The light : nutrient ratio in lakes: a test of hypothesized trends in bacterial nutrient limitation

The light-to-nutrient hypothesis explores how the balance between energy (as light energy) and nutrients (as total phosphorus) shapes aquatic ecosystem structure and process. The balance of energy and nutrients is thought to regulate ecosystem structure and process such that, in a "high" light-to-nutrient environment, bacteria would probably be driven towards phosphorus (P) limitation, whereas, in a "low" light-to-nutrient environment, bacteria would be driven towards carbon (C) limitation. We assessed the growth limitation of bacteria in two reservoirs of the southern U.S.A. using a mortality-corrected dilution-growth approach. We compared the frequency of P and C growth limitation with the intralake variation in the light-to-nutrient environment. As a metric of the light-to-nutrient environment, we used the ratio of the mean light in the surface mixed layer ( I _m) to the total phosphorus concentration ( TP ). In each lake, bacterial growth was more often P-limited when the I _m :  TP ratio was above the median ratio than below. We believe our data provide the first evidence supporting this aspect of the light-to-nutrient hypothesis.     

Heparanase affects food intake and regulates energy balance in mice

Mutation of the melanocortin-receptor 4 (MC4R) is the most frequent cause of severe obesity in humans. Binding of agouti-related peptide (AgRP) to MC4R involves the co-receptor syndecan-3, a heparan sulfate proteoglycan. The proteoglycan can be structurally modified by the enzyme heparanase. Here we tested the hypothesis that heparanase plays a role in food intake behaviour and energy balance regulation by analysing body weight, body composition and food intake in genetically modified mice that either lack or overexpress heparanase. We also assessed food intake and body weight following acute central intracerebroventricular administration of heparanase; such treatment reduced food intake in wildtype mice, an effect that was abolished in mice lacking MC4R. By contrast, heparanase knockout mice on a high-fat diet showed increased food intake and maturity-onset obesity, with up to a 40% increase in body fat. Mice overexpressing heparanase displayed essentially the opposite phenotypes, with a reduced fat mass. These results implicate heparanase in energy balance control via the central melanocortin system. Our data indicate that heparanase acts as a negative modulator of AgRP signaling at MC4R, through cleavage of heparan sulfate chains presumably linked to syndecan-3.     

The relative importance of light and nutrient limitation of phytoplankton growth: a simple index of coastal ecosystem sensitivity to nutrient enrichment

Anthropogenic nutrient enrichment of the coastal zone is now a well-established fact. However, there is still uncertainty about the mechanisms through which nutrient enrichment can disrupt biological communities and ecosystem processes in the coastal zone. For example, while some estuaries exhibit classic symptoms of acute eutrophication, including enhanced production of algal biomass, other nutrient-rich estuaries maintain low algal biomass and primary production. This implies that large differences exist among coastal ecosystems in the rates and patterns of nutrient assimilation and cycling. Part of this variability comes from differences among ecosystems in the other resource that can limit algal growth and production – the light energy required for photosynthesis. Complete understanding of the eutrophication process requires consideration of the interacting effects of light and nutrients, including the role of light availability as a regulator of the expression of eutrophication. A simple index of the relative strength of light and nutrient limitation of algal growth can be derived from models that describe growth rate as a function of these resources. This index can then be used as one diagnostic to classify the sensitivity of coastal ecosystems to the harmful effects of eutrophication. Here I illustrate the application of this diagnostic with light and nutrient measurements made in three California estuaries and two Dutch estuaries.     

Temperature affects the partitioning of absorbed light energy in freshwater phytoplankton

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Trophic structure in the pelagial of 25 shallow New Zealand lakes: changes along nutrient and fish gradients

To gain better insight into the importance of predator and resourcecontrol in New Zealand lakes we surveyed the late summer trophicstructure of 25 shallow South Island lakes with contrastingnutrient levels (6–603 µg TP l^–1) and fishdensities. Total catch of fish per net (CPUE) in multi-meshgillnets placed in the open water and the littoral zones waspositively related with the nutrient level. Trout CPUE was negativelycorrelated with total phosphorus (TP) and total nitrogen (TN).Zooplankton seemed largely influenced by fish, as high fishCPUE coincided with low zooplankton and Daphnia biomass, lowaverage weight of cladocerans, low contribution of Daphnia tototal cladoceran biomass, low ratio of calanoids to total copepodbiomass and low ratio of zooplankton biomass to phytoplanktonbiomass. However, chlorophyll a was only slightly negativelyrelated to Daphnia biomass and not to zooplankton biomass ina multiple regression that included TN and TP. Ciliate abundancewas positively related to chlorophyll a and negatively to Daphniabiomass, but not to total zooplankton biomass, while no relationshipswere found between heterotrophic nanoflagellates and zooplankton.The relationships between fish abundance and nutrients and fishabundance and zooplankton:phytoplankton ratio and between chlorophylla and TP largely followed the pattern obtained for 42 northtemperate Danish lakes. We conclude that fish, including trout,have a major effect on the zooplankton community structure andbiomass in the pelagial of the shallow oligotrophic to slightlyeutrophic New Zealand lakes, but that the cascading effectson phytoplankton and protist are apparently modest.     

Mild mitochondrial uncoupling in mice affects energy metabolism, redox balance and longevity

Caloric restriction is the most effective non-genetic intervention to enhance lifespan known to date. A major research interest has been the development of therapeutic strategies capable of promoting the beneficial results of this dietary regimen. In this sense, we propose that compounds that decrease the efficiency of energy conversion, such as mitochondrial uncouplers, can be caloric restriction mimetics. Treatment of mice with low doses of the protonophore 2,4-dinitrophenol promotes enhanced tissue respiratory rates, improved serological glucose, triglyceride and insulin levels, decrease of reactive oxygen species levels and tissue DNA and protein oxidation, as well as reduced body weight. Importantly, 2,4-dinitrophenol-treated animals also presented enhanced longevity. Our results demonstrate that mild mitochondrial uncoupling is a highly effective in vivo antioxidant strategy, and describe the first therapeutic intervention capable of effectively reproducing the physiological, metabolic and lifespan effects of caloric restriction in healthy mammals.     

Optimality and phenotypic plasticity of shoot-to-root ratio under variable light and nutrient availabilities

We studied the phenotypic plasticity of shoot-to-root ratio with a model of plant growth in different availabilities of light and nutrients. Optimal shoot-to-root ratio was defined as the equal limitation of growth by light and nutrients. An optimally growing plant had a curved relative growth rate (RGR) isoclines and a faster growth rate than a fixed-allocation plant having right-angled RGR isoclines. We assumed the plant be exposed to a unit standard deviation of bivariate normally distributed resources. Plants were more plastic in a low than in a high resource availability. Negative correlation between resources increased and positive correlation decreased plasticity. Plasticity was high in plants that saturate at low resource availabilities but independent of maximum growth rate. A trade-off between the maximum growth rate and plasticity of shoot-to-root allocation may rise indirectly from the tendency of fast-growing plants to have high resource requirements.     

高效经营雷竹林生态系统能量通量过程及闭合度

利用开路涡度相关系统和常规气象仪,对高效经营的雷竹林生态系统2011年的显热通量、潜热通量、净辐射、土壤热通量以及气温、地温、降雨量进行了观测,分析该生态系统能量通量的变化,以及各能量分量的分配特征,并计算了波文比及能量闭合.结果表明：雷竹林全年净辐射为2928.92 MJ·m^-2,潜热通量、显热通量和土壤热通量分别为1384.90、92754和-28.27 MJ·m^-2,各能量分量的日变化和月变化基本呈单峰曲线.潜热通量为能量散失主要形式,占净辐射的47.3%,显热通量占31.7%,波文比呈“U”型曲线,在0.285～2.062之间变化,土壤为热源.雷竹林年能量闭合度为0.782,月平均闭合度为0.808.
     

The role of polyphenols in terrestrial ecosystem nutrient cycling

Interspecific variation in polyphenol production by plants has been interpreted in terms of defense against herbivores. Several recent lines of evidence suggest that polyphenols also influence the pools and fluxes of inorganic and organic soil nutrients. Such effects could have far-ranging consequences for nutrient competition among and between plants and microbes, and for ecosystem nutrient cycling and retention. The significance of polyphenols for nutrient cycling and plant productivity is still uncertain, but it could provide an alternative or complementary explanation for the variability in polyphenol production by plants.     

Detritus and nutrient dynamics in the shore zone of lakes: a review

The importance of detritus varies greatly among shore zones of lakes, but in a large majority of these regions detrital pathways prevail. Aside from a great spatial and seasonal variability, macrophytes and bottom sediments appear to be dominant stores of nutrients in these habitats. Macrophytes hold a central position in nutrient cycling in the shore-littoral lake zones. They are the main source of autochthonous detritus as they prevail in the total biomass of littoral organisms, and they are only rarely available as direct food of consumers. Various processes and interactions determine the role of macrophytes in nutrients dynamics. These are: the intensity of nutrient uptake and translocation, release of nutrients by healthy plants and from decomposing plants, exchange of elements between macrophytes and their periphyton, as well as interception of seston by macrophyte stands. Particular plant species differ in their time of dying and susceptibility to decomposition. The changes in decomposing material (size structure of particles and nutrient content) mean that detritus in various stages of decomposition differs in its role in trophic dynamics of shore-littoral lake zones. Several types of shore regions as regards detritus sources and retention level are discussed.     

Effects of nutrients and light on periphytic biomass and nutrient stoichiometry in a tropical black-water aquatic ecosystem

This article aims to test the light-nutrient hypothesis (LNH) in a periphytic community in a tropical black-water lake. Individual and interactive effects of light and nutrient availability were assessed with periphyton biomass accrual, nutrient content, and nutrient stoichiometry. We performed a manipulative field experiment with a 4 × 2 factorial design. We used nutrient diffusing substrates to produce four different nutrients treatments: Control (no nutrient added), nitrogen amended (N), phosphorus amended (P) and combined N and P amendment (NP). Two light levels were also considered: high light (near surface water) and low light (near bottom water). Light and nutrients individually and interactively caused significant changes in aggregate periphyton community properties. Total and autotrophic biomasses were significantly higher in high light conditions and in nutrient enriched treatments. Autotrophic biomass was significantly higher in N enriched treatment whereas total biomass was mainly affected by the joint addition of N and P. At lower light availability periphyton growth was limited, even in enriched treatments. Light also strongly affected periphyton nutrient content. Periphyton C, N and P in general increased when subjected to high light conditions. As predicted by the LNH, light promoted an increase in periphyton C:P ratios in P deprived treatments, but an opposite effect was observed on C:N ratios, especially in N-enriched treatments. This experiment revealed that light availability strongly limits the propagation of nutrient effects on periphyton growth. Such complex interdependencies on basal resources affect the proportion of autotrophic to total periphytic biomass that can be an important mechanism to explain variation in the nutrient stoichiometry of periphyton in nature.     

Bud dormancy breaking affects respiration and energy balance of bilberry shoots in the initial stage of growth

Respiration and heat production in the shoots of bilberry (Vaccinium myrtillus L.) were studied at the beginning of growth after breaking bud dormancy by means of transfer of the shoots to indoor conditions (November–April) and upon natural sprouting in spring (May). The buds released from dormancy at the beginning of winter sprouted slower and showed lower respiratory activity than the buds that started growing in May. In May, cytochrome respiratory pathway in sprouting buds was 1.3 times more active than energetically ineffective alternative pathway, whereas activity of cytochrome pathway in December was 1.4 times lower as compared with the alternative. In November–December, the rate of heat evolution by the buds was 3–5 times lower than in April–May. In case of early breaking of bud dormancy, the share of respiration energy dissipated as heat was 30% on average. In the buds whose growth was induced later, the value of this parameter was twice as much. The ratio between heat evolution and respiration depended on temperature. High temperature more intensely activated heat evolution than respiration, which caused a decrease in the level of metabolic energy available for growth. In the temperature range of 5–15°C characteristic of the beginning of vegetation, the share of respiration energy dissipated as heat was 2–3 times lower than at 20–30°C, which reflects a great adaptability of V. myrtillus to climatic conditions of the region. Our data suggest that progression through a full cycle of winter dormancy is physiologically important for shoot growth. Early dormancy release brought about changes in respiration and energy balance of the shoots in the initial stage of extra-bud growth.     

Characterization of bacterial communities in four freshwater lakes differing in nutrient load and food web structure

The phylogenetic composition of bacterioplankton communities in the water column of four shallow eutrophic lakes was analyzed by partially sequencing cloned 16S rRNA genes and by PCR-DGGE analysis. The four lakes differed in nutrient load and food web structure: two were in a clearwater state and had dense stands of submerged macrophytes, while two others were in a turbid state characterized by the occurrence of phytoplankton blooms. One turbid and one clearwater lake had very high nutrient levels (total phosphorus > 100 microg/l), while the other lakes were less nutrient rich (total phosphorus < 100 microg/l). Cluster analysis, multidimensional scaling and ANOSIM (analysis of similarity) were used to investigate differences among the bacterial community composition in the four lakes. Our results show that each lake has its own distinct bacterioplankton community. The samples of lake Blankaart differed substantially from those of the other lakes; this pattern was consistent throughout the year of study. The bacterioplankton community composition in lake Blankaart seems to be less diverse and less stable than in the other three lakes. Clone library results reveal that Actinobacteria strongly dominated the bacterial community in lake Blankaart. The relative abundance of Betaproteobacteria was low, whereas this group was dominant in the other three lakes. Turbid lakes had a higher representation of Cyanobacteria, while clearwater lakes were characterized by more representatives of the Bacteroidetes. Correlating our DGGE data with environmental parameters, using the BIOENV procedure, suggests that differences are partly related to the equilibrium state of the lake.     

The Reeuwijk lakes: A five years water quality study in an eutrophic ecosystem

The Reeuwijk Lakes (The Netherlands) present a typical example of eutrophication in the lower Rhine catchment area. In 1986 restoration of these lakes started by reducing the external P-loading. Two lakes, Lake Elfhoeven and Lake Nieuwenbroek, differing in P-load and residence time were selected for monitoring water quality parameters before (1983–1985) and after (1986–1987) these restoration measures. Reduction of the external P-loading did not result in lower P-concentrations in both lakes. In contrast, P and N increased. This may have been caused by an increase in diffuse discharges. However, seasonal cycles of P and N point to a strong internal loading of nutrients. The concentrations of chlorophyll a and carotene decreased, indicating a lower phytoplankton biomass. However, as C-phycocyanine concentrations increased the relative abundance of cyanobacteria became higher. Seston concentrations and zooplankton densities did not change. Transparency in the lakes slightly decreased after P-reduction and is far too low for the development of any vegetation of submerged waterplants. The typical differences between both lakes remained after restoration measures. The inverse relationship between the concentrations of chlorophyll a and total phosphorus at the two sampled stations remained constant. The differences in phytoplankton composition and the zooplankton biomass give a plausible explanation for this inverse relationship, between the two stations. Restoring the lakes after four decades of P-loading can presumably, not simply be done by lowering the external P-load alone. Supplementary in-lake measures may accelerate the restoration process.     

The role of nutrient balance in shaping plant root-fungal interactions: facts and speculation

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Apparent P/O ratio and chemical energy balance in frog sartorius muscle in vitro

We tested the hypothesis that there is a constant relation between the amount of chemical energy used during a single tetanus and the extent of subsequent aerobic recovery metabolism. Breakdown of high energy compounds (Δ P) during contraction was measured by rapid freezing techniques using isometric contractions of unpoisoned frog sartorius muscles at 0 °C. A stable and sensitive method was designed to study recovery O₂ consumption of similar contractions. In some cases, initial chemical changes and recovery O₂ consumption were measured in the same muscle. The ratio of these chemical changes yields a value for the P/O ratio of whole muscle. This ratio was found to be 2 and was constant for the range of contraction durations studied (5–20 s). Splitting of phosphorylcreatine or ATP after mechanical relaxation and glycolytic resynthesis of ATP could not be detected. Thus, the tested hypothesis is valid; but there is no ready explanation why this estimate of the P/O ratio of whole muscle differs from the ratio measured in isolated mitochondria. Because of this constancy and the observed stoichiometry unknown energy yielding reactions, postulated to occur either early or late in a maintained tetanus, are excluded by these experiments.