Nitrogen sedimentation in a lake affected by massive nitrogen inputs: autochthonous versus allochthonous effects

1. Monthly changes to N loadings, in‐lake particulate organic nitrogen (PON), planktonic PON, and PON sedimentary flux were studied in a Spanish flowthrough, seepage lake subject to massive nitrogen inputs from June 2003 to December 2004 when water renewal was very rapid (0.09–0.17 year). 2. The distribution of in‐lake PON did not show a seasonal trend. Total nitrogen input flux ranged from 1.23 to 4.83 g N m^?2 day^?1, 71–76% of which is nitrate while PON represents 6–10%. PON sedimentation rates ranged from 9 to 90 mg N m^?2 day^?1 and fluctuated on a seasonal basis, reaching a minimum in winter and early spring and a maximum after thermal turnover had occurred. 3. This fluctuation was not related to either autochthonous planktonic production or allochthonous inputs. Since charophyte populations in Colgada Lake underwent a seasonal pattern of growth and decomposition, and ¹⁵δN values of settling material peaked at the end of that decomposition process, we suggest that PON sedimentary flux could be partly driven by decomposed charophyte particles. 4. However, the picture of PON sedimentation in this lake was more complex than anticipated because water residence time partly explained PON variability, albeit with a 1 month lag. Water residence time explained 40% of the overall variance of yearly averaged PON sedimentary flux in a meta‐analysis of 13 lakes worldwide. However, the factors such as phytoplankton composition, trophic structure, bottom communities, nutrient loading or productivity levels may also be influential on PON settling dynamics.     

Chimeras between Oilseed rape mosaic virus and Tobacco mosaic virus highlight the relevant role of the tobamoviral RdRp as pathogenicity determinant in several hosts

Oilseed rape mosaic virus (ORMV) is a tobamovirus taxonomically distinct from the type member of the genus, Tobacco mosaic virus (TMV). Both viruses display a specific host range, although they share certain hosts, such as Arabidopsis thaliana , Nicotiana benthamiana and N. tabacum , on which they induce different symptoms. Using a gain-of-symptom approach, we generated chimeric viruses, starting from a TMV infectious clone, over which different regions of ORMV were exchanged with their corresponding regions in the TMV genome. This approach allowed the association of pathogenicity determinants to certain genes within the ORMV genome. A general trend was observed associating the viral origin of the RNA-dependent RNA-polymerase ( RdRp ) gene and the gain of symptoms. In A. thaliana and N. benthamiana , chimeric viruses were unable to reproduce the symptoms induced by the parental viruses, leading to disease states which could be described as intermediate, and variable in some cases. In contrast, a hypersensitive reaction caused by both of these viruses on N -gene-bearing tobaccos could be found in resistance reactions to all chimeric viruses, suggesting that the avirulence determinant maps similarly in both viruses. A systemic necrotic spotting typical of non- N -gene tobaccos infected with ORMV was associated with the polymerase domain of RdRp . To our knowledge, this is the first time that this controversial portion of the tobamovirus genome has been identified directly as a pathogenicity determinant. None of the reactions of the chimeric viruses could be correlated with increases or decreases in virus titres in the infections.     

Interactive influences of fire intensity and vertical distribution of seed banks on post‐fire recolonization of a tall‐tussock grassland in Argentina

Abstract Recolonization patterns of a tall‐tussock grassland differ between low‐frequency and high‐intensity fires. A series of laboratory and field experiments were performed on the seed bank and on seeds of the main colonizing species (Carduus acanthoides, Cirsium vulgare, Lotus glaber) to determine and compare the effects of fire frequency and fire intensity on their recolonization potential. Seed‐bank size of colonizing species did not show a significant variation with contrasting fire frequencies, so overall interval‐dependent effects do not seem to affect their propagule pool at the time of fire. However, frequency of fire modified the proportion of viable seeds and their dormancy state according to species. Accumulated emergence from experimentally buried soft‐seeds of Lotus was lower after high‐intensity fires (following wind direction) than after back‐fires (against wind direction), but no significant effects of fire‐front direction on seedling emergence were observed for hard‐seeds of Lotus nor Carduus and Cirsium. Seedling emergence from buried seeds of Cirsium was increased with doubling fuel loads, particularly from deepen soil layers, where Cirsium had most of their viable seeds after a low frequency of fire. Emergence from hard‐seeds of Lotus was less promoted than Cirsium by doubling fuel loads, and according to analysis of excavated seedlings, it was completely inhibited from upper soil layers of the seed bank. Integration of species‐specific and burial depth‐specific responses of Cirsium and Lotus seeds according to fuel load, vertical distribution of the seed banks, seed dormant states and mean emergence depths, resulted in expected emergence values which agree with their previously observed patterns of recolonization. Thus, event‐dependent mechanisms had a better predictive value on recolonization success of the studied species than the observed cumulative effects of fire history on their seed‐bank size.     

NOTOLIMEA CLANDESTINA A NEW SPECIES OF NEOTENOUS BIVALVE (BIVALVIA: LIMIDAE) ENDEMIC TO THE STRAIT OF GIBRALTAR

Notolimea clandestina new species (Bivalvia: Limidae) is describedfrom the littoral of the Strait of Gibraltar. The small size(1 mm) and the persistence of the larval hinge suggest a neotenousspecies. Its area of distribution is likely to be restrictedto the Strait of Gibraltar. (Received 7 July 1993; accepted 2 December 1993)     

Genetic structure and population dynamics of a heteroecious plant pathogen Melampsora larici-epitea in short-rotation coppice willow plantations

Complex life strategies are common among plant pathogens belonging to rust fungi ( Uredinales ). The heteroecious willow rust Melampsora larici-epitea produces five spore stages and alternates on larch ( Larix ). To shed light on the epidemiology of this pathogen, amplified fragment length polymorphisms (AFLPs) were used to determine the genetic diversity and genetic structure of rust samples collected from coppice willow ( Salix ) plantations at three UK sites (LA, CA and MC) over three sampling dates (September 2000, July 2001 and September 2001). Of the total of 819 isolates, 465 were unique AFLP phenotypes and there was a shift in genotype diversity between the two seasons (0.67 in 2000 and 0.87–0.89 in 2001). No phenotypes were common between the two seasons within a site, suggesting that the rust did not overwinter as an asexual stage within plantations. A temporal analysis detected large amounts of genetic drift ( F _S = 0.15–0.26) between the two seasons and very small effective population sizes ( N _e  =  2–3) within sites. These results all point to a new colonization of the plantations by the rust in the second season (2001). The F _ST-analogue values were Φ_CT = 0.121, Weir and Cockerham's θ = 0.086 and the Bayesian estimate θ^B = 0.087–0.096. The results suggest that the sources of inoculum were somewhat localized and the same sources were mainly responsible for disease epidemics in LA and CA over the two seasons. The relatively low F _ST-values among sites (0.055–0.13) suggest the existence of significant gene flow among the three sampled sites.     

Comparison Between the Effect of the Fruit and of Exogenous GA3-Applications on Source-Sink Relationships in Citrus Sprouts

Inflorescence leaves of citrus (Citrus sinensis L. Osbeck) accumulatecarbohydrate reserves at the beginning of the fruit set period.This effect is mimicked by exogenous GA₃ applications in deflorateinflorescences. The effect of the hormone on two processes thatcould result in carbohydrate accumulation, i.e. increased ribulosebisphosphate carboxylase (RuBPCase) activity and changes inthe import/export balance of assimilates in the sprouts, wasstudied. Neither RuBPCase activity nor respiration rates were affectedby the hormonal treatment. On the other hand, blocking of thephloem connections with the rest of the plant induced an increasein carbohydrate contents in the leaves that was proportionallylower in GA₃-treated sprouts than in control deflorate inflorescences.This increase was minimal in leaves from inflorescences at laterstages of fruit development. This was in spite of a lower fruitgrowth rate that, in turn, brought about a decrease in totaldry weight of the sprout. In contrast, total dry weight washigher in girdled deflorate inflorescences, whether or not theywere treated with the hormone. The presence of the fruit induceda thickening of the stems of intact inflorescences that wasalso seen in ungirdled GA₂-treated sprouts. This increase indry weight per unit length, however, had a different chemicalbasis depending upon whether it was caused by the fruit or thehormone. From these data, the possible causes for the enhancement ofcarbohydrate accumulation in inflorescence leaves, and the involvementof gibberellin in the effect of fruit on the sprouts is discussed. Carbohydrates, Citrus sinensis L. Osbeck, gibberellic acid, girdling, orange, ribulose bisphosphate carboxylase, source-sink     

Survival of caddis larvae in relation to their case material in a group of temporary and permanent pools

1. The occurrence of larvae of Limnephilus coenosus and L. vittatus (Trichoptera) was recorded in 103 rock pools both before and after the dry periods in spring–summer (April–May and July–early August), when a great number of the pools dried out. In early spring, 84% of the pools sampled contained larvae of at least one of the species. Limnephilus coenosus larvae were present in a higher proportion of pools that later dried out than in the permanent ones, while L. vittatus larvae were predominantly found in permanent pools. Larvae of both species were found together in 31% of the inhabited pools.
2. Laboratory experiments, designed to elucidate differential survival under drought conditions, demonstrated that the organic cases of L. coenosus larvae held water more efficiently than did the mineral L. vittatus cases. Full-grown (fifth instar) L. coenosus larvae also survived dry conditions better than did L. vittatus larvae. The higher survival of L. coenosus was due to a combination of drought-resistant case material and a higher survival ability of the larvae themselves.
3. Limnephilus vittatus larvae were restricted by their low capability to withstand desiccation, although case material and food were available for L. vittatus larvae in all pools. Absence of case material and high-quality food in many pools, however, restricted the presence of L. coenosus larvae, but this species did not suffer from heavy mortality because of desiccation and all pool populations of L. coenosus survived until pupation.
4. The differences in species composition in these temporary and permanent pools can be explained by the differential site selection by ovipositing females, as well as by larval survival. The intrinsically greater survival of L. coenosus larvae during drought, together with the water-retaining properties of the cases, allowed this species to exploit stressful and risky habitats, such as temporary pools.     

A Novel Opioid Mechanism Seems to Modulate Phagocytosis in Tetrahymena

ABSTRACT. We have previously reported that a β-endorphin-like substance inhibits phagocytosis in Tetrahymena perhaps by a mu-like opioid receptor. We now report a further characterization of the elements involved in the signal transduction mechanism of this opioid. Affinity chromatography followed by immunoblots of both intracellular extracts and extracellular medium reveal the presence of two main proteins of 64 and 75 kDa. These molecular weights are much higher than that of any known opioid peptide or precursor protein and suggest that we may be dealing with either a novel opioid or with proteins that by chance cross-react with anti-β-endorphin antibody. Nevertheless, when the biological activity of these proteins was tested it was found that they had an effect similar to that of mammalian β-endorphin, namely inhibition of phagocytosis by a naloxone-reversible mechanism. We have probed a size-selected Tetrahymena library with a pro-opiomelanocortin probe and have obtained several positive clones; the sequencing of their inserts should establish whether we are dealing with a bona fide member of the opioid family. Another aspect we have been studying is the G-proteins which appear to be involved in the modulation of phagocytosis. We have found, by means of Western blotting (using an antibody against the conserved GTP-binding region of the α-subunit), two bands of 51 and 59 kDa; no α-subunit of 59 kDa had been reported previously and may represent a novel G-protein. In spite of these differences, the opioid signal transduction mechanism appears to remarkably resemble that present in more complex organisms.