Experimental whole‐lake increase of dissolved organic carbon concentration produces unexpected increase in crustacean zooplankton density

The observed pattern of lake browning, or increased terrestrial dissolved organic carbon (DOC) concentration, across the northern hemisphere has amplified the importance of understanding how consumer productivity varies with DOC concentration. Results from comparative studies suggest these increased DOC concentrations may reduce crustacean zooplankton productivity due to reductions in resource quality and volume of suitable habitat. Although these spatial comparisons provide an expectation for the response of zooplankton productivity as DOC concentration increases, we still have an incomplete understanding of how zooplankton respond to temporal increases in DOC concentration within a single system. As such, we used a whole‐lake manipulation, in which DOC concentration was increased from 8 to 11 mg L^?1 in one basin of a manipulated lake, to test the hypothesis that crustacean zooplankton production should subsequently decrease. In contrast to the spatially derived expectation of sharp DOC‐mediated decline, we observed a small increase in zooplankton densities in response to our experimental increase in DOC concentration of the treatment basin. This was due to significant increases in gross primary production and resource quality (lower seston carbon‐to‐phosphorus ratio; C:P). These results demonstrate that temporal changes in lake characteristics due to increased DOC may impact zooplankton in ways that differ from those observed in spatial surveys. We also identified significant interannual variability across our study region, which highlights potential difficulty in detecting temporal responses of organism abundances to gradual environmental change (e.g., browning).     

Embryonic Lethality of Mitochondrial Pyruvate Carrier 1 Deficient Mouse Can Be Rescued by a Ketogenic Diet

Mitochondrial import of pyruvate by the mitochondrial pyruvate carrier (MPC) is a central step which links cytosolic and mitochondrial intermediary metabolism. To investigate the role of the MPC in mammalian physiology and development, we generated a mouse strain with complete loss of MPC1 expression. This resulted in embryonic lethality at around E13.5. Mouse embryonic fibroblasts (MEFs) derived from mutant mice displayed defective pyruvate-driven respiration as well as perturbed metabolic profiles, and both defects could be restored by reexpression of MPC1. Labeling experiments using ¹³C-labeled glucose and glutamine demonstrated that MPC deficiency causes increased glutaminolysis and reduced contribution of glucose-derived pyruvate to the TCA cycle. Morphological defects were observed in mutant embryonic brains, together with major alterations of their metabolome including lactic acidosis, diminished TCA cycle intermediates, energy deficit and a perturbed balance of neurotransmitters. Strikingly, these changes were reversed when the pregnant dams were fed a ketogenic diet, which provides acetyl-CoA directly to the TCA cycle and bypasses the need for a functional MPC. This allowed the normal gestation and development of MPC deficient pups, even though they all died within a few minutes post-delivery. This study establishes the MPC as a key player in regulating the metabolic state necessary for embryonic development, neurotransmitter balance and post-natal survival.     

Study of Plasmodium falciparum DHHC palmitoyl transferases identifies a role for PfDHHC9 in gametocytogenesis

Palmitoylation is the post‐translational reversible addition of the acyl moiety, palmitate, to cysteine residues of proteins and is involved in regulating protein trafficking, localization, stability and function. The Aspartate‐Histidine‐Histidine‐Cysteine (DHHC) protein family, named for their highly conserved DHHC signature motif, is thought to be responsible for catalysing protein palmitoylation. Palmitoylation is widespread in all eukaryotes, including the malaria parasite, Plasmodium falciparum, where over 400 palmitoylated proteins are present in the asexual intraerythrocytic schizont stage parasites, including proteins involved in key aspects of parasite maturation and development. The P. falciparum genome includes 12 proteins containing the conserved DHHC motif. In this study, we adapted a palmitoyl‐transferase activity assay for use with P. falciparum proteins and demonstrated for the first time that P. falciparum DHHC proteins are responsible for the palmitoylation of P. falciparum substrates. This assay also reveals that multiple DHHCs are capable of palmitoylating the same substrate, indicating functional redundancy at least in vitro. To test whether functional redundancy also exists in vivo, we investigated the endogenous localization and essentiality of a subset of schizont‐expressed PfDHHC proteins. Individual PfDHHC proteins localized to distinct organelles, including parasite‐specific organelles such as the rhoptries and inner membrane complex. Knock‐out studies identified individual DHHCs that may be essential for blood‐stage growth and others that were functionally redundant in the blood stages but may have functions in other stages of parasite development. Supporting this hypothesis, disruption of PfDHHC9 had no effect on blood‐stage growth but reduced the formation of gametocytes, suggesting that this protein could be exploited as a transmission‐blocking target. The localization and stage‐specific expression of the DHHC proteins may be important for regulating their substrate specificity and thus may provide a path for inhibitor development.     

Three‐dimensional microscale modelling of CO2 transport and light propagation in tomato leaves enlightens photosynthesis

We present a combined three‐dimensional (3‐D) model of light propagation, CO₂ diffusion and photosynthesis in tomato (Solanum lycopersicum L.) leaves. The model incorporates a geometrical representation of the actual leaf microstructure that we obtained with synchrotron radiation X‐ray laminography, and was evaluated using measurements of gas exchange and leaf optical properties. The combination of the 3‐D microstructure of leaf tissue and chloroplast movement induced by changes in light intensity affects the simulated CO₂ transport within the leaf. The model predicts extensive reassimilation of CO₂ produced by respiration and photorespiration. Simulations also suggest that carbonic anhydrase could enhance photosynthesis at low CO₂ levels but had little impact on photosynthesis at high CO₂ levels. The model confirms that scaling of photosynthetic capacity with absorbed light would improve efficiency of CO₂ fixation in the leaf, especially at low light intensity.     

Land use,rangeland degradation and ecological changes in the southern Kalahari,Botswana

Dual‐scale analyses assessing farm‐scale patterns of ecological change and landscape‐scale patterns of change in vegetation cover and animal distribution are presented from ecological transect studies away from waterpoints, regional remotely sensed analysis of vegetation cover and animal numbers across the southern Kalahari, Botswana. Bush encroachment is prevalent in semi‐arid sites where Acacia mellifera Benth. is widespread in communal areas and private ranches, showing that land tenure changes over the last 40 years have not avoided rangeland degradation. Herbaceous cover is dominated in intensively grazed areas by the annual grass Schmidtia kalahariensis Stent and in moderately grazed areas by the perennial grass Eragrostis lehmanniana Nees. Nutritious perennial grass species including Eragrostis pallens Hack. Ex Schinz remain prevalent in Wildlife Management Areas. Other ecological changes include the invasion of the exotic Prosopis glandulosa Torr. and dense stands of Rhigozum trichotomum Kuntze. in the arid southwest. Regional patterns of wildlife species show that the expansion of cattleposts and fenced ranches has led to large areas of low wildlife conservation value even in areas where cattle production is not practiced. Findings show the need for integrated landscape‐scale planning of land use if the ecological value and biodiversity of the southern Kalahari is to be retained.