Mineralization, pore water chemistry and phosphorus release from peaty sediments in the eutrophic Loosdrecht lakes, The Netherlands

SUMMARY. 1. Pore water chemistry in peaty sediment was monitored for a year at two representative locations of the eutrophic shallow Loosdrecht lakes. The Netherlands. Phosphorus fluxes over the sediment-water interface were calculated using measured concentration gradients in the pore water and compared to fluxes measured under laboratory conditions. Results were analysed with Redundancy Analysis to detect patterns of variation in pore water chemistry and in measured and calculated fluxes, that could be ascribed to environmental variables.
2. It was demonstrated that phosphorus fluxes measured in long-term laboratory incubations were not correlated to any of the pore water characteristics.
3. Initial phosphorus fluxes measured in sediment columns, which varied between −7.7 and 1330 μmol m⁻²: day⁻¹, were correlated significantly to the calculated phosphorus flux over the sediment-water interface.
4. The high correlation between calculated fluxes of ammonia, phosphorus and methane and measured initial flux of phosphorus, conclusively pointed to mineralization of organic matter as the driving force for phosphorus release from the sediment.
5. Redundancy Analysis demonstrated that the rates of mineralization and phosphorus release were only weakly related to temperature. They appeared to be especially stimulated by the autumnal decrease in temperature which was probably related to an extra input of organic matter.     

How river structure and biological traits influence gene flow: a population genetic study of two stream invertebrates with differing dispersal abilities

1. Determined by landscape structure as well as dispersal‐related traits of species, connectivity influences various key aspects of population biology, ranging from population persistence to genetic structure and diversity. Here, we investigated differences in small‐scale connectivity in terms of gene flow between populations of two ecologically important invertebrates with contrasting dispersal‐related traits: an amphipod (Gammarus fossarum) with a purely aquatic life cycle and a mayfly (Baetis rhodani) with a terrestrial adult stage. 2. We used highly polymorphic markers to estimate genetic differentiation between populations of both species within a Swiss pre‐alpine catchment and compared these results to the broader‐scale genetic structure within the Rhine drainage. Landscape genetic approaches were used to test for correlations of genetic and geographical structures and in‐stream barrier effects. 3. We found overall very weak genetic structure in populations of B. rhodani. In contrast, G. fossarum showed strong genetic differentiation, even at spatial scales of a few kilometres, and a clear pattern of isolation by distance. Genetic diversity decreased from downstream towards upstream populations of G. fossarum, suggesting asymmetric gene flow. Correlation of genetic structure with landscape topography was more pronounced in the amphipod. Our study also indicates that G. fossarum might be capable of dispersing overland in headwater regions and of crossing small in‐stream barriers. 4. We speculate that differences in dispersal capacity but also habitat specialisation and potentially the extent of local adaptation could be responsible for the differences in genetic differentiation found between the two species. These results highlight the importance of taking into account dispersal‐related traits when planning management and conservation strategies.     

Mutants impaired in vacuolar metal mobilization identify chloroplasts as a target for cadmium hypersensitivity in Arabidopsis thaliana

Cadmium (Cd) is highly toxic to plants causing growth reduction and chlorosis. It binds thiols and competes with essential transition metals. It affects major biochemical processes such as photosynthesis and the redox balance, but the connection between cadmium effects at the biochemical level and its deleterious effect on growth has seldom been established. In this study, two Cd hypersensitive mutants, cad1‐3 impaired in phytochelatin synthase (PCS1), and nramp3nramp4 impaired in release of vacuolar metal stores, have been compared. The analysis combines genetics with measurements of photosynthetic and antioxidant functions. Loss of AtNRAMP3 and AtNRAMP4 function or of PCS1 function leads to comparable Cd sensitivity. Root Cd hypersensitivities conferred by cad1‐3 and nramp3nramp4 are cumulative. The two mutants contrast in their tolerance to oxidative stress. In nramp3nramp4, the photosynthetic apparatus is severely affected by Cd, whereas it is much less affected in cad1‐3. In agreement with chloroplast being a prime target for Cd toxicity in nramp3nramp4, the Cd hypersensitivity of this mutant is alleviated in the dark. The Cd hypersensitivity of nramp3nramp4 mutant highlights the critical role of vacuolar metal stores to supply essential metals to plastids and maintain photosynthetic function under Cd and oxidative stresses.     

Effect of latitude on flavonoid biosynthesis in plants

The growth conditions in different latitudes vary markedly with season, day length, light quality and temperature. Many plant species have adapted well to the distinct environments through different strategies, one of which is the production of additional secondary metabolites. Flavonoids are a widely spread group of plant secondary metabolites that are involved in many crucial functions of plants. Our understanding of the biosynthesis, occurrence and function of flavonoids has increased rapidly in recent decades. Numerous studies have been published on the influence of environmental factors on the biosynthesis of flavonoids. However, extensive long‐term studies that examine the effect of the characteristics of northern climates on flavonoid biosynthesis are still scarce. This review focuses on the current knowledge about the effect of light intensity, photoperiod and temperature on the gene–environment interaction related to flavonoid biosynthesis in plants.     

Water relations and carbon gain are closely related to cushion size in the moss Grimmia pulvinata

The present study of structural and physiological changes during the development of the cushion moss, Grimmia pulvinata , quantifies the size-dependence of various parameters of water relations such as changes in surface: volume ratio ( S/V ) or water loss rates, and also measures net CO₂ gas exchange in the light and the dark. Larger cushions had lower S/V values than smaller ones and featured lower rates of area-based evapotranspiration, owing to higher boundary-layer resistance, but did not differ in relative water storage capacity (expressed as a percentage of d. wt). In combination, this leads to considerably longer hydration periods in larger cushions. By contrast, CO₂ gas-exchange parameters were negatively correlated with size : larger cushions showed significantly lower (mass-based) rates of net photosynthesis and dark respiration. Using these data, we estimated carbon budgets during a drying cycle as a function of cushion size. When including alternations of dark and light periods, the relationship proved to be rather complicated. Depending on the time of hydration, net carbon budgets not only varied quantitatively with size but sometimes took on both positive and negative values depending on cushion size. We conclude that neglecting plant size can lead to unrepeatable or even misleading results in comparative ecophysiological studies, and therefore urge for adequate attention to be paid to size in these studies.     

MORPHOLOGY, ANATOMY AND HISTOLOGY OF FLABELLINA AFFINIS (GMELIN, 1791) (NUDIBRANCHIA, AEOLIDOIDEA, FLABELLINIDAE) AND ITS RELATION TO OTHER MEDITERRANEAN FLABELLINA SPECIES

The Atlantic and Mediterranean flabellinid Flabellina affinis(Gmelin, 1791) (Opisthobranchia, Nudi-branchia, Aeolidoidea)is examined morphologically, anatomically and histologicallywith special emphasis on characters which have been widely neglectedin recent literature (e.g. the histological structure of theoral glands, typhlosole). The study provides detailed data aboutall organ systems which are compared to existing data of otherauthors. The species described as Flabellina affinis by Bergh(1875; 1886) is considered not to be conspecific with the Flabellinaaffinis examined in this and other studies. Furthermore, Flabellinaaffinis is compared to other Mediterranean species, especiallyF. ischitana (Hirano & Thompson, 1990). F. ischitana differsfrom F. affinis mainly by the structure of the genital system.The phylogenetic trees presented for the genus by Gosliner &Kuzirian (1990) and Gosliner & Willan (1991) are discussed. (Received 5 August 1996; accepted 9 June 1997)     

Use of fluorescently labelled algae to measure the clearance rate of the rotifer Keratella cochlearis

1. Fluorescently labelled algae (FLA) were used to measure clearance rates of the rotifer Keratella cochlearis. The freshwater algae Chlorella vulgaris and Stichococcus bacillaris were labelled with a fluorescent dye, 5-(4,6-dichlorotriazin-2-yl) aminofluorescein (DTAF), following a modified staining procedure. 2. Keratella cochlearis ingested both algal species. Clearance rates on tracer foods varied between 2.4 and 6.9 μl ind^?1 h^?1, which are comparable with those determined using other techniques. 3. The main drawback of the FLA technique was that only a little more than one-third of the total amount of algal cells of both C, vulgaris and S. bacillaris were well stained with the dye (DTAF), despite the use of a higher concentration of dye and a longer staining period than recommended in the literature. 4. The FLA method can be successfully applied in grazing studies involving size selection and competition for food among zooplankton. The method complements existing techniques for measuring the clearance and ingestion rates of filter-feeders.     

Plasma membrane H+-ATPase-dependent citrate exudation from cluster roots of phosphate-deficient white lupin

White lupin ( Lupinus albus L.) is able to grow on soils with sparingly available phosphate (P) by producing specialized structures called cluster roots. To mobilize sparingly soluble P forms in soils, cluster roots release substantial amounts of carboxylates and concomitantly acidify the rhizosphere. The relationship between acidification and carboxylate exudation is still largely unknown. In the present work, we studied the linkage between organic acids (malate and citrate) and proton exudations in cluster roots of P-deficient white lupin. After the illumination started, citrate exudation increased transiently and reached a maximum after 5 h. This effect was accompanied by a strong acidification of the external medium and alkalinization of the cytosol, as evidenced by in vivo nuclear magnetic resonance (NMR) analysis. Fusicoccin, an activator of the plasma membrane (PM) H⁺-ATPase, stimulated citrate exudation, whereas vanadate, an inhibitor of the H⁺-ATPase, reduced citrate exudation. The burst of citrate exudation was associated with an increase in expression of the LHA1 PM H⁺-ATPase gene, an increased amount of H⁺-ATPase protein, a shift in pH optimum of the enzyme and post-translational modification of an H⁺-ATPase protein involving binding of activating 14-3-3 protein. Taken together, our results indicate a close link in cluster roots of P-deficient white lupin between the burst of citrate exudation and PM H⁺-ATPase-catalysed proton efflux.     

Net biome production of the Amazon Basin in the 21st century

Global change includes multiple stressors to natural ecosystems ranging from direct climate and land‐use impacts to indirect degradation processes resulting from fire. Humid tropical forests are vulnerable to projected climate change and possible synergistic interactions with deforestation and fire, which may initiate a positive feedback to rising atmospheric CO₂. Here, we present results from a multifactorial impact analysis that combined an ensemble of climate change models with feedbacks from deforestation and accidental fires to quantify changes in Amazon Basin carbon cycling. Using the LPJmL Dynamic Global Vegetation Model, we modelled spatio‐temporal changes in net biome production (NBP); the difference between carbon fluxes from fire, deforestation, soil respiration and net primary production. By 2050, deforestation and fire (with no CO₂ increase or climate change) resulted in carbon losses of 7.4–20.3 Pg C with the range of uncertainty depending on socio‐economic storyline. During the same time period, interactions between climate and land use either compensated for carbon losses due to wetter climate and CO₂ fertilization or exacerbated carbon losses from drought‐induced forest mortality (?20.1 to +4.3 Pg C). By the end of the 21st century, depending on climate projection and the rate of deforestation (including its interaction with fire), carbon stocks either increased (+12.6 Pg C) or decreased (?40.6 Pg C). The synergistic effect of deforestation and fire with climate change contributed up to 26–36 Pg C of the overall decrease in carbon stocks. Agreement between climate projections (n=9), not accounting for deforestation and fire, in 2050 and 2098 was relatively low for the directional change in basin‐wide NBP (19–37%) and aboveground live biomass (13–24%). The largest uncertainty resulted from climate projections, followed by implementation of ecosystem dynamics and deforestation. Our analysis partitions the drivers of tropical ecosystem change and is relevant for guiding mitigation and adaptation policy related to global change.     

Metabolism of Arginine and Ornithine in Suspension-Cultured Cells and Aseptic Roots of Intact Plants of Atropa belladonna

Suspension-cultured cells and aseptically cultured roots ofintact plants of Atropa belladonna L. removed tropane alkaloidprecursors arginine (Arg) and ornithine (Orn) at nearly an equalrate from the feeding medium. A great part of Arg- and Orn-derived¹⁴C-label was found in ethanol-insoluble compounds, mostly inproteins already after 2 h feeding. Ethanol-soluble label inthe roots was found mainly in amino acids (e.g. glutamine, Gln)after 2 h feeding, and after 20 h also in some intermediatesof the urea cycle (e.g. argininosuccinate). In suspension cultures, subculturing of the initiation callusdecreased both the uptake of the basic amino acids tested andtheir binding on to the apoplastic space. After 20 h feedingwith Arg more label was found in organic acids in stationaryphase suspension cultures with repressed alkaloid synthesisthan in roots producing alkaloids. The growth phase and passagenumber also affected into which amino acids the label was incorporated.When the initiation callus was young (the 3rd passage), theintermediates of the urea cycle were actively labelled, butwhen the initiation callus was older (the 8th passage) and thesuspension formed roots, especially Gln was labelled. Only tracesof -N-methylornithine were detected in feeding experiments withOrn and Arg. Considerable arginase activity with a high pH optimumwas observed in cell suspensions and roots of A. belladonna. Key words: Atropa, arginine, ornithine, roots, suspension culture