Is nutrient availability related to plant nutrient use in humid tropical forests?

Data on soil nutrient availability for humid tropical forests are often reported, but are rarely integrated in an ecologically meaningful way with other measures of nutrient cycling. In this paper, estimates of soil nutrient availability and the inverse of litterfall nutrient concentrations (as an index of plant nutrient use) were compared, using data from 36 sites throughout the humid tropics, to determine if relationships exist between commonly used indices of nutrient cycling for plants and soils. Measures of both extractable and total soil P were significantly and positively correlated with the ratio of litterfall mass/P, particularly for montane tropical forests. Extractable soil P was also significantly correlated with litterfall mass for lowland humid tropical forests, explaining 58% of the variability in litterfall mass. A weak, albeit significant correlation was found between exchangeable soil Ca and litterfall mass/Ca, even though soil extraction techniques vary greatly. No significant relationship was found for total soil N, the most commonly measured soil N pool, and the inverse of litterfall N concentrations. The results suggest that our indices of soil P are related to litterfall processes, but that other measures, particularly total soil N, may not be as relevant to nutrient cycling by the vegetation.     

How do plants respond to nutrient shortage by biomass allocation?

Plants constantly sense the changes in their environment; when mineral elements are scarce, they often allocate a greater proportion of their biomass to the root system. This acclimatory response is a consequence of metabolic changes in the shoot and an adjustment of carbohydrate transport to the root. It has long been known that deficiencies of essential macronutrients (nitrogen, phosphorus, potassium and magnesium) result in an accumulation of carbohydrates in leaves and roots, and modify the shoot-to-root biomass ratio. Here, we present an update on the effects of mineral deficiencies on the expression of genes involved in primary metabolism in the shoot, the evidence for increased carbohydrate concentrations and altered biomass allocation between shoot and root, and the consequences of these changes on the growth and morphology of the plant root system.     

Is the relation between nutrient supply and biodiversity co‐determined by the type of nutrient limitation?

Correlative studies have shown a ‘hump‐backed’ relation between the vegetation N:P ratio and plant species diversity with the highest diversity at balanced N:P ratios (between 10 and 14). We tested the hypothesis that adding growth‐limiting nutrients to mesotrophic grasslands that were in shortage of either N (N:P ratio<10) or P (N:P ratio>14) would lead to an increase of plant diversity. Thereto, we studied the effects of long‐term (11 yr) experimentally increased N and/or P supply on soil nutrient pools, vegetation nutrient dynamics and biodiversity in a riverine grassland in the Netherlands with a low soil N:P ratio (N shortage) and a peat grassland with a high soil N:P ratio (P shortage), respectively. Eleven years of nutrient addition hardly had any effects on the total stocks of C, N and P in the soils of both sites, due to the large size of the soil nutrient pools already present and to the management at both sites (annual hay‐making and ‐removal). However, in the riverine grassland the treatments increased the cycling of the small pool of labile N and P compounds resulting in large increases in annual fluxes of especially N. In the unfertilised controls, species establishments balanced more or less species losses during an 11 year period, thus leading to a dynamic equilibrium of the species pool. However, contrary to our hypothesis, addition of the growth‐limiting nutrient led at both sites to a reduction of species diversity even when total biomass remained below critical levels. Species diversity and species evenness were strongly determined by N mineralisation and to a lesser extent by total soil N and extractable P, respectively. Total aboveground biomass of the vegetation was determined by total soil N. Our study shows that patterns found in correlative studies of the relation between plant diversity and soil and vegetation N:P ratio can not be translated into successful experimental manipulations to enhance biodiversity. The most likely explanation is that colonization limitation occurred in the fertilized plots and that not sufficient diaspores of potentially new species could reach and/or colonize the plots to compensate for the species extinctions as a result of increased nutrient supply.     

Is the scaling of swim speed in sharks driven by metabolism?

The movement rates of sharks are intrinsically linked to foraging ecology, predator–prey dynamics and wider ecosystem functioning in marine systems. During ram ventilation, however, shark movement rates are linked not only to ecological parameters, but also to physiology, as minimum speeds are required to provide sufficient water flow across the gills to maintain metabolism. We develop a geometric model predicting a positive scaling relationship between swim speeds in relation to body size and ultimately shark metabolism, taking into account estimates for the scaling of gill dimensions. Empirical data from 64 studies (26 species) were compiled to test our model while controlling for the influence of phylogenetic similarity between related species. Our model predictions were found to closely resemble the observed relationships from tracked sharks, providing a means to infer mobility in particularly intractable species.     

Is population growth conducive to the sustainability of cooperation?

This paper asks whether population growth is conducive to the sustainability of cooperation. A simple model is developed in which farmers who live around a circular lake engage in trade with their adjacent neighbors. The payoffs from this activity are governed by a prisoner's dilemma “rule of engagement.” Every farmer has one son when the population is not growing, or two sons when it is growing. In the former case, the son takes over the farm when his father dies. In the latter case, one son stays on his father's farm, whereas the other son settles around another lake, along with the “other” sons of the other farmers. During his childhood, each son observes the strategies and the payoffs of his father and of the trading partners of his father, and imitates the most successful strategy when starting farming on his own. Then mutant defectors are introduced into an all-cooperator community. The defector strategy may spread. A comparison is drawn between the impact in terms of the sustainability of cooperation of the appearance of the mutants in a population that is not growing, and in one that is growing. It is shown that the ex-ante probability of sustaining the cooperation strategy is higher for a community that is growing than for a stagnant community.     

Is growth of soil microorganisms in boreal forests limited by carbon or nitrogen availability?

To study whether the biomass of soil microorganisms in a boreal Pinus sylvestris-Vaccinium vitis-idaea forest was limited by the availability of carbon or nitrogen, we applied sucrose from sugar cane, a C₄ plant, to the organic mor-layer of the C₃–C dominated soil. We can distinguish between microbial mineralization of the added sucrose and respiration of endogenous carbon (root and microbial) by using the C₄-sucrose as a tracer, exploiting the difference in natural abundance of ¹³C between the added C₄-sucrose (¹³C –10.8) and the endogenous C₃–carbon (¹³C –26.6 ). In addition to sucrose, NH₄Cl (340 kg N ha^–1) was added factorially to the mor-layer. We followed the microbial activity for nine days after the treatments, by in situ sampling of CO₂ evolved from the soil and mass spectrometric analyses of ¹³C in the CO₂. We found that microbial biomass was limited by the availability of carbon, rather than nitrogen availability, since there was a 50% increase in soil respiration in situ between 1 h and 5 days after adding the sucrose. However, no further increase was observed unless nitrogen was also added. Analyses of the ¹³C ratios of the evolved CO₂ showed that increases in respiration observed between 1 h and 9 days after the additions could be accounted for by an increase in mineralization of the added C₄–C.     

Is the response of coral calcification to seawater acidification related to nutrient loading?

The effect of decreasing aragonite saturation state (Ω_Arag) of seawater (elevated pCO₂) on calcification rates of Acropora muricata was studied using nubbins prepared from parent colonies located at two sites of La Saline reef (La Réunion Island, western Indian Ocean): a back-reef site (BR) affected by nutrient-enriched groundwater discharge (mainly nitrate), and a reef flat site (RF) with low terrigenous inputs. Protein and chlorophyll a content of the nubbins, as well as zooxanthellae abundance, were lower at RF than BR. Nubbins were incubated at ~27°C over 2 h under sunlight, in filtered seawater manipulated to get differing initial pCO₂ (1,440–340 μatm), Ω_Arag (1.4–4.0), and dissolved inorganic carbon (DIC) concentrations (2,100–1,850 μmol kg⁻¹). Increasing DIC concentrations at constant total alkalinity (A_T) resulted in a decrease in Ω_Arag and an increase in pCO₂. A_T at the beginning of the incubations was kept at a natural level of 2,193 ± 6 μmol kg⁻¹ (mean ± SD). Net photosynthesis (NP) and calcification were calculated from changes in pH and A_T during the incubations. Calcification decrease in response to doubling pCO₂ relative to preindustrial level was 22% for RF nubbins. When normalized to surface area of the nubbins, (1) NP and calcification were higher at BR than RF, (2) NP increased in high pCO₂ treatments at BR compared to low pCO₂ treatments, and (3) calcification was not related to Ω_Arag at BR. When normalized to NP, calcification was linearly related to Ω_Arag at both sites, and the slopes of the relationships were not significantly different. The increase in NP at BR in the high pCO₂ treatments may have increased calcification and thus masked the negative effect of low Ω_Arag on calcification. Removing the effect of NP variations at BR showed that calcification declined in a similar manner with decreased Ω_Arag (increased pCO₂) whatever the nutrient loading.     

Is it possible to calibrate the pollution level of the region of Algiers (Mediterranean Sea) by exploiting marine macrophytes?

In the absence of water-quality data, a biological indicator is the only way to estimate the pollution level. Samples of macrophytes from exposed shallow rocky substrata of the region of Algiers (Algeria, Mediterranean Sea) were collected in supposedly (in the absence of available pollution data) polluted and pristine waters. These samples were compared to a set of samples spanning a known pollution gradient found near Marseilles (France) and to some samples from a variety of other Mediterranean localities. All samples were collected in similar conditions. The diversity point (i.e., the number of species per sample) was not greater at Cherchell (control site) than at the three sites in the Bay of Algiers. Analysis of the dataset was successful in ranking the Algerian sites, but failed to calibrate the pollution level of the Algerian sites by inserting Algerian samples within the pollution gradient of Marseilles. In contrast, regional characteristics of the macrophyte communities appear to be largely prevalent. This means that water-quality biological indicators and indices based upon marine macrophytes, at least for the open waters and exposed shallow Mediterranean habitats studied here, could be reliable within a given region, but may require validation and/or adjustment, perhaps considerable, for other regions.     

Nitrogen, macrophytes, shallow lakes and nutrient limitation: resolution of a current controversy?

Geomorphic research across the semi-arid and wet-dry tropics of northern Australia has highlighted the role of the regions’ estuaries as a “canary in the coalmine” for climate variations, with dramatic structural shifts over the Holocene. This behaviour highlights the region’s potential sensitivity to climate change and suggests the need for careful identification and interpretation of dynamics in the tropical and sub-tropical regions. However, analysis of change in these regions requires care, as a number of the basic tools applied to interpreting estuarine change in temperate regions are obscured, invalid or simply unavailable when applied in lower latitudes. This study provides a synthesis from a range of investigations across northern Australia and identifies characteristics to be considered when interpreting or predicting sub-millennial estuarine change in these regions.     

Does rapid utilization of elevated nutrient availability allow eucalypts to dominate in the tropical savannas of Australia?

Northern Australia's savannas are among the most fire‐prone biomes on Earth and are dominated by eucalypts (Eucalyptus and Corymbia spp.). It is not clear what processes allow this group to dominate under such extreme fire frequencies and whether a superior ability to compete for nutrients and water might play a role. There is evidence that eucalypts are adapted to frequent fires; juvenile eucalypts escape the fire trap by growing rapidly in height between fires. However, non‐eucalypts are less able to escape the fire trap and tend to have stand structures strongly skewed toward suppressed juveniles. The mechanisms that drive these contrasting fire responses are not well understood. Here, we describe the results of a controlled glasshouse seedling experiment that evaluated the relative importance of nutrient and water availability in determining height growth and biomass growth of two eucalypt and one noneucalypt tree species, common in northern Australian savannas. We demonstrate that growth of eucalypt seedlings is particularly responsive to nutrient addition. Eucalypt seedlings are able to rapidly utilize soil nutrients and accumulate biomass at a much greater rate than noneucalypt seedlings. We suggest that a seasonal spike in nutrient availability creates a nutrient‐rich microsite that allows eucalypt seedlings to rapidly gain height and biomass, increasing their likelihood of establishing successfully and reaching a fire‐resistant size. Our results extend our understanding of how eucalypts dominate northern Australian savannas under extremely high fire frequencies.     

Is the intrasomal phase of fast axonal transport driven by oscillations of intracellular calcium?

An hypothesis is presented suggesting that the delivery of vesicle-packaged protein from the neuronal soma to the axonal transport system is physiologically coupled to spontaneous fluctuations of intracellular calcium (Ca_i). Evidence is reviewed that oscillations of Ca_i, commonly detected as agonist-or voltage-triggered waves and spikes propagating through the cytosol, also occur as spontaneous events. Endogenously-generated oscillations are examined since intrasomal transport persists in the absence of extracellular signals or nerve impulse activity. Vesicle budding from the endoplasmic reticulum (ER) may be a key step at which anterograde transport is regulated by events related to the release and reuptake of ER stores of Ca²⁺.Special-issue dedicated to Dr. Sidney Ochs.     

Is the development of falciparum malaria in the human host limited by the availability of uninfected erythrocytes?

Background  

The development and propagation of malaria parasites in their vertebrate host is a complex process in which various host and parasite factors are involved. Sometimes the evolution of parasitaemia seems to be quelled by parasite load. In order to understand the typical dynamics of evolution of parasitaemia, various mathematical models have been developed. The basic premise ingrained in most models is that the availability of uninfected red blood cells (RBC) in which the parasite develops is a limiting factor in the propagation of the parasite population.     

Seasonal dynamics of macrophytes and phytoplankton in shallow lakes: a eutrophication‐driven pathway from plants to plankton?

1. Seasonal relationships between macrophyte and phytoplankton populations may alter considerably as lakes undergo eutrophication. Understanding of these changes may be key to the interpretation of ecological processes operating over longer (decadal‐centennial) timescales. 2. We explore the seasonal dynamics of macrophytes (measured twice in June and August) and phytoplankton (measured monthly May–September) populations in 39 shallow lakes (29 in the U.K. and 10 in Denmark) covering broad gradients for nutrients and plant abundance. 3. Three site groups were identified based on macrophyte seasonality; 16 lakes where macrophyte abundance was perennially low and the water generally turbid (‘turbid lakes’); 7 where macrophyte abundance was high in June but low in August (‘crashing’ lakes); and 12 where macrophyte abundance was high in both June and August (‘stable’ lakes). The seasonal behaviour of the crashing and turbid lakes was extremely similar with a consistent increase in nutrient concentrations and chlorophyll‐a over May–September. By contrast in the stable lakes, seasonal changes were dampened with chlorophyll‐a consistently low (<10–15 μg L^?1) over the entire summer. The crashing lakes were dominated by one or a combination of Potamogeton pusillus, Potamogeton pectinatus and Zannichellia palustris, whereas Ceratophyllum demersum and Chara spp. were more abundant in the stable lakes. 4. A long‐term loss of macrophyte species diversity has occurred in many shallow lakes affected by eutrophication. One common pathway is from a species‐rich plant community with charophytes to a species‐poor community dominated by P. pusillus, P. pectinatus and Z. palustris. Such compositional changes may often be accompanied by a substantial reduction in the seasonal duration of plant dominance and a greater tendency for incursions by phytoplankton. We hypothesise a slow‐enacting (10–100 s years) feedback loop in nutrient‐enriched shallow lakes whereby increases in algal abundance are associated with losses of macrophyte species and hence different plant seasonal strategies. In turn such changes may favour increased phytoplankton production thus placing further pressure on remaining macrophytes. This study blurs the distinction between so‐called turbid phytoplankton‐dominated and clear plant‐dominated shallow lakes and suggests that plant loss from them may be a gradual process.     

Quality or quantity: is nutrient transfer driven more by symbiont identity and productivity than by symbiont abundance?

By forming symbiotic interactions with microbes, many animals and plants gain access to the products of novel metabolic pathways. We investigated the transfer of symbiont-derived carbon and nitrogen to the sponges Aplysina cauliformis, Aplysina fulva, Chondrilla caribensis, Neopetrosia subtriangularis and Xestospongia bocatorensis, all of which host abundant microbial populations, and Niphates erecta, which hosts a sparse symbiont community. We incubated sponges in light and dark bottles containing seawater spiked with ¹³C- and ¹⁵N-enriched inorganic compounds and then measured ¹³C and ¹⁵N enrichment in the microbial (nutrient assimilation) and sponge (nutrient transfer) fractions. Surprisingly, although most sponges hosting abundant microbial communities were more enriched in ¹³C than N. erecta, only N. subtriangularis was more enriched in ¹⁵N than N. erecta. Although photosymbiont abundance varied substantially across species, ¹³C and ¹⁵N enrichment was not significantly correlated with photosymbiont abundance. Enrichment was significantly correlated with the ratio of gross productivity to respiration (P:R), which varied across host species and symbiont phylotype. Because irradiance impacts P:R ratios, we also incubated A. cauliformis in ¹³C-enriched seawater under different irradiances to determine whether symbiont carbon fixation and transfer are dependent on irradiance. Carbon fixation and transfer to the sponge host occurred in all treatments, but was greatest at higher irradiances and was significantly correlated with P:R ratios. Taken together, these results demonstrate that nutrient transfer from microbial symbionts to host sponges is influenced more by host–symbiont identities and P:R ratios than by symbiont abundance.     

Mycorrhizal colonizations of ferns and lycophytes on the island of La Réunion in relation to nutrient availability

Very little is known about the degree and ecological correlates of mycorrhization of lycophytes and ferns, although mycorrhizae are believed to be crucial for the majority of land plants. We screened the degree of mycorrhizal colonization for all 75 fern species recorded in nine survey plots across a wide range of soil conditions on the island of La Réunion, including five plots on lava flows of different age. Overall, 66% of all samples had mycorrhizae, but ground-dwelling species had significantly higher colonization levels (78%) than epiphytes (58%). Among ground-dwelling species, 98% of the individuals belonged to species with mycorrhizae, which was significantly higher than at the species level. Most samples had glomeromycetes as fungal partners, but septate endophytes were found in 41% of the colonized samples. While we found no significant relationship between the percentage of fern species with mycorrhizae per plot and seven ecological parameters, the percentage of fern individuals with mycorrhizal colonizations per plot significantly increased on shallow soils with high pH values and high base-saturation. This supports the idea that mycorrhizal colonizations confer an ecological advantage to colonized individuals, and that this advantage is more pronounced on nutrient-deficient sites. Our study thus provides evidence for an ecological advantage of mycorrhizae for ferns, but raises the question why, despite this advantage, species level mycorrhization among ferns (68%) is so much lower than the average for land plants (85%).     

Is mammalian chromosomal evolution driven by regions of genome fragility?

Background

A fundamental question in comparative genomics concerns the identification of mechanisms that underpin chromosomal change. In an attempt to shed light on the dynamics of mammalian genome evolution, we analyzed the distribution of syntenic blocks, evolutionary breakpoint regions, and evolutionary breakpoints taken from public databases available for seven eutherian species (mouse, rat, cattle, dog, pig, cat, and horse) and the chicken, and examined these for correspondence with human fragile sites and tandem repeats.

Results

Our results confirm previous investigations that showed the presence of chromosomal regions in the human genome that have been repeatedly used as illustrated by a high breakpoint accumulation in certain chromosomes and chromosomal bands. We show, however, that there is a striking correspondence between fragile site location, the positions of evolutionary breakpoints, and the distribution of tandem repeats throughout the human genome, which similarly reflect a non-uniform pattern of occurrence.

Conclusion

These observations provide further evidence that certain chromosomal regions in the human genome have been repeatedly used in the evolutionary process. As a consequence, the genome is a composite of fragile regions prone to reorganization that have been conserved in different lineages, and genomic tracts that do not exhibit the same levels of evolutionary plasticity.     

Is mammalian chromosomal evolution driven by regions of genome fragility?

Background  

A fundamental question in comparative genomics concerns the identification of mechanisms that underpin chromosomal change. In an attempt to shed light on the dynamics of mammalian genome evolution, we analyzed the distribution of syntenic blocks, evolutionary breakpoint regions, and evolutionary breakpoints taken from public databases available for seven eutherian species (mouse, rat, cattle, dog, pig, cat, and horse) and the chicken, and examined these for correspondence with human fragile sites and tandem repeats.     

Is GC bias in the nuclear genome of the carnivorous plant Utricularia?driven by ROS-based mutation and biased gene conversion?

At less than 90 Mbp, the tiny nuclear genome of the carnivorous bladderwort plant Utricularia is an attractive model system for studying molecular evolutionary processes leading to genome miniaturization. Recently, we reported that expression of genes encoding DNA repair and reactive oxygen species (ROS) detoxification enzymes is highest in Utricularia traps, and we argued that ROS mutagenic action correlates with the high nucleotide substitution rates observed in the Utricularia plastid, mitochondrial, and nuclear genomes. Here, we extend our analysis of 100 nuclear genes from Utricularia and related asterid eudicots to examine nucleotide substitution biases and their potential correlation with ROS-induced DNA lesions. We discovered an unusual bias toward GC nucleotides, most prominently in transition substitutions at the third position of codons, which are presumably silent with respect to adaptation. Given the general tendency of biased gene conversion to drive GC bias, and of ROS to induce double strand breaks requiring recombinational repair, we propose that some of the unusual features of the bladderwort and its genome may be more reflective of these nonadaptive processes than of natural selection.