Warming the phycosphere: Differential effect of temperature on the use of diatom-derived carbon by two copiotrophic bacterial taxa

Heterotrophic bacteria associated with microphytoplankton, particularly those colonizing the phycosphere, are major players in the remineralization of algal-derived carbon. Ocean warming might impact dissolved organic carbon (DOC) uptake by microphytoplankton-associated bacteria with unknown biogeochemical implications. Here, by incubating natural seawater samples at three different temperatures, we analysed the effect of experimental warming on the abundance and C and N uptake activity of Rhodobacteraceae and Flavobacteria, two bacterial groups typically associated with microphytoplankton. Using a nano-scale secondary ion mass spectrometry (nanoSIMS) single-cell analysis, we quantified the temperature sensitivity of these two taxonomic groups to the uptake of algal-derived DOC in the microphytoplankton associated fraction with ¹³C-bicarbonate and ¹⁵N-leucine as tracers. We found that cell-specific ¹³C uptake was similar for both groups (~0.42 fg C h⁻¹ μm⁻³), but Rhodobacteraceae were more active in ¹⁵N-leucine uptake. Due to the higher abundance of Flavobacteria associated with microphytoplankton, this group incorporated fourfold more carbon than Rhodobacteraceae. Cell-specific ¹³C uptake was influenced by temperature, but no significant differences were found for ¹⁵N-leucine uptake. Our results show that the contribution of Flavobacteria and Rhodobacteraceae to C assimilation increased up to sixfold and twofold, respectively, with an increase of 3°C above ambient temperature, suggesting that warming may differently affect the contribution of distinct copiotrophic bacterial taxa to carbon cycling.     

Coronin 1A promotes a cytoskeletal-based feedback loop that facilitates Rac1 translocation and activation

The activation of the Rac1 GTPase during cell signalling entails its translocation from the cytosol to membranes, release from sequestering Rho GDP dissociation inhibitors (RhoGDI), and GDP/GTP exchange. In addition to those steps, we show here that optimal Rac1 activation during cell signalling requires the engagement of a downstream, cytoskeletal-based feedback loop nucleated around the cytoskeletal protein coronin 1A and the Rac1 exchange factor ArhGEF7. These two proteins form a cytosolic complex that, upon Rac1-driven F-actin polymerization, translocates to juxtamembrane areas where it expands the pool of activated, membrane-bound Rac1. Such activity requires the formation of an F-actin/ArhGEF7-dependent physical complex of coronin 1A with Pak1 and RhoGDIα that, once assembled, promotes the Pak1-dependent dissociation of Rac1 from the Rac1/RhoGDIα complex and subsequent Rac1 activation. Genetic evidence demonstrates that this relay circuit is essential for generating sustained Rac1 activation levels during cell signalling.     

Relationships Between Coastal Bacterioplankton Growth Rates and Biomass Production: Comparison of Leucine and Thymidine Uptake with Single-Cell Physiological Characteristics

Specific growth rates of heterotrophic bacterioplankton have been frequently estimated from in situ bacterial production (BP) to biomass (BB) ratios, using a series of assumptions that may result in serious discrepancies with values obtained from predator-free cultures. Here, we used both types of approaches together with a comprehensive assessment of single-cell physiological characteristics (membrane integrity, nucleic acid content, and active respiration) of coastal bacterioplankton during a complete annual cycle (February 2007–January 2008) in the southern Bay of Biscay off Xixón, Spain. Both leucine and thymidine incorporation rates were used in conjunction with empirical tracer to carbon or cells conversion factors (eCFs) to accurately derive BP. Leu and TdR incorporation rates covaried year-round, as did the corresponding eCFs at 0 and 50 m depth. eCFs peaked in autumn, with mean annual values close to the theoretical ones (3.4 kg C mol Leu⁻¹ and 2.0 × 10¹⁸ cells mol TdR⁻¹). Bacterial abundance (0.2–1.5 × 10⁶ cells L⁻¹) showed a bimodal distribution with maxima in May and October and minima in March. Live (membrane-intact) cells dominated year-round (79–97%), with high nucleic acid cells (42–88%) and actively respiring bacteria (CTC+, 1–16%) showing distinct surface maxima in April and July, respectively. BB (557–1,558 mg C m⁻²) and BP (7–139 mg C m⁻² day⁻¹) presented two distinct peaks in spring and autumn, both of similar size due to a strong upwelling event observed in September. Specific growth rates (0.35–3.8 day⁻¹) were one order of magnitude higher in predator-free incubations than bacterial turnover rates derived from integrated BP:BB ratios (0.01–0.16 and 0.01–0.09 day⁻¹, for Leu and TdR, respectively) and were not correlated, probably due to a significant contribution of low activity cells to total standing stocks. The Leu:TdR molar ratio averaged for the water column (6.6–25.5) decreased significantly with higher integrated BB, indicating that low standing stocks tend to present unbalanced growth. Discrepancies about the true magnitude of specific growth rates must be solved before fully appreciating the role of bacteria in the ocean carbon cycle.     

Net production and consumption of fluorescent colored dissolved organic matter by natural bacterial assemblages growing on marine phytoplankton exudates

An understanding of the distribution of colored dissolved organic matter (CDOM) in the oceans and its role in the global carbon cycle requires a better knowledge of the colored materials produced and consumed by marine phytoplankton and bacteria. In this work, we examined the net uptake and release of CDOM by a natural bacterial community growing on DOM derived from four phytoplankton species cultured under axenic conditions. Fluorescent humic-like substances exuded by phytoplankton (excitation/emission [Ex/Em] wavelength, 310 nm/392 nm; Coble's peak M) were utilized by bacteria in different proportions depending on the phytoplankton species of origin. Furthermore, bacteria produced humic-like substances that fluoresce at an Ex/Em wavelength of 340 nm/440 nm (Coble's peak C). Differences were also observed in the Ex/Em wavelengths of the protein-like materials (Coble's peak T) produced by phytoplankton and bacteria. The induced fluorescent emission of CDOM produced by prokaryotes was an order of magnitude higher than that of CDOM produced by eukaryotes. We have also examined the final compositions of the bacterial communities growing on the exudates, which differed markedly depending on the phytoplankton species of origin. Alteromonas and Roseobacter were dominant during all the incubations on Chaetoceros sp. and Prorocentrum minimum exudates, respectively. Alteromonas was the dominant group growing on Skeletonema costatum exudates during the exponential growth phase, but it was replaced by Roseobacter afterwards. On Micromonas pusilla exudates, Roseobacter was replaced by Bacteroidetes after the exponential growth phase. Our work shows that fluorescence excitation-emission matrices of CDOM can be a helpful tool for the identification of microbial sources of DOM in the marine environment, but further studies are necessary to explore the association of particular bacterial groups with specific fluorophores.     

Decrease in the autotrophic-to-heterotrophic biomass ratio of picoplankton in oligotrophic marine waters due to bottle enclosure

We investigated the effects of bottle enclosure on autotrophic and heterotrophic picoplankton in North and South subtropical Atlantic oligotrophic waters, where the biomass and metabolism of the microbial community are dominated by the picoplankton size class. We measured changes in both autotrophic (Prochlorococcus, Synechococcus, and picoeukaryotes) and heterotrophic picoplankton biomass during three time series experiments and in 16 endpoint experiments over 24 h in light and dark treatments. Our results showed a divergent effect of bottle incubation on the autotrophic and heterotrophic components of the picoplankton community. The biomass of picophytoplankton showed, on average, a >50% decrease, mostly affecting the picoeukaryotes and, to a lesser extent, Prochlorococcus. In contrast, the biomass of heterotrophic bacteria remained constant or increased during the incubations. We also sampled 10 stations during a Lagrangian study in the North Atlantic subtropical gyre, which enabled us to compare the observed changes in the auto- to heterotrophic picoplankton biomass ratio (AB:HB ratio) inside the incubation bottles with those taking place in situ. While the AB:HB ratio in situ remained fairly constant during the Lagrangian study, it decreased significantly during the 24 h of incubation experiments. Thus, the rapid biomass changes observed in the incubations are artifacts resulting from bottle confinement and do not take place in natural conditions. Our results suggest that short (<1 day) bottle incubations in oligotrophic waters may lead to biased estimates of the microbial metabolic balance by underestimating primary production and/or overestimating bacterial respiration.     

The Spanish tourist sector facing extreme climate events: a case study of domestic tourism in the heat wave of 2003

This research explores, by means of a questionnaire-based survey, public knowledge and perception as well as the behaviour of young Spanish tourists before, during and after the summer holiday period affected by an episode of extreme heat in 2003. The survey was administered between November and December 2004. The extraordinary heat wave of the summer of 2003 can be seen as an example of a normal episode in terms of the predicted intensity and duration of European summers towards the end of the twenty-first century. It can therefore be used as the laboratory setting for this study. In this context, the use of the climate analogue approach allows us to obtain novel perspectives regarding the future impact that this type of event could have on tourist demand, based on a real experience. Likewise, such an approach allows the strategies of adaptation implemented by the different elements in the tourist system in order to cope with the atmospheric episode to be evaluated. Such strategies could prove useful in reducing vulnerability when faced with similar episodes in the future. The main results indicate that Spanish tourists (young segment market) are flexible in adapting to episodes of extremely high temperatures. Their personal perception of the phenomenon, their behaviour and the adaptation measures implemented to a greater or lesser extent before that time, reduce the vulnerability of the sector when faced with this type of event, at least from the point of view of this young segment of the internal national market. In Spain, the episode of extreme heat of 2003 has led to the implementation or improvement of some adaptive measures after the event, especially in the fields of management, policy and education.     

Reduction of NADPH-oxidase activity ameliorates the cardiovascular phenotype in a mouse model of Williams-Beuren Syndrome

A hallmark feature of Williams-Beuren Syndrome (WBS) is a generalized arteriopathy due to elastin deficiency, presenting as stenoses of medium and large arteries and leading to hypertension and other cardiovascular complications. Deletion of a functional NCF1 gene copy has been shown to protect a proportion of WBS patients against hypertension, likely through reduced NADPH-oxidase (NOX)-mediated oxidative stress. DD mice, carrying a 0.67 Mb heterozygous deletion including the Eln gene, presented with a generalized arteriopathy, hypertension, and cardiac hypertrophy, associated with elevated angiotensin II (angII), oxidative stress parameters, and Ncf1 expression. Genetic (by crossing with Ncf1 mutant) and/or pharmacological (with ang II type 1 receptor blocker, losartan, or NOX inhibitor apocynin) reduction of NOX activity controlled hormonal and biochemical parameters in DD mice, resulting in normalized blood pressure and improved cardiovascular histology. We provide strong evidence for implication of the redox system in the pathophysiology of the cardiovascular disease in a mouse model of WBS. The phenotype of these mice can be ameliorated by either genetic or pharmacological intervention reducing NOX activity, likely through reduced angII-mediated oxidative stress. Therefore, anti-NOX therapy merits evaluation to prevent the potentially serious cardiovascular complications of WBS, as well as in other cardiovascular disorders mediated by similar pathogenic mechanism.     

Vav1 and Rac control chemokine-promoted T lymphocyte adhesion mediated by the integrin alpha4beta1

The chemokine CXCL12 promotes T lymphocyte adhesion mediated by the integrin alpha4beta1. CXCL12 activates the GTPase Rac, as well as Vav1, a guanine-nucleotide exchange factor for Rac, concomitant with up-regulation of alpha4beta1-dependent adhesion. Inhibition of CXCL12-promoted Rac and Vav1 activation by transfection of dominant negative Rac or Vav1 forms, or by transfection of their siRNA, remarkably impaired the increase in T lymphocyte attachment to alpha4beta1 ligands in response to this chemokine. Importantly, inhibition of Vav1 expression by RNA interference resulted in a blockade of Rac activation in response to CXCL12. Adhesions in flow chambers and soluble binding assays using these transfectants indicated that initial ligand binding and adhesion strengthening mediated by alpha4beta1 were dependent on Vav1 and Rac activation by CXCL12. Finally, CXCL12-promoted T-cell transendothelial migration involving alpha4beta1-mediated adhesion was notably inhibited by expression of dominant negative Vav1 and Rac. These results indicate that activation of Vav1-Rac signaling pathway by CXCL12 represents an important inside-out event controlling efficient up-regulation of alpha4beta1-dependent T lymphocyte adhesion.