Why we should conserve ‘limiting factors’, at least sometimes!

Körner's recent paper provided an excellent initial discussion of the term ‘limitation’ in ecology. I accept most of his arguments but would argue that there are some circumstances in applied ecology where the concept of limitation can be useful; these are (1) where there is a deficiency of a factor that prevents growth, and (2) where there is a need to manage an ecosystem so that a limitation is enforced.     

Fire effects on ecosystem nitrogen cycling in a Californian bishop pine forest

Fire can cause severe nitrogen (N) losses from grassland, chaparral, and temperate and boreal forest ecosystems. Paradoxically, soil ammonium levels are markedly increased by fire, resulting in high rates of primary production in re-establishing plant communities. In a manipulative experiment, we examined the influence of wild-fire ash residues on soil, microbial and plant N pools in a recently burned Californian bishop pine (Pinus muricata D. Don) forest. Ash stimulated post-fire primary production and ecosystem N retention through direct N inputs from ash to soils, as well as indirect ash effects on soil N availability to plants. These results suggest that redistribution of surface ash after fire by wind or water may cause substantial heterogeneity in soil N availability to plants, and could be an important mechanism contributing to vegetation patchiness in fire-prone ecosystems. In addition, we investigated the impact of fire on ecosystem N cycling by comparing ¹⁵N natural abundance values from recently burned and nearby unburned P. muricata forest communities. At the burned site, ¹⁵N natural abundance in recolonising species was similar to that in bulk soil organic matter. By contrast, there was a marked ¹⁵N depletion in the same species relative to the total soil N pool at the unburned site. These results suggest that plant uptake of nitrate (which tends to be strongly depleted in ¹⁵N because of fractionation during nitrification) is low in recently burned forest communities but could be an important component of eco- system N cycling in mature conifer stands. Received: 29 June 1999 / Accepted: 24 October 1999     

Site-directed mutations altering methyl-accepting residues of a sensory transducer protein

The Trg protein is one of a family of transducer proteins that mediate chemotactic response in Escherichia coli. Transducers are methyl-accepting proteins that gain or lose methyl esters on specific glutamyl residues during sensory adaptation. In this study, the significance of multiple sites of methylation on transducer proteins was addressed by using oligonucleotide-directed, site-specific mutagenesis to substitute an alanyl residue at each of the five methyl-accepting sites in Trg. The resulting collection of five mutations, each inactivating a single site, was analyzed for effects on covalent modification at the remaining sites on Trg and for the ability of the altered proteins to mediate sensory adaptation. Most of the alanyl substitutions had substantial biochemical effects, enhancing or reducing methyl-accepting activity of other sites, including one case of activation of a site not methylated in wild-type protein. Analysis of the altered proteins provided explanations for many features of the complex pattern of electrophoretic forms exhibited by Trg. The mutant proteins were less efficient than normal Trg in mediating adaptation. Correlation of biochemical and behavioral data indicated that reduction in the number of methyl-accepting sites on the transducer lengthened the time required to reach an adapted state.     

Photosynthesis and respiration of a diatom biofilm cultured in a new gradient growth chamber

Abstract A diatom biofilm was grown in a chamber developed for culture of biofilms in chemical gradients. The diatoms grew on a polycarbonate membrane filter which separated a sterile reservoir, with added phosphate, from a reservoir without phosphate. Within 3 weeks of inoculation, a thick biofilm developed on the surface of the filter. The biofilms were homogeneous and therefore suitable for calculations of O₂ diffusion fluxes from concentration profiles of O₂. Profiles of O₂, pH, and gross photosynthesis at different light intensities and liquid medium concentrations of dissolved inorganic carbon and O₂ were measured with microelectrodes. Respiratory activity in a layer of the biofilm was determined as the difference between gross photosynthesis and outflux of O₂ from that layer. The photosynthetic activity in a well-developed biofilm grown at 360 μEinst m⁻² s⁻¹ and 2.4 mM HCO₃⁻ was limited by the supply of inorganic carbon. Exposure to light above 360 μEinst m⁻² s⁻¹ stimulated gross photosynthesis as well as respiratory processes without affecting net outflux of O₂. Higher concentrations of inorganic carbon, on the other hand, enhanced gross photosynthesis without concurrent increase in respiratory rate, resulting in an increased outflux of O₂. High concentrations of O₂ in the liquid medium decreased the net outflux of O₂ with little effect on the gross photosynthesis. The effects of inorganic carbon and O₂ on the metabolic activities of the biofilm were consistent with the presence of photorespiratory activity.     

Scanning and transmission electron microscopy of oral sucker papillae of Philophthalmus megalurus

Edwards H. H., Nollen P. M. &; Nadakavukaren M. J. 1977. Scanning and transmission electron microscopy of the oral sucker papillae of Philophthalmus megalurus. International Journal for Parasitology7: 429–437. Scanning electron microscopy reveals papillae on both the inside and outside of the oral sucker of the eyefluke, Philophthalmus megalurus. In the transmission electron microscope the papillae can be divided into 3 different types with 2 of these types occurring on the outside of the oral sucker. One type of outer papilla contains a bulb cell which is terminated by a cilium having an apparent typical arrangement of microtubules. This type is a presumed sensory receptor having tango- and/or rheoreceptor function. The second type of outer papilla contains a gland cell which secretes electron-dense granules to the outside of the fluke. The third type of papilla is found only on the inside of the oral sucker and contains a bulb cell terminated by a cilium which does not have the typical 9 + 2 arrangement of microtubules. Instead, their numbers increase to over 60 randomly arranged microtubules. This inner papilla is felt to have chemoreceptor function because of its location and the presence of the modified cilium. The bulb cell of the inner papilla contains 2 newly described features. The first is granular, and associated with microtubules; it may be a possible nucleating site for microtubule synthesis. The second structure is a crystalline inclusion of unknown function and origin.     

The sensory basis of trail following in some lepidopterous larvae: contact chemoreception

ABSTRACT. . Caterpillars of the genus Malacosoma follow trails of the chemical 5-beta-cholestane-3,24-dione, but nothing is known of how they perceive this compound, or more generally about the sensory basis of trail following in caterpillars. By selective ablations of chemosensory organs we show that, in Malacosoma , the trail chemical is perceived by the maxillary palpi. In another lepidopteran species, Yponomeuta cagnagellus , the palpi are needed to discriminate their own trails from a trail of Malacosoma. Malacosoma larvae also lose their specificity for conspecific trails when their palpi are ablated. Volatile cues evidently do not play a role in trail-following behaviour, since neither Malacosoma nor Yponomeuta can orient on a trail covered with fine nylon mesh. These data indicate that for Malacosoma , and probably also for Yponomeuta , contact chemoreception mediated by the maxillary palpi is the primary mode of pheromone perception. The evolution of receptor sensitivity to trail chemicals in caterpillars is discussed.     

Modelling the influence of nutrient loads on Portuguese estuaries

The effects of implementing Directive 91/271/EEC of 21 May 1991 (Waste Water Treatment Plan Directive) and Directive 91/676/EEC of 12 December (Nitrates Directive) are analysed in 7 Portuguese estuaries (Minho, Lima, Douro, Mondego, Tagus, Sado and Guadiana) and two coastal lagoons (Ria de Aveiro and Ria Formosa), with a modelling approach. MOHID Water Modelling System was used to perform simulations with three nitrogen load scenarios for each system: a reference scenario, a 50% nitrate removal by agriculture scenario and another with a 100% nutrients removal by waste water treatment plants (WWTP). It is shown that the interaction between hydrodynamic and ecological processes is an important feature to study trophic problems in estuaries. Ecological processes such as primary production only occur inside the system if the residence time of water is high enough to enable organismal activity and if the adequate conditions are found (e.g. light, nutrients, temperature). From the model results it is possible to conclude: (i) in systems with short residence time a reduction in nutrient load will only produce a decrease in nutrient transit and will not affect the system’s global ecological status (e.g. Douro Estuary); (ii) in systems with long residence time the effects will range from significant, when primary production is mostly limited by nutrients (e.g. Ria de Aveiro), to non-significant, when primary production in the system is light-limited (e.g. Tagus Estuary).     

Divergent Effects of Acute Depolarization on Somatostatin Release and Protein Synthesis in Cultured Fetal and Neonatal Rat Brain Cells

The influence of membrane depolarization on somatostatin secretion and protein synthesis by fetal and neonatal cerebrocortical neurons was studied. Cortical cells obtained by mechanical dispersion were maintained as monolayer cultures for 8 days. The ability of fetal cerebrocortical and hypothalamic cells to release immunoreactive somatostatin (IR-SRIF) was confirmed. Total protein synthesis was determined by the incorporation of [3H]phenylalanine into trichloroacetic acid-precipitable proteins. To study the effect of acute depolarization on protein synthesis, cells were incubated for 30 min with [3H]phenylalanine or [3H]leucine and the depolarizing agent. In fetal cerebrocortical cells, potassium (30 and 56 mM) decreased protein synthesis and RNA levels and increased IR-SRIF release. Depolarization by veratridine, a sodium channel activator, induced a similar effect. The effect of veratridine on IR-SRIF and protein synthesis was reversed by tetrodotoxin, a sodium channel blocker, or verapamil, a calcium channel blocker. These findings suggest that protein synthesis by cerebrocortical cells is decreased in fetal brain cells by membrane depolarization and is dependent on Na+ and Ca2+ entry into cells. In postnatal (day 7) cerebrocortical cells, depolarization induced by high potassium concentrations led to a concomitant increase in protein synthesis, RNA content, and somatostatin release. These findings indicate that depolarization of the cellular membrane is coupled to an increase in protein synthesis in neonatal, but not in fetal, dispersed brain cells.