Biological soil crusts (biocrusts) as a model system in community,landscape and ecosystem ecology

Model systems have had a profound influence on the development of ecological theory and general principles. Compared to alternatives, the most effective models share some combination of the following characteristics: simpler, smaller, faster, general, idiosyncratic or manipulable. We argue that biological soil crusts (biocrusts) have unique combinations of these features that should be more widely exploited in community, landscape and ecosystem ecology. In community ecology, biocrusts are elucidating the importance of biodiversity and spatial pattern for maintaining ecosystem multifunctionality due to their manipulability in experiments. Due to idiosyncrasies in their modes of facilitation and competition, biocrusts have led to new models on the interplay between environmental stress and biotic interactions and on the maintenance of biodiversity by competitive processes. Biocrusts are perhaps one of the best examples of micro-landscapes—real landscapes that are small in size. Although they exhibit varying patch heterogeneity, aggregation, connectivity and fragmentation, like macro-landscapes, they are also compatible with well-replicated experiments (unlike macro-landscapes). In ecosystem ecology, a number of studies are imposing small-scale, low cost manipulations of global change or state factors in biocrust micro-landscapes. The versatility of biocrusts to inform such disparate lines of inquiry suggests that they are an especially useful model system that can enable researchers to see ecological principles more clearly and quickly.     

Are soil lichen communities structured by biotic interactions? A null model analysis

Question: Are soil lichen communities structured by biotic interactions? Location: Gypsum outcrops located next to Belmonte del Tajo, central Spain. Methods: We sampled a total of 68 (50 cm × 50 cm) plots in gypsum outcrops from central Spain. Each plot was divided into 100 (5 cm × 5 cm) sampling quadrats, and the presence of all lichen species in every quadrat was recorded (6800 quadrats in total). We used two realistic null models to generate random communities unstructured by biotic interactions, and used them to test the hypothesis that soil lichen species co‐occur less often than expected by chance. Results: We found fewer species combinations and less co‐occurrence than expected by chance. However, the latter result was dependent on the null model selected. The number of checkerboard pairs did not differ significantly from the null expectation. Conclusions: Overall, our results suggest that gypsiferous soil lichen communities are structured by competitive interactions. They are consistent with studies conducted with a wide variety of taxa, and fill a gap in our knowledge of the factors driving the small‐scale distribution of these important organisms.     

Heavy metals fractionation and organic matter mineralisation in contaminated calcareous soil amended with organic materials

Degradation of organic matter (OM) from organic amendments used in the remediation of metal contaminated soils leads to changes in soil chemical properties shortly after their addition, which may affect the soil metal distribution. The effects of two differing organic amendments on OM mineralisation and fractionation of heavy metals in a contaminated soil were investigated in an incubation experiment. The treatments were: control unamended soil, soil amended with fresh cow manure, and soil amended with a compost having a high maturity degree. The soil used was characteristic of the mining area at La Unión (Murcia, Spain) with 28% CaCO(3) and sandy-loam texture (pH 7.7; 2602 mg kg(-1)Zn; 1572 mg kg(-1)Pb). Manure and compost C-mineralisation after 56 days (24% and 3.8%, respectively) were below values reported previously for uncontaminated soils. Both amendments favoured Zn and Pb fixation, particularly the manure. Mn solubility increased at the beginning of the experiment due to a pH effect, and only Cu solubility increased through organic matter chelation in both amended soils.     

Mobility of GPIIb-IIIa receptors within membranes of surface- and suspension-activated platelets does not depend on assembly and contraction of cytoplasmic actin

Investigations using fibrinogen coupled to colloidal gold (Fgn/Au) have shown that glycoprotein IIb-IIIa (GPIIb-IIIa) receptors are mobile and undergo centripetal reorganization on spread platelets following surface activation. The assembly of cytoplasmic actin and its constriction into an inner filamentous zone have been proposed as the mechanism driving the Fgn/Au receptor complexes across the plasma membrane. The present study has used cytochalasin B (CB), an agent known to inhibit actin assembly and cause breakdown of newly formed actin filaments to test that hypothesis. At a concentration which inhibited pseudopod formation and spreading, CB did not block movement of Fgn/Au receptor complexes toward platelet centers or into the open canalicular system (OCS). Channels of the OCS filled with Fgn/Au receptor complexes were evident in over 90% of the CB-treated, surface-activated platelets. The findings do not support the concept that assembly of cytoplasmic actin and its contraction move receptor-ligand complexes on the platelet plasma membrane.     

Platelet membrane responses to surface and suspension activation

Exposure of blood platelets to foreign surfaces or to potent agonists in suspension results in dramatic changes in physical appearance and conversion from a nonsticky state. The transformation to the sticky state is associated with exposure of the fibrinogen receptor, GPIIB-IIIa, which is hidden in resting, discoid platelets. Recent studies employing fibrinogen coupled to gold (Fgn-Au) as an electron-dense probe have suggested that GPIIb-IIIa receptors not only become exposed in surface-activated platelets, but undergo a reorganization not observed in suspension-activated cells. Discoid platelets do not bind Fgn-Au; however, the bodies and extended pseudopods of dendritic forms are covered with Fgn-Au particles. Conversion of dendritic platelets to spread forms is accompanied by movement of receptor-ligand complexes away from peripheral margins into a concentrated mass in cell centers over the inner filamentous zone of the cytoplasm. Movement of the Fgn-Au particles to cell centers during spreading was considered due to the transmembrane action of the newly assembled actin filaments. We have carried out similar experiments on surface- and suspension-activated platelets with Fgn-Au and latex particles. GPIIb-IIIa receptors move Fgn-Au particles on outer membranes of surface- and suspension-activated platelets to channels of the open canalicular system. Treatment with cytochalasin B prevents assembly of actin filaments in surface- and suspension-activated platelets, and dissociates residual actin from the cell membranes, circumferential microtubules and organelles with which they interact. However, cytochalasin B does not prevent removal of Fgn-Au to channels of the open canalicular system. Thus, reorganization of fibrinogen receptors on surface- and suspension-activated platelets is due to the particles, and not to the fibrinogen, although fibrinogen is required to link gold to the receptor. The surface membrane has its own detergent and cytochalasin B-resistant cytoskeleton for directed transport of ligand-receptor complexes, independent of the internal assembly and contraction of actin into an inner filamentous zone.     

Soil organic matter and the indigenous nitrogen supply of intensive irrigated rice systems in the tropics

Soil organic matter (SOM) has been proposed as an index of N supply in paddy soils although field validations are few. We evaluated the relationship between the indigenous N supply (N _i ) of the soil-floodwater system and soil organic carbon (SOC) or total N (N _t ) in surface soil of long-term fertility experiments (LTFEs) at 11 sites, in 42 farmer's fiels with similar soil type, and in the same field in ten consecutive rice (Oryza sativa L.) crops. The N _i was estimated by crop N uptake from plots without applied N (N _o plots) under otherwise favorable growth conditions. There was a tight linear correlation between yields and N uptake in N _o plots and tremendous variation in both parameters among LTFE sites, farmer's fields, and in the same field over time. Correlation between N _i and SOC or N _t explained little of this variation. Factors likely to contribute to the poor correlation were: (1) inputs of N from sources other than N mineralization of SOM in surface soil, (2) degree of congruence between soil N supply and crop demand, which is sensitive to soil drying, length of fallow, crop rotation, and residue management, and (3) differences in SOM quality related to intensive cropping in submerged soil. Better understanding of the processes governing the N _i of tropical lowland rice systems would contribute to the development of crop management practices that optimize utilization of indigenous N resources.     

TRPC6 participates in the regulation of cytosolic basal calcium concentration in murine resting platelets

Cytosolic-free Ca^2 + plays a crucial role in blood platelet function and is essential for thrombosis and hemostasis. Therefore, cytosolic-free Ca^2 + concentration is tightly regulated in this cell. TRPC6 is expressed in platelets, and an important role for this Ca^2 + channel in Ca^2 + homeostasis has been reported in other cell types. The aim of this work is to study the function of TRPC6 in platelet Ca^2 + homeostasis. The absence of TRPC6 resulted in an 18.73% decreased basal [Ca^2 +]_c in resting platelets as compared to control cells. Further analysis confirmed a similar Ca^2 + accumulation in wild-type and TRPC6-deficient mice; however, passive Ca^2 + leak rates from agonist-sensitive intracellular stores were significantly decreased in TRPC6-deficient platelets. Biotinylation studies indicated the presence of an intracellular TRPC6 population, and subcellular fractionation indicated their presence on endoplasmic reticulum membranes. Moreover, the presence of intracellular calcium release in platelets stimulated with 1-oleoyl-2-acetyl-sn-glycerol further suggested a functional TRPC6 population located on the intracellular membranes surrounding calcium stores. However, coimmunoprecipitation assay confirmed the absence of STIM1–TRPC6 interactions in resting conditions. This findings together with the absence of extracellular Mn^2 + entry in resting wild-type platelets indicate that the plasma membrane TRPC6 fraction does not play a significant role in the maintenance of basal [Ca^2 +]_c in mouse platelets. Our results suggest an active participation of the intracellular TRPC6 fraction as a regulator of basal [Ca^2 +]_c, controlling the passive Ca^2 + leak rate from agonist-sensitive intracellular Ca^2 + stores in resting platelets.