Coralline photosynthetic physiology across a steep light gradient

Photosynthesis Research - Coralline algae (CA) are globally distributed and fulfil many important roles within coastal ecosystems. In this study, photosynthetically active radiation (PAR) measured...     

Contribution of photosynthetic sulphur bacteria to primary production in Lake Vechten

The role of photosynthetic sulphur bacteria as primary producers in monomictic Lake Vechten (The Netherlands) is described. Lake Vechten has a surface area of 4.7 ha, a maximum depth of 11.9 m and a mean depth of 6 m.Bacterial populations, appearing at the boundary layer of the oxidative and reductive zone from early June till late October, were composed of cyanobacteria, Chromatiaceae and green and brown coloured Chlorobiaceae. Predominating genera were Synechococcus, Chloronema, Chromatium and Thiopedia. The photosynthetic sulphur bacteria accounted for a primary production rate of 13.6–106.1 mg C.m^–2 day^–1, which corresponded to 3.9–17.5% of total daily productivity in the pelagial zone. The percentage of photosynthetic bacterial production of total annual planktonic primary production calculated for the entire pelagial zone, taking into account compensation for decreasing volume of lower strata, was 3.6% (i.e. 127 against a total production of 3 510 kg C.lake^–1yr^–1).     

Gender-Related developmental instability and herbivory ofPistacia atlantica across a steep environmental gradient

To examine gender-related developmental instability and herbivory of a plant across a steep climatic gradient, we studiedPistacia atlantica DESF., a dioecious tree of the Irano-Turanian zone in the Middle East. Leaves were sampled from five populations along a climatic gradient in Israel from mesic Mediterranean to xeric (and geographically peripheral) sites. Leaves of the xeric populations were ca. 25% smaller in size (length) than leaves of other populations. Leaf developmental instability (an indicator of stress) was examined using the measurement of leaf asymmetry (LA), estimated by the deviation from the origin of paired lateral leaflets and by the presence vs. absence of the terminal leaflet. LA was not correlated with the climatic (site) gradient and had the highest, though not significantly, levels in the intermediate ecotonal zone between the Mediterranean and the arid ecosystems. Male trees were 13% higher in LA than females. Density of two specialist gall-forming aphids varied with site but was not affected by plant gender. Gender-related developmental instability is discussed in relation to resource allocation in early spring. Because the study was limited to five populations further research is clearly needed.     

Mammal mycophagy and fungal spore dispersal across a steep environmental gradient in eastern Australia

We examined changes in the types of fungi consumed by six species of small mammals across a habitat gradient in north‐eastern New South Wales that graded from swamp, to woodland, to open forest and then to rainforest. All mammals ate some fungus, but only bush rats (Rattus fuscipes) regularly did so, and their diet included most of the fungal taxa that we identified across all mammals in the study. The composition of bush rat diet changed significantly with each change in habitat from woodland, to forest, to rainforest. In particular, there was a significant difference in the diets of rats caught either side of the open forest‐rainforest ecotone, which marks the change in fungal community from one dominated by ectomycorrhizal fungi, to a community dominated by arbuscular mycorrhizal fungi. Movement patterns of bush rats living around the open forest‐rainforest ecotone suggest that they transport fungal spores between these contrasting fungal communities. Therefore, bush rats have the potential, by way of spore dispersal, to influence the structure of vegetation communities.     

Microaerobic hydrogen production by photosynthetic bacteria in a double-phase photobioreactor

The rate of hydrogen production by the marine nonsulfur photosynthetic bacterium, Rhodovulum sp., increased with increasing light intensity. A light intensity of 1800 W/m(2) hydrogen production rate was achieved at the rate of 9.4 micromol/mg dry weight/h. The hydrogen production of this strain was enhanced by the addition of a small amount of oxygen (12 micromol O(2)/reactor). Intracellular ATP content was most efficiently accumulated under microaerobic, dark conditions. Hydrogen production rate by Rhodovulum sp. was investigated using a double-phase photobioreactor consisting of light and dark compartments. This rate was compared with data obtained using a conventional photobioreactor. Rhodovulum sp. produced hydrogen at a rate of 0.38+/-0.03 micromol/mg dry weight/h under microaerobic conditions using the double-phase photobioreactor. The hydrogen production rate was four times greater under microaerobic conditions, as compared with anaerobic conditions using either type of photobioreactor. Hydrogen production using a double-phase photobioreactor was demonstrated continuously at the same rate for 150 h.     

Impact of rainfall manipulations and biotic controls on soil respiration in Mediterranean and desert ecosystems along an aridity gradient

Spatially heterogeneous ecosystems form a majority of land types in the vast drylands of the globe. To evaluate climate‐change effects on CO₂ fluxes in such ecosystems, it is critical to understand the relative responses of each ecosystem component (microsite). We investigated soil respiration (R_s) at four sites along an aridity gradient (90–780 mm mean annual precipitation, MAP) during almost 2 years. In addition, R_s was measured in rainfall manipulations plots at the two central sites where ～30% droughting and ～30% water supplementation treatments were used over 5 years. Annual R_s was higher by 23% under shrub canopies compared with herbaceous gaps between shrubs, but R_s at both microsites responded similarly to rainfall reduction. Decreasing precipitation and soil water content along the aridity gradient and across rainfall manipulations resulted in a progressive decline in R_s at both microsites, i.e. the drier the conditions, the larger was the effect of reduction in water availability on R_s. Annual R_s on the ecosystem scale decreased at a slope of 256/MAP g C m^?2 yr^?1 mm^?1 (r²=0.97). The reduction in R_s amounted to 77% along the aridity gradient and to 16% across rainfall manipulations. Soil organic carbon (SOC) decreased with declining precipitation, and variation in SOC stocks explained 77% of the variation in annual R_s across sites, rainfall manipulations and microsites. This study shows that rainfall manipulations over several years are a useful tool for experimentally predicting climate‐change effects on CO₂ fluxes for time scales (such as approximated by aridity gradients) that are beyond common research periods. Rainfall reduction decreases rates of R_s not only by lowering biological activity, but also by drastically reducing shrub cover. We postulate that future climate change in heterogeneous ecosystems, such as Mediterranean and deserts shrublands will have a major impact on R_s by feedbacks through changes in vegetation structure.     

The fertile island effect varies with aridity and plant patch type across an extensive continental gradient

Plant and Soil - Perennial plants play important roles in maintaining ecosystem functions by forming fertile islands beneath their canopies. Little is known about how the fertile island effect...     

Environmental effects on molecular and phenotypic variation in populations of Eruca sativa across a steep climatic gradient

In Israel Eruca sativa has a geographically narrow distribution across a steep climatic gradient that ranges from mesic Mediterranean to hot desert environments. These conditions offer an opportunity to study the influence of the environment on intraspecific genetic variation. For this, we combined an analysis of neutral genetic markers with a phenotypic evaluation in common‐garden experiments, and environmental characterization of populations that included climatic and edaphic parameters, as well as geographic distribution. A Bayesian clustering of individuals from nine representative populations based on amplified fragment length polymorphism (AFLP) divided the populations into a southern and a northern geographic cluster, with one admixed population at the geographic border between them. Linear mixed models, with cluster added as a grouping factor, revealed no clear effects of environment or geography on genetic distances, but this may be due to a strong association of geography and environment with genetic clusters. However, environmental factors accounted for part of the phenotypic variation observed in the common‐garden experiments. In addition, candidate loci for selection were identified by association with environmental parameters and by two outlier methods. One locus, identified by all three methods, also showed an association with trichome density and herbivore damage, in net‐house and field experiments, respectively. Accordingly, we propose that because trichomes are directly linked to defense against both herbivores and excess radiation, they could potentially be related to adaptive variation in these populations. These results demonstrate the value of combining environmental and phenotypic data with a detailed genetic survey when studying adaptation in plant populations.     

Temperature effects on fish production across a natural thermal gradient

Global warming is widely predicted to reduce the biomass production of top predators, or even result in species loss. Several exceptions to this expectation have been identified, however, and it is vital that we understand the underlying mechanisms if we are to improve our ability to predict future trends. Here, we used a natural warming experiment in Iceland and quantitative theoretical predictions to investigate the success of brown trout as top predators across a stream temperature gradient (4–25 °C). Brown trout are at the northern limit of their geographic distribution in this system, with ambient stream temperatures below their optimum for maximal growth, and above it in the warmest streams. A five‐month mark‐recapture study revealed that population abundance, biomass, growth rate, and production of trout all increased with stream temperature. We identified two mechanisms that contributed to these responses: (1) trout became more selective in their diet as stream temperature increased, feeding higher in the food web and increasing in trophic position; and (2) trophic transfer through the food web was more efficient in the warmer streams. We found little evidence to support a third potential mechanism: that external subsidies would play a more important role in the diet of trout with increasing stream temperature. Resource availability was also amplified through the trophic levels with warming, as predicted by metabolic theory in nutrient‐replete systems. These results highlight circumstances in which top predators can thrive in warmer environments and contribute to our knowledge of warming impacts on natural communities and ecosystem functioning.     

Acetate as a carbon source for hydrogen production by photosynthetic bacteria

Hydrogen is a clean energy alternative to fossil fuels. Photosynthetic bacteria produce hydrogen from organic compounds by an anaerobic light-dependent electron transfer process. In the present study hydrogen production by three photosynthetic bacterial strains (Rhodopseudomonas sp., Rhodopseudomonas palustris and a non-identified strain), from four different short-chain organic acids (lactate, malate, acetate and butyrate) was investigated. The effect of light intensity on hydrogen production was also studied by supplying two different light intensities, using acetate as the electron donor. Hydrogen production rates and light efficiencies were compared. Rhodopseudomonas sp. produced the highest volume of H2. This strain reached a maximum H2 production rate of 25 ml H2 l(-1) h(-1), under a light intensity of 680 micromol photons m(-2) s(-1), and a maximum light efficiency of 6.2% under a light intensity of 43 micromol photons m(-2) s(-1). Furthermore, a decrease in acetate concentration from 22 to 11 mM resulted in a decrease in the hydrogen evolved from 214 to 27 ml H2 per vessel.     

Seasonal and spatial variation in photosynthetic response of the kelp Ecklonia radiata across a turbidity gradient

Photosynthesis Research - Understanding the photoacclimation response of macroalgae across broad spatial and temporal scales is necessary for predicting their vulnerability to environmental changes...     

Methylmercury concentrations and production rates across a trophic gradient in the northern Everglades

Methylmercury (MeHg) concentrations and production rates were examined along with sulfur biogeochemistry in Everglades sediments in March, July and December, 1995, as part of a large, multi-investigator study, the Aquatic Cycling of Mercury in the Everglades (ACME) project. The sites examined constitute a trophic gradient, generated from agricultural runoff, across the Everglades Nutrient Removal (ENR) Area, which is a re-constructed wetland, and Water Conservation Areas (WCA) 2A, 2B and 3 in the northern Everglades. MeHg concentrations and %MeHg (MeHg as a percent of total Hg) were lowest in the more eutrophic areas and highest in the more pristine areas in the south. MeHg concentrations ranged from <0.1 ng gdw^-1 sediment in the ENR to 5 ng gdw^-1 in WCA3 sediments; and MeHg constituted <0.2% of total Hg (Hg_T) in ENR, but up to about 2% in two sites in WCA2B and WCA3. Methylation rates in surficial sediments, estimated using tracer-level injections of²⁰³ Hg(II) into intact sediment cores, ranged from 0 to 0.12 d^-1, or about 1 to 10 ng g^-1 d^-1when the per day values are multiplied by the ambient total Hg concentration. Methylation was generally maximal at or within centimeters of the sediment surface, and was never observed in water overlying cores. The spatial pattern of MeHg production generally matched that of MeHg concentration. The coincident distributions of MeHg and its production suggest that in situ production controls concentration, and that MeHg concentration can be used as an analog for MeHg production. In addition, the spatial pattern of MeHg in Everglades sediments matches that in biota, suggesting that MeHg bioaccumulation may be predominantly a function of the de novo methylation rate in surficial sediments.Sulfate concentrations in surficial pore waters (up to 400 µm), microbial sulfate-reduction rates (up to 800 nm cc^-1 d^-1) and resultant pore water sulfide concentrations (up to 300 µm) at the eutrophic northern sites were all high relative to most freshwater systems. All declined to the south, and sulfate concentrations in WCA2B and in central WCA3 resembled those in oligotrophic lakes (50–100 µm). MeHg concentration and production were inversely related to sulfate reduction rate and pore water sulfide. Control of MeHg production in the northern Everglades appears to mimic that in an estuary, where sulfate concentrations are high and where sulfide produced by microbial sulfate reduction inhibits MeHg production.     

Virus morphological diversity and relationship to bacteria and chlorophyll across a freshwater trophic gradient in the Lake Michigan watershed

The effect of starvation on sexual reproduction in cyclic parthenogenetic rotifers has been studied using life history experiment. Short-time starvation of rotifers that experienced starvation immediately after hatching from resting eggs can cause high induction of sexual reproduction up to the 10th generation. However, it is not clear whether the induction of sexual reproduction can occur beyond the 10th generation. To investigate this phenomenon, we conducted a sex induction study using the monogonont rotifer Brachionus manjavacas. Newborn stem females were starved for 12 h, while controls were supplied with 7.0 × 10⁶ cells ml^?1 of Nannochloropsis oculata. In a life history experiment, the rotifers were individually cultured in 96-well microplates containing 0.2 ml of seawater (22 ppt) in each well at 25 °C with daily feeding thereafter. Mixis induction in offspring from starved stem females was significantly higher than in those from non-starved stem females up to the 40th generation. The effect of accumulative generations increased mixis induction up to the 20th generation. Effect on future generations of the rise in mixis ratio by the starvation to stem females may facilitate colonization by favoring population growth via female parthenogenesis and by decreasing food requirements for survival and reproduction.     

Microbial activity across a boreal peatland nutrient gradient: the role of fungi and bacteria

Microbes involved in decomposition within peatlands and the conditions that influence their activities have implications for C and greenhouse gas exchange. The objectives of this research were to characterize the role of fungal and bacterial activities in peatlands using selective antibiotic inhibition techniques across a nutrient gradient (rich to poor fens) and to search for environmental controls on the activity of each group. Bacterial activities predominated across a range of rich to poor boreal peatlands in central Ontario, Canada, although fungal activity became increasingly important in the poor sites. Linkages between soil pH and nutrient status and fungal and bacterial activities were found. However, they did not confirm our initial hypotheses that bacterial activity would be low in poor sites due to proton stress and low nutrient (particularly N) availability, whereas, fungal activity would be low in rich sites due to increased competitive ability of bacteria under near neutral pH conditions and high nutrient availability. Further work across these sites aimed at characterizing the phylogeny of the rhizosphere fungi is needed to determine if increased presence of mycorrhizae in poor sites could have explained our observed patterns. However, regardless of fungal: bacterial activity ratio differences across sites and its associated controls, microbial CO₂ production rates across fen types did not vary significantly, suggesting that the proportion of bacteria and fungi may not matter to broader carbon cycling and greenhouse gas emissions in peat soils.     

Sensitivity of primary production to precipitation across the United States

Primary production, a key regulator of the global carbon cycle, is highly responsive to variations in climate. Yet, a detailed, continental‐scale risk assessment of climate‐related impacts on primary production is lacking. We combined 16 years of MODIS NDVI data, a remotely sensed proxy for primary production, with observations from 1218 climate stations to derive values of ecosystem sensitivity to precipitation and aridity. For the first time, we produced an empirically‐derived map of ecosystem sensitivity to climate across the conterminous United States. Over this 16‐year period, annual primary production values were most sensitive to precipitation and aridity in dryland and grassland ecosystems. Century‐long trends measured at the climate stations showed intensifying aridity and climatic variability in many of these sensitive regions. Dryland ecosystems in the western US may be particularly vulnerable to reductions in primary production and consequent degradation of ecosystem services as climate change and variability increase in the future.     

Gain and loss learning differentially contribute to life financial outcomes

Emerging findings imply that distinct neurobehavioral systems process gains and losses. This study investigated whether individual differences in gain learning and loss learning might contribute to different life financial outcomes (i.e., assets versus debt). In a community sample of healthy adults (n = 75), rapid learners had smaller debt-to-asset ratios overall. More specific analyses, however, revealed that those who learned rapidly about gains had more assets, while those who learned rapidly about losses had less debt. These distinct associations remained strong even after controlling for potential cognitive (e.g., intelligence, memory, and risk preferences) and socioeconomic (e.g., age, sex, ethnicity, income, education) confounds. Self-reported measures of assets and debt were additionally validated with credit report data in a subset of subjects. These findings support the notion that different gain and loss learning systems may exert a cumulative influence on distinct life financial outcomes.     

An unfortunate alliance: Native shrubs increase the abundance,performance, and apparent impacts of Bromus tectorum across a regional aridity gradient

Interspecific facilitation contributes to the assembly of desert plant communities. However, we know little of how desert communities invaded by exotic species respond to facilitation along regional-scale aridity gradients. These measures are essential for predicting how desert plant communities might respond to concomitant plant invasion and environmental change. Here, we evaluated the potential for Bromus tectorum (a dominant invasive plant species) and the broader herbaceous plant community to form positive associations with native shrubs along a substantial aridity gradient across the Great Basin, Mojave, and San Joaquin Deserts in North America. Along this gradient, we sampled metrics of abundance and performance for B. tectorum, all native herbaceous species combined, all exotic herbaceous species combined, and the total herbaceous community using 180 pairs of shrub and open microsites. Across the gradient, B. tectorum formed strong positive associations with native shrubs, achieving 1.6–2.2 times greater abundance, biomass, and reproductive output under native shrubs than away from shrubs, regardless of relative aridity. In contrast, the broader herbaceous community was not positively associated with native shrubs. Interestingly, increasing B. tectorum abundance corresponded to decreasing native abundance, native species richness, exotic species richness, and total species richness under but not away from shrubs. Taken together, these findings suggest that native shrubs have considerable potential to directly (by increasing abundance and performance) and indirectly (by increasing competitive effects on neighbors) facilitate B. tectorum invasion across a large portion of the non-native range.     

Patterns of litter production across a salinity gradient in a Pterocarpus officinalis tropical wetland

Historically, Pterocarpus officinalisJacq. (Leguminoseae) dominated freshwater wetlands in the coastal plains of Puerto Rico, but deforestation has reduced its distribution to small patches adjacent to mangrove forests in areas of higher salinity. The objective of this study was to determine how a gradient in soil salinity affected litter, flower, and fruit production in a Pterocarpus officinalis.Three 100 m² plots were established in each of three sites along a salinity gradient: pasture/Pterocarpus edge (low salinity, mean salinity at 60 cm–9.7 g Kg^–1), Pterocarpus forest (intermediate salinity, 11.5 g Kg^–1) and a Pterocarpus/mangrove ecotone (high salinity, 15.0 g Kg^–1). Across this gradient, P. officinalis accounted for 100% of the relative basal area in the low and intermediate sites and 43% in the high salinity site which was domimated by Laguncularia racemosa. The basal area of P. officinalis decreased along the gradient from 73.5 m² ha^–1 in the low salinity site to 42.0 m² ha^–1 in the high salinity site. Litterfall was sampled on average every 23 days in 45 0.25 m² traps (5 traps per plot) for two years. Annual litterfall for the forest was 11.9 Mg ha^–1 yr^–1. Peaks in litterfall were associated with high precipitation in May 1995 and tropical storms in September 1995. Leaf fall of P. officinalis was significantly higher in the low salinity site (4.8 Mg ha^–1 yr^–1) than the high salinity site (1.8 Mg ha^–1 yr^–1), but total stand litterfall was greatest in the area of high salinity due to the greater contribution of L. racemosa. Pterocarpus flower and fruit production was approximately 10 times greater in low and intermediate salinity sites in comparison with the high salinity site. An increase in global temperature, will lead to higher sea level and higher soil salinity in costal wetlands. To conserve this wetland forest type it is critical to expand the distribution into areas of lower salinity where this species occurred historically.