection were 55 μM and 18 μM, respectively, during the experiment. NO3 and PO4 concentrations were below the detection limit of 1 to 2 μM at a site 497 m below the injection during the Cl tracer arrival, indicating a high capacity for nutrient uptake by algal communities. NO2 and NH4 were present at sites 226 and 327 m below the injection, indicating that, in addition to denitrification and algal uptake, dissimilatory NO3 reduction to NO2 and NH4 may be a NO3 sink during transport. Transport modelling with nutrient uptake represented as a 1st-order process yielded reach-scale parameters of 4.3 × 10−5 to 3.9 × 10−4/s and 1.4 × 10−4 to 3.8 × 10−4/s for uptake of NO3 and PO4, respectively. The best match with the observed data was a model in which PO4 uptake occurred only in the main channel and NO3 uptake occurred in the main channel and in the hyporheic zone. Hyporheic NO3 uptake was 7 to 16% of the total uptake for the different stream reaches. These results demonstrate that nutrient flux to the lakes is controlled by hyporheic exchange and nutrient uptake by algal mats in dry valley streams. Streams without algal mats contribute more nutrients to the lakes than streams with algal mats.

Effects of increasing ultraviolet B radiation on decomposition and soil organic matter dynamics: a synthesis and modelling study

The net effect of increasing ultraviolet B radiation levels on ecosystems is unknown. Most of the relevant ecological research has focused on the responses of living plants and algae to ultraviolet B exposure, with little attention directed toward other groups. However, research in such diverse areas of study as the degradation of textiles, pigments, synthetic polymers, paper, cellulose, wood, and museum artifacts show that ultraviolet light is a significant factor in the decay of many organic compounds. In aquatic ecosystems, the photochemical degradation of recalcitrant, dissolved organic compounds is increased by ultraviolet B exposure, and similar reactions could make important contributions to organic matter turnover in terrestrial ecosystems. This hypothesis is supported by observed patterns of decomposition of exposed surface litter in arid and semi-arid environments. Since plant lignins are both photochemically reactive and form a significant component of soil organic matter, ultraviolet B-induced lignin degradation could alter material cycling in terrestrial ecosystems. However, results of a model simulating the potential effects of ultraviolet B-induced lignin degradation suggest that higher rates of litter turnover may have only slight effects on soil organic matter dynamics.

A general model of litter decomposition in the northern Chihuahuan Desert

Abstract Numerous empirical studies have described the pathways of mass, C and N flows during decomposition, but there remains a paucity of data on underlying mechanisms in arid ecosystems. In the northern Chihuahuan Desert, termites remove large quantities of litter and act as carbon and nitrogen sinks, contributing to low soil fertility. In their absence, decomposition at the soil surface is primarily driven by abiotic weathering, but studies suggest buried litter decay occurs through microbiological activities. We develop a general, synthetic model to examine the interactions between buried litter, decomposer microorganisms, and C and N pools in this ecosystem. Our goal is to explore the mechanisms underlying observed patterns of decomposition in arid systems using a modelling approach that balances simplicity with enough detail to suggest the reasons for system behavior. To this end, we utilize elements of existing models, interfacing microbial physiology and population dynamics with empirical observations of C and N pool dynamics, litter mass loss and changing C:N ratios. Good agreement was achieved between simulated and observed patterns of mass loss and nitrogen concentrations once a time lag describing the microbial colonization of litter was included. Model results indicate nutrient availabilities may be determined by relatively short-term carbon dynamics mediated by microflora since soil organic matter and nitrogen content are low. Model behavior also suggests decomposer organisms immobilize nitrogen from surrounding soils, accounting for the elevated quantities observed within decaying materials. Past hypotheses have proposed that soil flora and fauna partially decouple decomposition processes from abiotic constraints in this system. This study indicates that the pattern of microbial activities, accounting for the decomposition of buried materials in the absence of termites, is primarily determined by climatic conditions.

The enzymic basis of plant litter decomposition: emergence of an ecological process

Abstract Plant litter decomposition is an integral process in the macronutrient cycles of ecosystems. The absence of fine-scale models for this process hampers attempts to simulate ecosystem responses to disturbance. Two recent studies have suggested that mass loss from decomposing plant litter can be directly related to the temporally-integrated activity of extracellular lignocellulose-degrading enzymes. To evaluate the generality and potential application of such relationships, we surveyed the literature and found eight studies that included mass loss data and some enzymic measure of lignocellulose degradation potential. Although the number of suitable studies was small, they encompassed a broad range of ecosystem and litter types. For all studies, there were strong linear relationships between temporally integrated enzyme activity, expressed as cumulative activity-days, and mass loss. No single enzyme gave the best fit in all cases; multiple linear regressions that included all enzymes measured within a particular study generally yielded better goodness of fit statistics than single enzyme models. Where methodological compatibility permitted, direct comparisons of apparent enzymatic efficiency (relative mass loss/activity-day) were made between studies; values for particular enzymes varied by a factor of ten and were strongly correlated with mean exposure temperature ( r 2 = 0.88). Activation energy for enzymatic decomposition was estimated at 58 kJ mol −1 . Principal components analysis (PCA) was used to generate a composite lignocellulase variable from each study, providing a common format for comparison. The results suggested that the three microcosm studies differed from the field investigations: enzymatic efficiency was approximately half that estimated for field studies and the data were more stochastic. We attributed these differences to disruptions in microbial succession caused by the lack of exogenous sources of colonists, nutrients and grazers. PCA also permitted the calculation of a global regression model for mass loss as a function of cumulative lignocellulase activity. For the five field studies, this model had an r 2 value of 0.73 in linear form and 0.76 in logarithmic form. Our analyses suggest that enzymatic decomposition models retain predictive value even when viewed from the ecosystem perspective. This hierarchal penetrance suggests useful applications: as a monitoring tool for the estimation of litter turnover in the field and as a basis for the simulation of decomposition processes at the microbial community level.