Soil enzyme kinetics and thermodynamics in response to long-term vegetation succession

Our current knowledge regarding soil organic matter (SOM) turnover during vegetation succession is often limited to conventional C decomposition models. However, microbial enzyme-mediated SOM degradation and nutrient cycling are mainly reflected in the kinetic parameters of these enzymes. Changes in the composition and structure of plant communities are typically accompanied by alterations in soil ecological functions. Therefore, it is important to clarify the kinetic parameters of soil enzymes and their temperature sensitivity in response to vegetation succession, especially under the current trend of climate change-related global warming; however, these are still understudied. Here, we examined the kinetic parameters of soil enzymes, their temperature sensitivity, and their associations with environmental variables over long-term (approximately 160 years) vegetation succession on the Loess Plateau using a space-for-time substitution method. We found that the kinetic parameters of soil enzymes changed significantly during vegetation succession. Specific response characteristics varied depending on the enzyme. Overall, the temperature sensitivity (Q₁₀, 0.79-1.87) and activation energy (E_a, 8.69-41.49 kJ·mol^-1) remained stable during long-term succession. Compared with N-acetyl-glucosaminidase and alkaline phosphatase, β-glucosidase was more sensitive to extreme temperatures. In particular, two kinetic parameters (i.e., maximum reaction rate, V_max; half-saturation constant, K_m) of β-glucosidase were decoupled at low (5 °C) and high (35 °C) temperatures. Overall, V_max was the primary determinant of variations of enzyme catalytic efficiency (K_cat) during succession, and soil total nutrients had a greater impact on K_cat than available nutrients. Our results suggested that soil ecosystems played an increasingly important role as a C source during long-term vegetation succession, as indicated by the positive responses of the C cycling enzyme K_cat, while the factors related to soil N and P cycling remained relatively stable.

The Coupling Response between Different Bacterial Metabolic Functions in Water and Sediment Improve the Ability to Mitigate Climate Change

Extreme climatic events, such as heat wave and large temperature fluctuations, are predicted to increase in frequency and intensity during the next hundred years, which may rapidly alter the composition and function of lake bacterial communities. Here, we conducted a year-long experiment to explore the effect of warming on bacterial metabolic function of lake water and sediment. Predictions of the metabolic capabilities of these communities were performed with FAPROTAX using 16S rRNA sequencing data. The results indicated that the increase in temperature changed the structure of bacterial metabolic functional groups in water and sediment. During periods of low temperature, the carbon degradation pathway decreased, and the synthesis pathway increased, under the stimulation of warming, especially under the conditions temperature fluctuation. We also observed that nitrogen fixation ability was especially important in the warming treatments during the summer season. However, an elevated temperature significantly led to reduced nitrogen fixation abilities in winter. Compared with the water column, the most predominant functional groups of nitrogen cycle in sediment were nitrite oxidation and nitrification. Variable warming significantly promoted nitrite oxidation and nitrification function in winter, and constant warming was significantly inhibited in spring, with control in sediments. Co-occurrence network results showed that warming, especially variable warming, made microbial co-occurrence networks larger, more connected and less modular, and eventually functional groups in the water column and sediment cooperated to resist warming. We concluded that warming changed bacterial functional potentials important to the biogeochemical cycling in the experimental mesocosms in winter and spring with low temperature. The effect of different bacteria metabolism functions in water column and sediment may change the carbon and nitrogen fluxes in aquatic ecosystems. In conclusion, the coupling response between different bacterial metabolic functions in water and sediment may improve the ability to mitigate climate change.

Decomposing decomposition: isolating direct effects of temperature from other drivers of detrital processing

Understanding the observed temperature dependence of decomposition (i.e., its "apparent" activation energy) requires separation of direct effects of temperature on consumer metabolism (i.e., the "inherent" activation energy) from those driven by indirect seasonal patterns in phenology and biomass, and by longer-term, climate-driven shifts in acclimation, adaptation, and community assembly. Such parsing is important because studies that relate temperature to decomposition usually involve multi-season data and/or spatial proxies for long-term shifts, and so incorporate these indirect factors. The various effects of such factors can obscure the inherent temperature dependence of detrital processing. Separating the inherent temperature dependence of decomposition from other drivers is important for accurate prediction of the contribution of detritus-sourced greenhouse gases to climate warming and requires novel approaches to data collection and analysis. Here, we present breakdown rates of red maple litter incubated in coarse- and fine-mesh litterbags (the latter excluding macroinvertebrates) for serial approximately one-month increments over one year in nine streams along a natural temperature gradient (mean annual: 12.8°-16.4°C) from north Georgia to central Alabama, USA. We analyzed these data using distance-based redundancy analysis and generalized additive mixed models to parse the dependence of decomposition rates on temperature, seasonality, and shredding macroinvertebrate biomass. Microbial decomposition in fine-mesh bags was significantly influenced by both temperature and seasonality. Accounting for seasonality corrected the temperature dependence of decomposition rate from 0.25 to 0.08 eV. Shredder assemblage structure in coarse-mesh bags was related to temperature across both sites and seasons, shifting from "cold" stonefly-dominated communities to "warm" communities dominated by snails or crayfish. Shredder biomass was not a significant predictor of either coarse-mesh or macroinvertebrate-mediated (i.e., coarse- minus fine-mesh) breakdown rates, which were also jointly influenced by temperature and seasonality. Unlike fine-mesh bags, however, temperature dependence of litter breakdown did not differ between models with and without seasonality for either coarse-mesh (0.36 eV) or macroinvertebrate-mediated (0.13 eV) rates. We conclude that indirect (non-thermal) seasonal and site-level effects play a variable and potentially strong role in shaping the apparent temperature dependence of detrital breakdown. Such effects should be incorporated into studies designed to estimate inherent temperature dependence of slow ecological processes.

Temperature Responses of Heterotrophic Bacteria in Co-culture With a Red Sea Synechococcus Strain

Interactions between autotrophic and heterotrophic bacteria are fundamental for marine biogeochemical cycling. How global warming will affect the dynamics of these essential microbial players is not fully understood. The aims of this study were to identify the major groups of heterotrophic bacteria present in a Synechococcus culture originally isolated from the Red Sea and assess their joint responses to experimental warming within the metabolic ecology framework. A co-culture of Synechococcus sp. RS9907 and their associated heterotrophic bacteria, after determining their taxonomic affiliation by 16S rRNA gene sequencing, was acclimated and maintained in the lab at different temperatures (24-34°C). The abundance and cellular properties of Synechococcus and the three dominant heterotrophic bacterial groups (pertaining to the genera Paracoccus, Marinobacter, and Muricauda) were monitored by flow cytometry. The activation energy of Synechococcus, which grew at 0.94-1.38 d-1, was very similar (0.34 ± 0.02 eV) to the value hypothesized by the metabolic theory of ecology (MTE) for autotrophs (0.32 eV), while the values of the three heterotrophic bacteria ranged from 0.16 to 1.15 eV and were negatively correlated with their corresponding specific growth rates (2.38-24.4 d-1). The corresponding carrying capacities did not always follow the inverse relationship with temperature predicted by MTE, nor did we observe a consistent response of bacterial cell size and temperature. Our results show that the responses to future ocean warming of autotrophic and heterotrophic bacteria in microbial consortia might not be well described by theoretical universal rules.

Depth-Dependent Variables Shape Community Structure and Functionality in the Prince Edward Islands

Physicochemical variables limit and control the distribution of microbial communities in all environments. In the oceans, this may significantly influence functional processes such the consumption of dissolved organic material and nutrient sequestration. Yet, the relative contributions of physical factors, such as water mass variability and depth, on functional processes are underexplored. We assessed microbial community structure and functionality in the Prince Edward Islands (PEIs) using 16S rRNA gene amplicon analysis and extracellular enzymatic activity assays, respectively. We found that depth and nutrients substantially drive the structural patterns of bacteria and archaea in this region. Shifts from epipelagic to bathypelagic zones were linked to decreases in the activities of several extracellular enzymes. These extracellular enzymatic activities were positively correlated with several phyla including several Alphaproteobacteria (including members of the SAR 11 clade and order Rhodospirillales) and Cyanobacteria. We show that depth-dependent variables may be essential drivers of community structure and functionality in the PEIs.

Changes in the Trophic Pathways within the Microbial Food Web in the Global Warming Scenario: An Experimental Study in the Adriatic Sea

A recent analysis of the Mediterranean Sea surface temperature showed significant annual warming. Since small picoplankton microorganisms play an important role in all major biogeochemical cycles, fluxes and processes occurring in marine systems (the changes at the base of the food web) as a response to human-induced temperature increase, could be amplified through the trophic chains and could also significantly affect different aspects of the structure and functioning of marine ecosystems. In this study, manipulative laboratory growth/grazing experiments were performed under in situ simulated conditions to study the structural and functional changes within the microbial food web after a 3 °C increase in temperature. The results show that a rise in temperature affects the changes in: (1) the growth and grazing rates of picoplankton, (2) their growth efficiency, (3) carrying capacities, (4) sensitivity of their production and grazing mortality to temperature, (5) satisfying protistan grazer carbon demands, (6) their preference in the selection of prey, (7) predator niche breadth and their overlap, (8) apparent uptake rates of nutrients, and (9) carbon biomass flow through the microbial food web. Furthermore, temperature affects the autotrophic and heterotrophic components of picoplankton in different ways.

Watershed geomorphology modifies the sensitivity of aquatic ecosystem metabolism to temperature

The regulation of aquatic carbon cycles by temperature is a significant uncertainty in our understanding of how watersheds will respond to climate change. Aquatic ecosystems transport substantial quantities of carbon to the atmosphere and ocean, yet we have limited understanding of how temperature modifies aquatic ecosystem metabolic processes and contributions to carbon cycles at watershed to global scales. We propose that geomorphology controls the distribution and quality of organic material that forms the metabolic base of aquatic ecosystems, thereby controlling the response of aquatic ecosystem metabolism to temperature across landscapes. Across 23 streams and four years during summer baseflow, we estimated variation in the temperature sensitivity of ecosystem respiration (R) among streams draining watersheds with different geomorphic characteristics across a boreal river basin. We found that geomorphic features imposed strong controls on temperature sensitivity; R in streams draining flat watersheds was up to six times more temperature sensitive than streams draining steeper watersheds. Further, our results show that this association between watershed geomorphology and temperature sensitivity of R was linked to the carbon quality of substrates that changed systematically across the geomorphic gradient. This suggests that geomorphology will control how carbon is transported, stored, and incorporated into river food webs as the climate warms.

Importance of Bacterial Maintenance Respiration in a Subarctic Estuary: a Proof of Concept from the Field

Bacterial respiration contributes to atmospheric carbon dioxide accumulation and development of hypoxia and is a critical, often overlooked, component of ecosystem function. This study investigates the concept that maintenance respiration is a significant proportion of bacterial respiration at natural nutrient levels in the field, advancing our understanding of bacterial living conditions and energy strategies. Two river-sea transects of respiration and specific growth rates were analyzed representing low- and high-productivity conditions (by in situ bacterial biomass production) in a subarctic estuary, using an established ecophysiological linear model (the Pirt model) estimating maintenance respiration. The Pirt model was applicable to field conditions during high, but not low, bacterial biomass production. However, a quadratic model provided a better fit to observed data, accounting for the maintained respiration at low μ. A first estimate of maintenance respiration was 0.58 fmol O₂ day^-1 cell^-1 by the quadratic model. Twenty percent to nearly all of the bacterial respiration was due to maintenance respiration over the observed range of μ (0.21-0.002 day^-1). In the less productive condition, bacterial specific respiration was high and without dependence on μ, suggesting enhanced bacterial energy expenditure during starvation. Annual maintenance respiration accounted for 58% of the total bacterioplankton respiration based on μ from monitoring data. Phosphorus availability occasionally, but inconsistently, explained some of the remaining variation in bacterial specific respiration. Bacterial maintenance respiration can constitute a large share of pelagic respiration and merit further study to understand bacterial energetics and oxygen dynamics in the aquatic environment.

Comprehensive analysis of the phylogeny and extracellular proteases in genus Vibrio strain

As one of the dominant bacteria in the ocean, Vibrio play important roles in maintaining the aquatic ecosystem. In this study, we studied the phylogenetic relationships of 32 Vibrio based on the 16S rRNA genes sequences and utilized substrate immersing zymography method to detect the trend of protease production and components of multiprotease system of Vibrio extracellular proteases. The result showed that different extracellular proteolytic profiles among various Vibrio strains demonstrated a large interspecific variation, and for strains from the same environments, the closer the evolutionary relationship of them, the more similar their zymograms were. In addition, these proteases displayed very different hydrolysis abilities to casein and gelatin. Moreover, the results of the inhibitor-substrate immersing zymography indicated that the proteases secreted by marine Vibrio mostly belonged to serine proteases or metalloproteases. These results implied that combined taxonomic information of the Vibrios with their extracellular protease zymograms maybe contributed to the study of the classification, phylogeny and pathogenic mechanism of Vibrio, and can serve as a theoretical basis for controlling the pathogenic Vibrio disease as well as exploiting proteases. More importantly, we can also eliminate many similar strains by this way, thus can greatly reduce the workload of the experiments for us.

Enzymatic origin and various curvatures of metabolic scaling in microbes

The famous and controversial power law is a basal metabolic scaling model mainly derived from the “surface rule” or a fractal transport network. However, this law neglects biological mechanisms in the important active state. Here, we hypothesized that the relative metabolic rate and growth rate of actively growing microbes are driven by the changeable rate of their rate-limiting enzymes and concluded that natural logarithmic microbial metabolism (lnλ) and growth (or biomass) (lnM) are both dependent on limiting resources, and then developed novel models with interdependence between lnλ and lnM. We tested the models using the data obtained from the literature. We explain how and why the scaling is usually curved with the difference between microbial metabolic and growth (or biomass’s) half-saturation constants (K_M, K_λ) in the active state and agree that the linear relationship of the power law is a particular case under the given condition: K_M = K_λ, which means that the enzyme dynamics may drive active and basal metabolic scaling relationships. Our interdependent model is more general than the power law, which is important for integrating the ecology and biochemical processes.

Temperature sensitivities of microbial plankton net growth rates are seasonally coherent and linked to nutrient availability

Recent work suggests that temperature effects on marine heterotrophic bacteria are strongly seasonal, but few attempts have been made to concurrently assess them across trophic levels. Here, we estimated the temperature sensitivities (using activation energies, E) of autotrophic and heterotrophic microbial plankton net growth rates over an annual cycle in NE Atlantic coastal waters. Phytoplankton grew in winter and late autumn (0.41 ± 0.16 SE d^-1 ) and decayed in the remaining months (-0.42 ± 0.10 d^-1 ). Heterotrophic microbes shared a similar seasonality, with positive net growth for bacteria (0.14-1.48 d^-1 ), while nanoflagellates had higher values (> 0.4 d^-1 ) in winter and spring relative to the rest of the year (-0.46 to 0.29 d^-1 ). Net growth rates activation energies showed similar dynamics in the three groups (-1.07 to 1.51 eV), characterized by maxima in winter, minima in summer and resumed increases in autumn. Microbial plankton E values were significantly correlated with nitrate concentrations as a proxy for nutrient availability. Nutrient-sufficiency (i.e., > 1 μmol l^-1 nitrate) resulted in significantly higher activation energies of phytoplankton and heterotrophic nanoflagellates relative to nutrient-limited conditions. We suggest that only within spatio-temporal windows of both moderate bottom-up and top-down controls will temperature have a major enhancing effect on microbial growth.

Testing the metabolic theory of ecology with marine bacteria: different temperature sensitivity of major phylogenetic groups during the spring phytoplankton bloom

Although temperature is a key driver of bacterioplankton metabolism, the effect of ocean warming on different bacterial phylogenetic groups remains unclear. Here, we conducted monthly short-term incubations with natural coastal bacterial communities over an annual cycle to test the effect of experimental temperature on the growth rates and carrying capacities of four phylogenetic groups: SAR11, Rhodobacteraceae, Gammaproteobacteria and Bacteroidetes. SAR11 was the most abundant group year-round as analysed by CARD-FISH, with maximum abundances in summer, while the other taxa peaked in spring. All groups, including SAR11, showed high temperature-sensitivity of growth rates and/or carrying capacities in spring, under phytoplankton bloom or post-bloom conditions. In that season, Rhodobacteraceae showed the strongest temperature response in growth rates, estimated here as activation energy (E, 1.43 eV), suggesting an advantage to outcompete other groups under warmer conditions. In summer E values were in general lower than 0.65 eV, the value predicted by the Metabolic Theory of Ecology (MTE). Contrary to MTE predictions, carrying capacity tended to increase with warming for all bacterial groups. Our analysis confirms that resource availability is key when addressing the temperature response of heterotrophic bacterioplankton. We further show that even under nutrient-sufficient conditions, warming differentially affected distinct bacterioplankton taxa.

Temperature dependences of growth rates and carrying capacities of marine bacteria depart from metabolic theoretical predictions

Using the metabolic theory of ecology (MTE) framework, we evaluated over a whole annual cycle the monthly responses to temperature of the growth rates (μ) and carrying capacities (K) of heterotrophic bacterioplankton at a temperate coastal site. We used experimental incubations spanning 6ºC with bacterial physiological groups identified by flow cytometry according to membrane integrity (live), nucleic acid content (HNA and LNA) and respiratory activity (CTC+). The temperature dependence of μ at the exponential phase of growth was summarized by the activation energy (E), which was variable (-0.52 to 0.72 eV) but followed a seasonal pattern, only reaching the hypothesized value for aerobic heterotrophs of 0.65 eV during the spring bloom for the most active bacterial groups (live, HNA, CTC+). K (i.e. maximum experimental abundance) peaked at 4 × 10(6) cells mL(-1) and generally covaried with μ but, contrary to MTE predictions, it did not decrease consistently with temperature. In the case of live cells, the responses of μ and K to temperature were positively correlated and related to seasonal changes in substrate availability, indicating that the responses of bacteria to warming are far from homogeneous and poorly explained by MTE at our site.

Interactions between temperature and nutrients across levels of ecological organization

Temperature and nutrient availability play key roles in controlling the pathways and rates at which energy and materials move through ecosystems. These factors have also changed dramatically on Earth over the past century as human activities have intensified. Although significant effort has been devoted to understanding the role of temperature and nutrients in isolation, less is known about how these two factors interact to influence ecological processes. Recent advances in ecological stoichiometry and metabolic ecology provide a useful framework for making progress in this area, but conceptual synthesis and review are needed to help catalyze additional research. Here, we examine known and potential interactions between temperature and nutrients from a variety of physiological, community, and ecosystem perspectives. We first review patterns at the level of the individual, focusing on four traits--growth, respiration, body size, and elemental content--that should theoretically govern how temperature and nutrients interact to influence higher levels of biological organization. We next explore the interactive effects of temperature and nutrients on populations, communities, and food webs by synthesizing information related to community size spectra, biomass distributions, and elemental composition. We use metabolic theory to make predictions about how population-level secondary production should respond to interactions between temperature and resource supply, setting up qualitative predictions about the flows of energy and materials through metazoan food webs. Last, we examine how temperature-nutrient interactions influence processes at the whole-ecosystem level, focusing on apparent vs. intrinsic activation energies of ecosystem processes, how to represent temperature-nutrient interactions in ecosystem models, and patterns with respect to nutrient uptake and organic matter decomposition. We conclude that a better understanding of interactions between temperature and nutrients will be critical for developing realistic predictions about ecological responses to multiple, simultaneous drivers of global change, including climate warming and elevated nutrient supply.

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.

Linking hydrolytic activities to variables influencing a submerged membrane bioreactor (MBR) treating urban wastewater under real operating conditions

The seasonal variation of the hydrolytic activities acid and alkaline phosphatase, α-glucosidase and protease, was studied in both the aerated and anoxic phases of a full-scale membrane bioreactor (MBR) (total operational volume = 28.2 m(3)), operated in pre-denitrification mode and fed real urban wastewater. Non-metric multidimensional scaling (MDS) and BIO-ENV analysis were used to study the distribution of enzyme activities in different seasons of the year (spring, summer and autumn) and unveil their relationships with changes in variables influencing the system (composition of influent wastewater, activated sludge temperature and biomass concentration in the bioreactors). The activities of all the tested hydrolases were remarkably dynamic, and each enzyme showed complex and diverse patterns of variation. Except in the summer season, the variables included in this study gave a good explanation of those patterns and displayed high and consistent correlations with them; however, markedly different correlation trends were found in each season, indicating dissimilar adaptation responses of the community to the influence of changing conditions. A consistent and highly negative correlation between protease and α-glucosidase was revealed in all the experiments. The variables included in this study showed contrary influences on these activities, suggesting an alternation of the major groups of carbon-degrading hydrolases in connection to changes in temperature and the availability and composition of nutrients in the different seasons. Sampling over a long period of time was required to adequately lay down the links between hydrolytic activities and the variables influencing the MBR system. These results highlight the complexity of the regulation of substrate degradation by the mixed microbial sludge communities under real operating conditions.

Disconnect of microbial structure and function: enzyme activities and bacterial communities in nascent stream corridors

A fundamental issue in microbial and general ecology is the question to what extent environmental conditions dictate the structure of communities and the linkages with functional properties of ecosystems (that is, ecosystem function). We approached this question by taking advantage of environmental gradients established in soil and sediments of small stream corridors in a recently created, early successional catchment. Specifically, we determined spatial and temporal patterns of bacterial community structure and their linkages with potential microbial enzyme activities along the hydrological flow paths of the catchment. Soil and sediments were sampled in a total of 15 sites on four occasions spread throughout a year. Denaturing gradient gel electrophoresis (DGGE) was used to characterize bacterial communities, and substrate analogs linked to fluorescent molecules served to track 10 different enzymes as specific measures of ecosystem function. Potential enzyme activities varied little among sites, despite contrasting environmental conditions, especially in terms of water availability. Temporal changes, in contrast, were pronounced and remarkably variable among the enzymes tested. This suggests much greater importance of temporal dynamics than spatial heterogeneity in affecting specific ecosystem functions. Most strikingly, bacterial community structure revealed neither temporal nor spatial patterns. The resulting disconnect between bacterial community structure and potential enzyme activities indicates high functional redundancy within microbial communities even in the physically and biologically simplified stream corridors of early successional landscapes.

Regulation and spatiotemporal patterns of extracellular enzyme activities in a coastal, sandy aquifer system (Doñana, SW Spain)

A seasonal study of extracellular enzyme activities (EEA) was conducted in the coastal, sandy aquifer system located in the greater fluvial-littoral ecosystem of Doñana (SW, Spain). Glucosidase, leucine aminopeptidase, alkaline phosphatase, and phenol oxidase activities were determined over a 2-year period in 30 piezometers spread in an area of approximately 100 km(2). Taking into account all enzymes, piezometers, and seasons, EEA ranged over several orders of magnitude, from 1.01 × 10(-5) ± 2.92 × 10(-6) to 1.37 ± 0.13 nmol (methylumbelliferyl, amido-4-methylcoumarin, or dihydroxyphenylalanine) mL(-1) h(-1). The quality, much more than the quantity, of organic matter and nutrients seemed to be the major variables that controlled the spatiotemporal patterns showed by EEA. EEA patterns obtained in this study agree with several functional models of microbial communities, such as optimal resource allocation and nutrient co-limitation. This study probably represents the first one in which these functional models have been tested in subsurface systems. Results obtained in this study seem to suggest that microbial communities inhabiting groundwaters in Doñana are not dead or compromised cells. By contrast, these communities play relevant roles in carbon and nutrient cycling, continue with the decomposition process that begins in the sediments of the shallow lakes located in the area, provide remineralized carbon and nutrients to producers of these surface aquatic systems, and close energy and matter cycles. This study proposes that groundwater systems should be considered dynamic systems, comparable in functional complexity to surface systems.

Ecoenzymatic stoichiometry in relation to productivity for freshwater biofilm and plankton communities

The degradation of detrital organic matter and assimilation of carbon (C), nitrogen (N), and phosphorus (P) by heterotrophic microbial communities is mediated by enzymes released into the environment (ecoenzymes). For the attached microbial communities of soils and freshwater sediments, the activities of β-glucosidase, β-N-acetylglucosaminidase, leucine aminopeptidase, and phosphatase show consistent stoichiometric patterns. To determine whether similar constraints apply to planktonic communities, we assembled data from nine studies that include measurements of these enzyme activities along with microbial productivity. By normalizing enzyme activity to productivity, we directly compared the ecoenzymatic stoichiometry of aquatic biofilm and bacterioplankton communities. The relationships between β-glucosidase and α-glucosidase and β-glucosidase and β-N-acetylglucosaminidase were statistically indistinguishable for the two community types, while the relationships between β-glucosidase and phosphatase and β-glucosidase and leucine aminopeptidase significantly differed. For β-glucosidase vs. phosphatase, the differences in slope (biofilm 0.65, plankton 1.05) corresponded with differences in the mean elemental C:P ratio of microbial biomass (60 and 106, respectively). For β-glucosidase vs. leucine aminopeptidase, differences in slope (0.80 and 1.02) did not correspond to differences in the mean elemental C:N of biomass (8.6 and 6.6). β-N-Acetylglucosaminidase activity in biofilms was significantly greater than that of plankton, suggesting that aminosaccharides were a relatively more important N source for biofilms, perhaps because fungi are more abundant. The slopes of β-glucosidase vs. (β-N-acetylglucosaminidase + leucine aminopeptidase) regressions (biofilm 1.07, plankton 0.94) corresponded more closely to the estimated difference in mean biomass C:N. Despite major differences in physical structure and trophic organization, biofilm and plankton communities have similar ecoenzymatic stoichiometry in relation to productivity and biomass composition. These relationships can be integrated into the stoichiometric and metabolic theories of ecology and used to analyze community metabolism in relation to resource constraints.