Distribution-free two-sample comparisons in the case of heterogeneous variances

Behavioral ecologists are often faced with a situation where they need to compare the central tendencies of two samples. The standard tools of the t test and Mann–Whitney U test (equivalent to the Wilcoxon rank-sum test) are unreliable when the variances of the groups are different. The problem is particularly severe when sample sizes are different between groups. The unequal-variance t test (Welch test) may not be suitable for nonnormal data. Here, we propose the use of Brunner and Munzel’s generalized Wilcoxon test followed by randomization to allow for small sample sizes. This tests whether the probability of an individual from one population being bigger than an individual from the other deviates from random expectation. This probability may sometimes be a more clear and informative measure of difference between the groups than a difference in more commonly used measures of central tendency (such as the mean). We provide a recipe for carrying out a statistical test of the null hypothesis that this probability is 50% and demonstrate the effectiveness of this technique for sample sizes typical in behavioral ecology. Although the test is not available in any commercial software package, it is relatively straightforward to implement for anyone with some programming ability. Furthermore, implementations in R and SAS are freely available on the internet.     

Sulfide assimilation by ectosymbionts of the sessile ciliate, <Emphasis Type="Italic">Zoothamnium niveum</Emphasis>

We investigated the constraints on sulfide uptake by bacterial ectosymbionts on the marine peritrich ciliate Zoothamnium niveum by a combination of experimental and numerical methods. Protists with symbionts were collected on large blocks of mangrove-peat. The blocks were placed in a flow cell with flow adjusted to in situ velocity. The water motion around the colonies was then characterized by particle tracking velocimetry. This shows that the feather-shaped colony of Z. niveum generates a unidirectional flow of seawater through the colony with no recirculation. The source of the feeding current was the free-flowing water although the size of the colonies suggests that they live partly submerged in the diffusive boundary layer. We showed that the filtered volume allows Z. niveum to assimilate sufficient sulfide to sustain the symbiosis at a few micromoles per liter in ambient concentration. Numerical modeling shows that sulfide oxidizing bacteria on the surfaces of Z. niveum can sustain 100-times higher sulfide uptake than bacteria on flat surfaces, such as microbial mats. The study demonstrates that the filter feeding zooids of Z. niveum are preadapted to be prime habitats for sulfide oxidizing bacteria due to Z. niveum’s habitat preference and due to the feeding current. Z. niveum is capable of exploiting low concentrations of sulfide in near norm-oxic seawater. This links its otherwise dissimilar habitats and makes it functionally similar to invertebrates with thiotrophic symbionts in filtering organs. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Application of a simplified mathematical model to estimate the effect of forced aeration on composting in a closed system

The aeration rate is a key process control parameter in the forced aeration composting process because it greatly affects different physico-chemical parameters such as temperature and moisture content, and indirectly influences the biological degradation rate. In this study, the effect of a constant airflow rate on vertical temperature distribution and organic waste degradation in the composting mass is analyzed using a previously developed mathematical model of the composting process. The model was applied to analyze the effect of two different ambient conditions, namely, hot and cold ambient condition, and four different airflow rates such as 1.5, 3.0, 4.5, and 6.0m(3)m(-2)h(-1), respectively, on the temperature distribution and organic waste degradation in a given waste mixture. The typical waste mixture had 59% moisture content and 96% volatile solids, however, the proportion could be varied as required. The results suggested that the model could be efficiently used to analyze composting under variable ambient and operating conditions. A lower airflow rate around 1.5-3.0m(3)m(-2)h(-1) was found to be suitable for cold ambient condition while a higher airflow rate around 4.5-6.0m(3)m(-2)h(-1) was preferable for hot ambient condition. The engineered way of application of this model is flexible which allows the changes in any input parameters within the realistic range. It can be widely used for conceptual process design, studies on the effect of ambient conditions, optimization studies in existing composting plants, and process control.     

Management, disposal and recycling of waste industrial organic solvents in Hong Kong.

An attempt has been made to establish a mass balance of industrial organic solvents in Hong Kong. It is estimated that only a small portion, less than 4%, of all the organic solvents consumed in Hong Kong are collected as waste solvents and properly treated, while the remainder are used either in the formulation of solvent containing products, or are lost to the environment through vapour emissions, leaks and spills, or dumped illegally. It was found that solvent recycling has been a common practice in some industries but the existing level of solvent recycling in Hong Kong is difficult to estimate. About 87.4% of all the waste organic solvents disposed of at the licensed facilities are potentially recyclable although whether they can be recycled in practice depends on many factors. Examples of existing waste organic solvent management and recycling practices from selected industries in Hong Kong are presented. The economic feasibility of current and future potential recycling systems is evaluated for a few selected cases. An integrated waste organic solvent management strategy is proposed to minimize adverse impacts of organic solvents to the environment and human health.     

Insights from complete-incomplete brood sex-ratio disparity

An assertion deeply rooted in the ornithological literature holds that sex-specific mortality causes a sex ratio disparity (SRD) between complete and incomplete broods. Complete broods are thought to reflect the primary sex ratio before any bias introduced by developmental mortality. Contrary to this view, however, complete and incomplete broods should exhibit identical sex ratio distributions even when the sexes experience differential mortality, as shown in the classic paper of Fiala (Am Nat 115: 442–444, 1980). Therefore, in partially unsexed samples, primary sex ratio biases cannot be distinguished from biases caused by differential mortality. In addition, complete broods do not represent primary sex ratio more accurately than incomplete ones and might even be misleading. Despite Fiala’s prediction, SRD does occur in some empirical studies. We show that this pattern could arise if (1) primary sex ratio affects chick mortality rates independently of sex (direct effect), (2) primary sex ratio covaries with a variable that also affects mortality rate, or (3) sex differential mortality covaries with overall mortality rate (indirect effects). Direct effects may cause stronger SRD than indirect ones with a smaller and opposite bias in the overall sex ratio and could also lead to highly inconsistent covariate effects on brood sex ratios. These features may help differentiate direct from indirect effects. Most interestingly, differences in covariate effects between complete and incomplete broods imply that influential variables are missing from the analysis.     

Interactions between chromium and sulfur metabolism in Brassica juncea

The effects of chromate on sulfate uptake and assimilation were investigated in the accumulator Brassica juncea (L.) Czern. Seven-day-old plants were grown for 2 d under the following combination of sulfate and chromate concentration: (i) no sulfate and no chromate (-S), (ii) no sulfate and 0.2 mmol L(-1) chromate (-S +Cr), (iii) 1 mmol L(-1) sulfate and no chromate (+S), or (iv) 1 mmol L(-1) sulfate and 0.2 mmol L(-1) chromate (+S +Cr). Despite the toxic effects exerted by chromate as indicated by altered level of reducing sugars and proteins in leaves, the growth of B. juncea was only weakly reduced by chromate, and no variation in chlorophyll a and b was measured, regardless of S availability. Chromium (Cr) was stored more in roots than in leaves, and the maximum Cr accumulation was measured in -S +Cr plants. The significant decrease of the sulfate uptake rates observed in Cr-treated plants was accompanied by a repression of the root low-affinity sulfate transporter (BjST1), suggesting that the transport of chromate in B. juncea may involve sulfate carriers. Once absorbed, chromate induced genes involved in sulfate assimilation (ATP-sulfurylase: atps6; APS-reductase: apsr2; Glutathione synthethase: gsh2) and accumulation of cysteine and glutathione, which may suggest that these reduced S compounds play a role in Cr tolerance. Together, our findings indicate that when phytoremediation technologies are used to recover Cr-contaminated areas, the concentration of sulfate in the plant growth medium must be considered because it may influence the ability of plants to accumulate and tolerate Cr.     

A new method to quantify the impact of soil carbon management on biophysical soil properties: the example of two apple orchard systems in New Zealand

A new method to diagnose the environmental sustainability of specific orchard management practices was derived and tested. As a significant factor for soil quality, the soil carbon (C) management in the topsoil of the tree-row of an integrated and organic apple orchard was selected and compared. Soil C management was defined as land management practices that maintain or increase soil C. We analyzed the impact of the soil C management on biological (microbial biomass C, basal respiration, dehydrogenase activity, respiratory quotient) and physical (aggregate stability, amount of plant-available water, conductive mean pore diameter near water saturation) soil properties. Soil in the alley acted as a reference for the managed soil in the tree row. The total and hot-water-extractable C amounts served as a combined proxy for the soil C management. The soil C management accounted for 0 to 81% of the degradation or enhancement of biophysical soil properties in the integrated and organic system. In the integrated system, soil C management led to a loss of C in the top 0.3 m of the tree row within 12 yr, causing a decrease in microbial activities. In the tree row of the organic orchard, C loss occurred in the top 0.1 m, and the decrease in microbial activities was small or not significant. Regarding physical soil properties, the C loss in the integrated system led to a decrease of the aggregate stability, whereas it increased in the organic system. Generally, the impact of soil C management was better correlated with soil microbial than with the physical properties. With respect to environmental soil functions that are sensitive to the decrease in microbial activity or aggregate stability, soil C management was sustainable in the organic system but not in the integrated system.     

Chemical composition of the slippery epicuticular wax blooms on Macaranga (Euphorbiaceae) ant-plants

Summary. The stems of many Macaranga ant-plants (Euphorbiaceae) are covered by epicuticular wax crystals rendering the surface very slippery for most insects. These wax blooms act as selective barriers protecting the symbiotic ant partners, which are specialized “wax-runners”, against the competition of other ants. Glaucous stems occur almost exclusively among the ant-plants of the genus Macaranga (). We analyzed the cuticular lipids of 16 Macaranga species by GC-MS and investigated the wax crystal morphology using SEM. Presence of crystalline wax blooms was strongly correlated with high concentrations (52%–88%) of triterpenoids. In contrast epicuticular waxes of glossy Macaranga surfaces contained only 0% to 36% of these dominant components. Therefore we conclude that triterpenoids are responsible for the formation of the thread-like Macaranga wax crystals. In all Macaranga ant-plants investigated, the principal components were epitaraxerol and taraxerone accompanied by smaller portions of taraxerol, β-amyrin and friedelin. Only in the case of the non-myrmecophytic M. tanarius did β-amyrin predominate. Moreover, we found that only in M. tanarius, the dense wax crystal lacework is torn into large mosaic-like pieces in the course of secondary stem diameter growth. Both chemical and macroscopic differences may contribute to a reduced slipperiness of M. tanarius stems and appear to be functionally important. The distribution of wax crystals and their composition amongst different sections of the genus suggests that glaucousness is a polyphyletic character within Macaranga. Received 7 October 1999; accepted 3 December 1999     

Modulation of steroidogenesis by coastal waters and sewage effluents of Hong Kong, China, using the H295R assay

BACKGROUND, AIM, AND SCOPE: The presence of a variety of pollutants in the aquatic environment that can potentially interfere with the production of sex steroid hormones in wildlife and humans has been of increasing concern. The aim of the present study was to investigate the effects of extracts from Hong Kong marine waters, and influents and effluents from wastewater treatment plants on steroidogenesis using the H295R cell bioassay. After exposing H295R cells to extracts of water, the expression of four steroidogenic genes and the production of three steroid hormones were measured. MATERIALS AND METHODS: Water samples were collected during the summer of 2005 from 24 coastal marine areas and from the influents and effluents of two major waste water treatment plants (WWTPs) in Hong Kong, China. Samples were extracted by solid phase extraction (SPE). H295R cells were exposed for 48 h to dilutions of these extracts. Modulations of the expression of the steroidogenic genes CYP19, CYP17, 3betaHSD2, and CYP11beta2 were determined by measuring mRNA concentrations by real-time polymerase chain reaction (Q-RT-PCR). Production of the hormones progesterone (P), estradiol (E2), and testosterone (T) was quantified using enzyme linked immunosorbent assays (ELISA). RESULTS: Extracts from samples collected in two fish culture areas inhibited growth and proliferation of H295R cells at concentrations greater or equal to 10(5) L equivalents. The cells were exposed to the equivalent concentration of active substances in 10,000 L of water. Thus, to observe the same level of effect as observed in vitro on aquatic organisms would require a bioaccumulation factor of this same magnitude. None of the other 22 marine samples affected growth of the cells at any dilution tested. Twelve of the marine water samples completely inhibited the expression of CYP19 without affecting E2 production; inhibition of CYP17 expression was observed only in one of the samples while expression of CYP11beta2 was induced as much as five- and ninefold after exposure of cells to extracts from two locations. The expression of the progesterone gene 3betaHSD2 was not affected by any of the samples; only one sample induced approximately fourfold the production of E2. Although more than twofold inductions were observed for P and T production, none of these values were statistically significant to conclude effects on the production of these two hormones. While influents from WWTPs did not affect gene expression, an approximately 30% inhibition in the production of E2 and a 40% increase in P occurred for the exposure with influents from the Sha Tin and Stonecutters WWTPs, respectively. Effluents from WWTPs did not affect the production of any of the studied hormones, but a decrement in the expression of the aldosterone gene CYP11beta2 was observed for the Sha Tin WWTP exposure. No direct correlation could be established between gene expression and hormone production. DISCUSSION: Observed cytotoxicity in the two samples from fish culture areas suggest the presence of toxic compounds; chemical analysis is required for their full identification. Although effluents from WWTPs did not affect hormone production, other types of endocrine activity such as receptor-mediated effects cannot be ruled out. Interactions due to the complexity of the samples and alternative steroidogenic pathways might explain the lack of correlation between gene expression and hormone production results. CONCLUSIONS: Changes observed in gene expression and hormone production suggest the presence in Hong Kong coastal waters of pollutants with endocrine disruption potential and others of significant toxic effects. The aromatase and aldosterone genes seem to be the most affected by the exposures, while E2 and P are the hormones with more significant changes observed. Results also suggest effectiveness in the removing of compounds with endocrine activity by the WWTPs studied, as effluent samples did not significantly affect hormone production. The H295R cell showed to be a valuable toll in the battery required for the analysis of endocrine disrupting activities of complex environmental samples. RECOMMENDATIONS AND PERSPECTIVES: Due to the intrinsic complexity of environmental samples, a combination of analytical tools is required to realistically assess environmental conditions, especially in aquatic systems. In the evaluation of endocrine disrupting activities, the H295R cell bioassay should be used in combination with other genomic, biological, chemical, and hydrological tests to establish viable modes for endocrine disruption and identify compounds responsible for the observed effects.     

Impact of mineral components and selected trace metals on ambient PM10 concentrations

PM10 levels of the mineral components Si, Al, Fe, Ca, Mg and some trace metals were measured at three different sites in the urban area of Vienna (Austria). Observed trace metal concentrations varied between less than 0.1 ng m^?3 (Cd) and approximately 200 ng m^?3 (Zn), mineral components showed enhanced concentrations ranging from 0.01 μg m^?3 (Ca) to 16.3 μg m^?3 (Si). The contribution of the respective mineral oxides to PM10 mass concentrations accounted on average for 26.4 ± 16% (n = 1090) of the PM10 mass, with enhanced rates in spring and autumn (monthly averages of up to 40%) and decreased contributions in the cold season (monthly averages below 10%). The atmospheric occurrence of Al, Ti and Sr could be assigned to crustal sources, whereas for the elements Ba, Ca, Fe, Mg, Mn and V an increased contribution of non-crustal origin was observed. PM10 levels of As, Cd, Co, Cr, Cu, Ni, Pb, Sb, Sn and Zn were predominantly derived from man-made emissions. Intersite comparison indicated that urban PM10 mass concentrations and PM10 levels of As, Pb and Zn were predominantly influenced from the transport of aerosols from outside into the city, whereas for the elements Ba, Mg, Ca, Cu and Fe a distinctly increased impact of local emissions was observed. The contribution of these urban emissions to total PM10 concentrations was estimated by calculating the so-called “urban impact”, which was found to be 32.7 ± 18% (n = 392) in the case of PM10 mass concentrations. The investigated elements accounted on average for 31.3 ± 19% (n = 392) of the observed PM10 mass increase. The mean values for the “urban impacts” of individual elements varied between 25.5% (As) and 77.0% (Ba).