Production of dissolved organic matter by phytoplankton and its uptake by heterotrophic prokaryotes in large tropical lakes

In pelagic ecosystems, phytoplankton extracellular release can extensively subsidize the heterotrophic prokaryotic carbon demand. Time-course experiments were carried out to quantify primary production, phytoplankton excretion, and the microbial uptake of freshly released dissolved organic carbon (DOC) derived from phytoplankton extracellular release (DOCp) in four large tropical lakes distributed along a productivity gradient: Kivu, Edward, Albert, and Victoria. The contributions of the major heterotrophic bacterial groups to the uptake of DOCp was also analyzed in Lake Kivu, using microautoradiography coupled to catalyzed reporter deposition fluorescent in situ hybridization. The percentage of extracellular release (PER) varied across the productivity gradient, with higher values at low productivity. Furthermore, PER was significantly related to high light and low phosphate concentrations in the mixed layer and was comparatively higher in oligotrophic tropical lakes than in their temperate counterparts. Both observations suggest that environmental factors play a key role in the control of phytoplankton excretion. Standing stocks of DOCp were small and generally contributed less than 1% to the total DOC because it was rapidly assimilated by prokaryotes. In other words, there was a tight coupling between the production and the heterotrophic consumption of DOCp. None of the major phylogenetic bacterial groups that were investigated differed in their ability to take up DOCp, in contrast with earlier results reported for standard labeled single-molecule substrates (leucine, glucose, adenosine triphosphate). It supports the idea that the metabolic ability to use DOCp is widespread among heterotrophic prokaryotes. Overall, these results highlight the importance of carbon transfer between phytoplankton and bacterioplankton in large African lakes.AFRIVAL - EAGLES East African Great Lake Ecosystem Sensitivity to Changes - CAKI Cycle du Carbone et des Nutriments au Lac Kivu - MICKI Microbial Diversity and Processes in Lake Kiv

Identification of a New Cholesterol-Binding Site within the IFN-gamma Receptor that is Required for Signal Transduction

[EN] The cytokine interferon-gamma (IFN-gamma) is a master regulator of innate and adaptive immunity involved in a broad array of human diseases that range from atherosclerosis to cancer. IFN-gamma exerts it signaling action by binding to a specific cell surface receptor, the IFN-gamma receptor (IFN-gamma R), whose activation critically depends on its partition into lipid nanodomains. However, little is known about the impact of specific lipids on IFN-gamma R signal transduction activity. Here, a new conserved cholesterol (chol) binding motif localized within its single transmembrane domain is identified. Through direct binding, chol drives the partition of IFN-gamma R2 chains into plasma membrane lipid nanodomains, orchestrating IFN-gamma R oligomerization and transmembrane signaling. Bioinformatics studies show that the signature sequence stands for a conserved chol-binding motif presented in many mammalian membrane proteins. The discovery of chol as the molecular switch governing IFN-gamma R transmembrane signaling represents a significant advance for understanding the mechanism of lipid selectivity by membrane proteins, but also for figuring out the role of lipids in modulating cell surface receptor function. Finally, this study suggests that inhibition of the chol-IFN gamma R2 interaction may represent a potential therapeutic strategy for various IFN-gamma-dependent diseases.This work was supported by grants from the Spanish Ministry of Science, Innovation, and Universities (BFU-2015-68981-P and PID2020-117405GB-I00) and the Basque Government (IT1264-19, IT1625-22) to F.-X.C. and M.L. F.-X.C. acknowledge the generous support of Fundacion Ramon Areces (grant CIVP20S11276). O.T. was supported by a Basque Government grant (IT1270-19) I.R.-B., O.M., J.A.N.-G., and D.C. were supported by the Fundacion Biofisica Bizkaia. The Lamaze laboratory was supported from Agence Nationale de la Recherche grants ANR-11-LABX-0038, ANR-10-IDEX-0001-02, and ANR NanoGammaR-15-CE11-0025-01. The Bernardino de la Serna Lab acknowledges support from Belinda and Bill Gates Foundation and BBSRC (INV-016631 and BB/V019791/1, respectively). This work was supported in part by the Fundacion Biofisica Bizkaia and the Basque Excellence Research Centre (BERC) program of the Basque Government. The authors thank J. M. Gonzalez Manas for helpful comments on the manuscript. The authors thank the technical and human support provided by the analytical and high-resolution microscopy facility (SGIker) of UPV/EHU and European funding (ERDF and ESF)

Bacterioplankton niche partitioning in the use of phytoplankton-derived dissolved organic carbon : quantity is more important than quality

Some prokaryotes are known to be specialized in the use of phytoplankton-derived dissolved organic carbon (DOCp) originated by exudation or cell lysis; however, direct quantification measurements are extremely rare. Several studies have described bacterial selectivity based on DOCp quality, but very few have focused on the quantity of DOCp, and the relative importance of each of these variables (for example, quantity versus quality) on prokaryote responses. We applied an adapted version of the MAR-FISH (microautoradiography coupled with catalyzed reporter deposition fluorescence in situ hybridization) protocol using radiolabelled exudates from axenic algal cultures to calculate a specialization index (d') for large bacterioplankton phylogenetic groups using DOCp from different phytoplankton species and at different concentrations to elucidate to what extent the bacterial response to DOCp is driven by resource quantity (different DOCp concentrations) or by quality (DOCp from different phytoplankton species). All bacterial phylogenetic groups studied had lower d' at higher DOCp concentration, indicating more generalist behavior at higher resource availabilities. Indeed, at increasing resource concentrations, most bacterial groups incorporated DOCp indiscriminately, regardless of its origin (or quality). At low resource concentrations, only some specialists were able to actively incorporate the various types of organic matter effectively. The variability of bacterial responses to different treatments was systematically higher at varying concentrations than at varying DOCp types, suggesting that, at least for this range of concentrations (10-100 μM), DOCp quantity affects bacterial responses more than quality does. Therefore, resource quantity may be more relevant than resource quality in the bacterial responses to DOCp and affect how bacterioplankton use phytoplankton-derived carbon.status: publishe

Bacterioplankton niche partitioning in the use of phytoplankton-derived dissolved organic carbon: quantity versus quality

Second EMBO Conference on Aquatic Microbial Ecology - SAME-14, 23-28 August 2015, UppsalaPeer Reviewe

Distribution of dissolved green-house gases (CO 2 , CH4, N2O) in Lakes Edward and George: Results from the first field cruise of the HIPE project

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Nitrous oxide and methane seasonal variability in the epilimnion of a large tropical meromictic lake (Lake Kivu, East-Africa)

© 2016, Springer International Publishing. We report a data-set of monthly vertical profiles obtained from January 2012 to October 2013, from the surface to 70 m depth of nitrous oxide (N 2 O) and dissolved methane (CH 4 ) in Lake Kivu, a large and deep meromictic tropical lake (East Africa). Vertical variations of N 2 O were modest, with ranges of 6–9 and 0–16 nmol L −1 in surface and bottom waters, respectively, and occasionally peaks of N 2 O (up to 58 nmol L −1 ) were observed at the oxic-anoxic interface. On the contrary, steep vertical gradients of CH 4 were observed with values changing several orders of magnitude from surface (19–103 nmol L −1 ) to 70 m (~113,000–520,000 nmol L −1 ). Seasonal variations of CH 4 were caused by annual cycles of mixing and stratification, during the dry and rainy seasons, respectively. This mixing allowed the establishment of a thick oxic layer (maximum 65 m deep), leading to decreased CH 4 concentrations (minimum of 8 nmol L −1 ), presumably due to bacterial CH 4 oxidation. During the stratification period, the oxic mixed layer was thinner (minimum 25 m deep), and an increase of CH 4 concentrations in surface waters was observed (maximum of 103 nmol L −1 ), probably due to a lower integrated CH 4 oxidation on the water column. Lake Kivu seasonally alternated between a source and a sink for atmospheric N 2 O, but on an annual scale was a small source of N 2 O to the atmosphere (on average 0.43 µmol m −2  day −1 ), while it was a small source of CH 4 to the atmosphere throughout the year (on average 86 µmol m −2  day −1 ). Vertical and seasonal variations of N 2 O are discussed in terms of nitrification and denitrification, although from the present data-set it is not possible to unambiguously identify the main drivers of N 2 O production.status: publishe

Carbon cycling of Lake Kivu (East Africa): net autotrophy in the epilimnion and emission of CO2 to the atmosphere sustained by geogenic inputs

We report organic and inorganic carbon distributions and fluxes in a large (>2000 km2) oligotrophic, tropical lake (Lake Kivu, East Africa), acquired during four field surveys, that captured the seasonal variations (March 2007–mid rainy season, September 2007–late dry season, June 2008–early dry season, and April 2009–late rainy season). The partial pressure of CO2 (pCO2) in surface waters of the main basin of Lake Kivu showed modest spatial (coefficient of variation between 3% and 6%), and seasonal variations with an amplitude of 163 ppm (between 579±23 ppm on average in March 2007 and 742±28 ppm on average in September 2007). The most prominent spatial feature of the pCO2 distribution was the very high pCO2 values in Kabuno Bay (a small sub-basin with little connection to the main lake) ranging between 11213 ppm and 14213 ppm (between 18 and 26 times higher than in the main basin). Surface waters of the main basin of Lake Kivu were a net source of CO2 to the atmosphere at an average rate of 10.8 mmol m−2 d−1, which is lower than the global average reported for freshwater, saline, and volcanic lakes. In Kabuno Bay, the CO2 emission to the atmosphere was on average 500.7 mmol m−2 d−1 (~46 times higher than in the main basin). Based on whole-lake mass balance of dissolved inorganic carbon (DIC) bulk concentrations and of its stable carbon isotope composition, we show that the epilimnion of Lake Kivu was net autotrophic. This is due to the modest river inputs of organic carbon owing to the small ratio of catchment area to lake surface area (2.15). The carbon budget implies that the CO2 emission to the atmosphere must be sustained by DIC inputs of geogenic origin from deep geothermal springs.status: publishe

Chronic hyperosmotic stress interferes with immune homeostasis in striped catfish (Pangasianodon hypophthalmus, S.) and leads to excessive inflammatory response during bacterial infection

Hyperosmotic stress has often been investigated from osmoregulation perspectives while the effects of such stress on the immune capacity remain largely unexplored. In this study, striped catfish were submitted to three salinity profiles (freshwater, low saline water, saline water) during 20 days, followed by infection with a virulent bacteria, Edwardsiella ictaluri, responsible for the enteric septicaemia of catfish. Osmoregulatory (plasma osmolality, gill Na(+)K(+)ATPase), immune (blood cells, lysozyme activity, complement activity, respiratory burst) parameters and mortality rate were investigated. In addition, abundances of heat shock protein 70 and high mobility group box 1 were explored. With elevated salinity, plasma osmolality severely increased while gill Na(+)K(+)ATPase slightly increased. Salinity alone stimulated the number of granulocytes, lysozyme activity and respiratory burst but depleted the number of thrombocytes. Salinity in combination with infection stimulated the number of monocytes and ACH50. On the contrary, erythrocytes, hematocrit, heat shock protein 70 and high mobility group box 1 did not significantly vary with salinity profiles. Then, salinity induced earlier onset on mortalities after E. ictaluri inoculation whereas cumulative mortality reach 79.2%, 67.0% and 91.7% respectively in freshwater, low saline water and saline water. In conclusion, salinity stimulates several immune functions in striped catfish but prolonged exposure to excessive hyperosmotic condition may lead to excessive inflammatory response and death.publisher: Elsevier articletitle: Chronic hyperosmotic stress interferes with immune homeostasis in striped catfish (Pangasianodon hypophthalmus, S.) and leads to excessive inflammatory response during bacterial infection journaltitle: Fish & Shellfish Immunology articlelink: http://dx.doi.org/10.1016/j.fsi.2016.06.031 content_type: article copyright: © 2016 Elsevier Ltd. All rights reserved.status: publishe

Dissolved primary production and heterotrophic prokaryote reassimilation in a large oligotrophic tropical lake (Lake Kivu, Eastern Africa)

2012 ASLO Aquatic Sciences Meeting: Voyages of Discovery, 8-13 July 2012, Lake Biwa, Shiga, JapanIn large lakes, phytoplankton extracellular release (dissolved primary production, dPP) can contribute substantially to the carbon requirement of heterotrophic prokaryote (HP). Nevertheless, the magnitude of dPP has never been measured in large tropical lakes and the identity of the micro-organisms responsible for the reassimilation of excreted compounds (DOCp) is unknown. We performed short-term (6h) radiotracer (14C) incubations to quantify dPP and the prokaryotic reassimilation of the labeled DOCp in Lake Kivu. We also examined the identity of the cells actively taking up DO14Cp using MAR-FISH (microautoradiography combined with fluorescent in-situ hybridization). The dPP rates were comparatively high, ranging from 40 to 65 % of total phytoplankton production, consistent with the view that tropical environmental conditions (high irradiance and temperature) favors high extracellular release. The DOCp was rapidly assimilated, less than 2 hours after the release. Members of Alphaproteobacteria, Betaproteobacteria and Bacteriodetes were involved in DO14Cp uptake proportionally to their abundance, but differed strongly in their ability to incorporate a standard tracer (3H-leucine). The latter suggests that bacterial production measurements (made with 3H-leucine) could be dramatically influenced by prokaryotic community compositionPeer Reviewe