Strict Autotrophs Research Articles

Vegetation restoration increased the diversity and network complexity of carbon-fixing functional bacteria in heavily eroded areas of southern China

Carbon-fixing bacteria are a key functional group in soil carbon fixation processes, but their relationship with soil multifunctionality during vegetation restoration in eroded areas remains unclear. This hinders our ability to assess the true impact of vegetation restoration on soil ecosystem services. Therefore, we studied the impacts of three vegetation communities (tree-shrub, tree-grass, and tree-shrub-grass) in the same small watershed in a severely eroded area in southern China on the community of carbon-fixing bacteria in the soil. Compared to eroded areas, vegetation restoration sites had more complex networks of carbon-fixing bacteria, more carbon-fixing bacterial taxa, and more associations among these bacteria. Particularly, in the tree-shrub communities, the Chao1 index, Shannon index, node number, and edge number increased significantly by 291.29%, 49.65%, 364.58%, and 530.89%, respectively. The vegetation community pattern shifted the dominant carbon-fixing bacteria in the eroded areas from Rhodovastum to Nocardia. In the tree-shrub, tree-grass, and tree-shrub-grass patterns, the relative abundance of Rhodovastum decreased by 90.11%, 96.76%, and 95.37%, respectively, compared to that in the control group (CK). This indicates that vegetation community patterns have the potential to shift the dominant carbon-fixing bacteria from strict autotrophs to facultative autotrophs. The changes in the characteristics of the carbon-fixing bacterial community are closely related to the changes in soil multifunctionality caused by vegetation restoration. These results demonstrate that vegetation communities have a significant positive impact on the soil carbon fixation capacity.

Mixotrophy in aquatic plants, an overlooked ability.

Aquatic Embryophytes play a key role in the proper functioning of aquatic ecosystems, where carbon (inorganic and organic forms) is pivotal in biogeochemical processes. There is growing awareness that mixotrophy, the direct use of exogenous organic carbon by autotrophs, is a widespread phenomenon and that it has emerged recurrently in the evolution of many autotrophic lineages. Despite living in an environment providing organic matter and presenting many favourable predispositions, aquatic plants from the Embryophytes, except carnivorous ones, have never been deeply investigated for mixotrophy. Here, we address the possibility that aquatic plants may exhibit mixotrophy, a prospect overlooked by research until now, and that this may be much more widespread than imagined under the conventional paradigm of plants considered as strict autotrophs.

Bolstering fitness via CO2 fixation and organic carbon uptake: mixotrophs in modern groundwater

Current understanding of organic carbon inputs into ecosystems lacking photosynthetic primary production is predicated on data and inferences derived almost entirely from metagenomic analyses. The elevated abundances of putative chemolithoautotrophs in groundwaters suggest that dark CO2 fixation is an integral component of subsurface trophic webs. To understand the impact of autotrophically fixed carbon, the flux of CO2-derived carbon through various populations of subsurface microbiota must first be resolved, both quantitatively and temporally. Here we implement novel Stable Isotope Cluster Analysis to render a time-resolved and quantitative evaluation of 13CO2-derived carbon flow through a groundwater community in microcosms stimulated with reduced sulfur compounds. We demonstrate that mixotrophs, not strict autotrophs, were the most abundant active organisms in groundwater microcosms. Species of Hydrogenophaga, Polaromonas, Dechloromonas, and other metabolically versatile mixotrophs drove the production and remineralization of organic carbon. Their activity facilitated the replacement of 43% and 80% of total microbial carbon stores in the groundwater microcosms with 13C in just 21 and 70 days, respectively. The mixotrophs employed different strategies for satisfying their carbon requirements by balancing CO2 fixation and uptake of available organic compounds. These different strategies might provide fitness under nutrient-limited conditions, explaining the great abundances of mixotrophs in other oligotrophic habitats, such as the upper ocean and boreal lakes.

Mixotrophy upgrades food quality for marine calanoid copepods

AbstractInorganic nutrient limitation affects the stoichiometry and nutritional quality of marine phytoplankton. Mixoplankton, able to photosynthesize and feed simultaneously in the one cell, can compensate shortage of nutrients by phagotrophy, theoretically upgrading their nutritional quality for their predators: the zooplankton. Yet, the additional value that phagotrophy in mixoplankton may provide to support zooplankton growth and recruitment has been poorly explored. Therefore, we investigated the feeding and reproductive performances of the copepods Paracartia grani and Centropages typicus on mono‐diets of the dinoflagellate Karlodinium veneficum grown as strict autotroph and as mixotroph, both under N and P depletion, and in nutrient‐balanced conditions (f/2 medium; only as autotroph). Feeding and reproduction outputs were generally higher in P. grani than in C. typicus. Both copepod species ingested the mixotrophic K. veneficum at similar rates than the autotrophic ones in either nutrient‐limited scenario. However, egg production and recruitment rates generally increased when feeding on mixotrophs, in P. grani on both N‐ and P‐limited diets, and in C. typicus under P limitation. In general, P limitation influenced copepod physiology more than N depletion. Our results show that phagotrophy upgrades nutritional quality in nutrient‐limited mixotrophs as prey for copepods compared to the strict autotrophic ones. These findings are among the first reported cases of copepod ingestion in the laboratory on actively feeding mixoplankton, and they highlight the importance of considering the trophic mode of the protist and the response by various zooplanktonic predators when attempting to understand the functioning of marine planktonic food webs.

Temporal and spatial variability of phytoplankton and mixotrophs in a temperate estuary

A significant proportion of phototrophic species are known to be mixotrophs: cells that obtain nutrients through a combination of photosynthesis and prey ingestion. Current methods to estimate mixotroph abundance in situ are known to be limited in their ability to help identify conditions that favor mixotrophs over strict autotrophs. For the first time, we combine microscopic analysis of phototrophic taxa with immunoprecipitated bromodeoxyuridine (BrdU)-labeled DNA amplicon sequencing to identify and quantify active and putative mixotrophs at 2 locations in a microtidal temperate estuary. We analyze these data to examine spatial and temporal variability of phytoplankton and mixotrophs. Microscopy-based phototrophic diversity and abundances reveal expected seasonal patterns for our 2 stations, with the start of growth in winter and highest abundances in summer. Diatoms tend to dominate at the site with less stratification, while dinoflagellates and euglenids are usually more prominent at the stratified station. The BrdU-based mixotroph identifications are translated to the microscopy identification and abundances to estimate the proportion of mixotrophs (cells >10 µm in size) at both sites. The average proportion of potential mixotrophs is higher at the station with higher stratification (51%) compared to the station with lower stratification (30%), and potential mixotrophs tend to be higher in summer, although we did not conduct BrdU experiments in any of the other seasons. Combining the identification of active mixotrophs through the uptake of BrdU-labeled bacteria with robust abundance measurements can expand our understanding of mixotrophs across systems.

Hydrogen peroxide detoxification is a key mechanism for growth of ammonia-oxidizing archaea

Ammonia-oxidizing archaea (AOA), that is, members of the Thaumarchaeota phylum, occur ubiquitously in the environment and are of major significance for global nitrogen cycling. However, controls on cell growth and organic carbon assimilation by AOA are poorly understood. We isolated an ammonia-oxidizing archaeon (designated strain DDS1) from seawater and used this organism to study the physiology of ammonia oxidation. These findings were confirmed using four additional Thaumarchaeota strains from both marine and terrestrial habitats. Ammonia oxidation by strain DDS1 was enhanced in coculture with other bacteria, as well as in artificial seawater media supplemented with α-keto acids (e.g., pyruvate, oxaloacetate). α-Keto acid-enhanced activity of AOA has previously been interpreted as evidence of mixotrophy. However, assays for heterotrophic growth indicated that incorporation of pyruvate into archaeal membrane lipids was negligible. Lipid carbon atoms were, instead, derived from dissolved inorganic carbon, indicating strict autotrophic growth. α-Keto acids spontaneously detoxify H2O2 via a nonenzymatic decarboxylation reaction, suggesting a role of α-keto acids as H2O2 scavengers. Indeed, agents that also scavenge H2O2, such as dimethylthiourea and catalase, replaced the α-keto acid requirement, enhancing growth of strain DDS1. In fact, in the absence of α-keto acids, strain DDS1 and other AOA isolates were shown to endogenously produce H2O2 (up to ∼4.5 μM), which was inhibitory to growth. Genomic analyses indicated catalase genes are largely absent in the AOA. Our results indicate that AOA broadly feature strict autotrophic nutrition and implicate H2O2 as an important factor determining the activity, evolution, and community ecology of AOA ecotypes.

Microbial planktonic communities of freshwater environments from Tierra del Fuego: dominant trophic strategies in lakes with contrasting features

We analysed the structure of the microbial plankton communities of different typesof freshwater environments from the southernmost region of South America (Tierradel Fuego). Water bodies were grouped in four categories: humic lakes, clear oligo-trophic lakes, beaver ponds and steppe shallow lakes, which differed in their nutrientand dissolved organic carbon (DOC) contents. We tested if microbial planktoniccommunities were different among lakes with dissimilar nutrient and DOC concen-trations, analysing to what extent the known large-scale patterns of lake trophicstructure applies to a diverse but localized set of lakes. We found that mixotrophsdominated over strict autotrophs in both humic and clear oligotrophic systems,whereas in eutrophic lakes autotrophy was a successful strategy. The functionalphytoplankton approach also allowed the separation between oligotrophic (clear andhumic) and eutrophic systems, with different functional groups. The lowest abun-dances of picoplankton were found in oligotrophic lakes, picoeukaryotes being moreabundant than picocyanobacteria in beaver ponds and humic lakes. Our resultsshow that in low nutrient environments, mixotrophic strategies thrive over strictautotrophs suggesting the paramount importance of the microbial loop when com-pared with high trophic status systems where the prevalence of autotrophy indicatesthat the energy flux depends on phytoplankton.

Interactions between Perchlorate and Nitrate Reductions in the Biofilm of a Hydrogen-Based Membrane Biofilm Reactor

We studied the microbial functional and structural interactions between nitrate (NO(3)(-)) and perchlorate (ClO(4)(-)) reductions in the hydrogen (H(2))-based membrane biofilm reactor (MBfR). When H(2) was not limiting, ClO(4)(-) and NO(3)(-) reductions were complete, and the MBfR's biofilm was composed mainly of bacteria from the ε- and β-proteobacteria classes, with autotrophic genera Sulfuricurvum, Hydrogenophaga, and Dechloromonas dominating the biofilm. Based on functional-gene and pyrosequencing assays, Dechloromonas played the most important role in ClO(4)(-) reduction, while Sulfuricurvum and Hydrogenophaga were responsible for NO(3)(-) reduction. When H(2) delivery was insufficient to completely reduce both electron acceptors, NO(3)(-) reduction out-competed ClO(4)(-) reduction for electrons from H(2), and mixotrophs become important in the MBfR biofilm. β-Proteobacteria became the dominant class, and Azonexus replaced Sulfuricurvum as a main genus. The changes suggest that facultative, NO(3)(-)-reducing bacteria had advantages over strict autotrophs when H(2) was limiting, because organic microbial products became important electron donors when H(2) was severely limiting.

Oligotrophication and emergence of picocyanobacteria and a toxic dinoflagellate in Thau lagoon, southern France

Time series data have been examined in Thau lagoon (Southern France) from 1972 to 2006 for water temperature, salinity, nutrients and from 1987 to 2006 for phytoplankton. A first main trend identified is an increase in mean annual water temperature (1.5 °C over 33 years or 0.045 °C/year) that was not evenly distributed among seasons. The highest rate of increase was in the spring (+ 3.0 °C over 33 years), followed by summer (+ 2.0 °C) and fall (+ 1.7 °C). In winter, no significant increase over the 33 year period could be found. A second clear trend is a large decrease in soluble reactive phosphorus (SRP) concentration over the same 33 year period (summer values decreased from 10 µM to 1 µM, while winter values decreased from 3 µM to undetectable at present). Nitrate concentrations depended mainly on rainfall events and watershed runoff. Ammonium data were too fragmentary to be useful. N/P ratios expressed the traditional way of DIN/SRP cannot be used for phytoplankton that are not strict autotrophs. The recent and almost simultaneous appearance of both picocyanobacteria (mostly Synechococcus) and the toxic dinoflagellate Alexandrium catenella in Thau seem to be related to reduced nutrient loading and the increase in water temperature. A. catenella blooms occur either in the spring or the fall when water temperature is near 20 °C and remains so for several weeks with winds speeds below 2–3 m s − 1 . Picocyanobacterial growth is stimulated by increased summer temperatures, and lowered SRP levels provide picocyanobacteria an ecological advantage over other phytoplankton classes, in particular diatoms such as Skeletonema costatum whose cell densities have decreased over the last 8 years in summer and fall, but not in winter. An hypothesis is presented according to which A. catenella is not stimulated by increased temperatures, but is able to use picocyanobacteria for growth, and this provides this organism an additional resource over other strictly autotrophic phytoplankton. On a more general level, our data do not support the hypothesis that increased nutrient loading leads to harmful blooms of dinoflagellates. Instead, a combination of habitat disturbance and species displacement seems to lead to such blooms.

Pathways of Carbon Assimilation and Ammonia Oxidation Suggested by Environmental Genomic Analyses of Marine Crenarchaeota

Marine Crenarchaeota represent an abundant component of oceanic microbiota with potential to significantly influence biogeochemical cycling in marine ecosystems. Prior studies using specific archaeal lipid biomarkers and isotopic analyses indicated that planktonic Crenarchaeota have the capacity for autotrophic growth, and more recent cultivation studies support an ammonia-based chemolithoautotrophic energy metabolism. We report here analysis of fosmid sequences derived from the uncultivated marine crenarchaeote, Cenarchaeum symbiosum, focused on the reconstruction of carbon and energy metabolism. Genes predicted to encode multiple components of a modified 3-hydroxypropionate cycle of autotrophic carbon assimilation were identified, consistent with utilization of carbon dioxide as a carbon source. Additionally, genes predicted to encode a near complete oxidative tricarboxylic acid cycle were also identified, consistent with the consumption of organic carbon and in the production of intermediates for amino acid and cofactor biosynthesis. Therefore, C. symbiosum has the potential to function either as a strict autotroph, or as a mixotroph utilizing both carbon dioxide and organic material as carbon sources. From the standpoint of energy metabolism, genes predicted to encode ammonia monooxygenase subunits, ammonia permease, urease, and urea transporters were identified, consistent with the use of reduced nitrogen compounds as energy sources fueling autotrophic metabolism. Homologues of these genes, recovered from ocean waters worldwide, demonstrate the conservation and ubiquity of crenarchaeal pathways for carbon assimilation and ammonia oxidation. These findings further substantiate the likely global metabolic importance of Crenarchaeota with respect to key steps in the biogeochemical transformation of carbon and nitrogen in marine ecosystems.

Predation limitation in the pelagic microbial food web: an in situ study in an oligotrophic aquatic system

Temporal and spatial variations of pelagic microorganisms in the northern Baltic Sea were studied, as well as factors influencing their abundance and growth rates. Three main questions were asked 1) How does increased productivity influence the structure of the microbial food web? 2) Does predation limitation vary between trophic levels? 3) What is the relative importance of resource and predation limitation at different trophic levels?A field study in the northern Baltic Sea showed that dominating protozoa, flagellates and ciliates, increased with increasing primary productivity from north to south. Furthermore, relatively small protozoan cells dominated in the low productive north, while larger cells became more dominant in the south. The relationship between plankton size structure and productivity was further studied in an experimental system. In agreement with present theories regarding nutrient status of pelagic food webs, increased productivity caused a lengthening of the food chain as well as a change in plankton size structure. While microplankton dominated in nutrient rich treatments pico- and nanoplankton dominated during nutrient poor treament. The flagellate community was dominated by a potentially mixotroph, Chrysochromulina sp., at low nutrient concentrations. To our knowledge this is the first experimental study showing that Chrysochromulina sp. in resemblance with other mixotrophs is favoured by nutrient poor conditions compared to strict autotrophs and heterotrophs.During a stratified summer period autotrophic microorganisms in the northern Baltic Sea did not respond to removal of potential predators, indicating that they were primarily limited by inorganic nutrients. An exception was small eucaryotic picoplankton that showed a large response to predator removal. Among the heterotrophic microorganisms direct effect of predation seemed to increase from ciliates, heterotrophic bacteria, small heterotrophic flagellates, medium flagellates to large flagellates. No quick indirect effect was observed, but after four days trophic cascades were detected.The relative importance of resource and predation limitation was studied among heterotrophic bacteria, flagellates and ciliates in the northern Baltic Sea. For all these groups, resource limitation seemed to prevail during the summer period. The results also indicated that the relative importance of predation increased with the productivity of the system. To our knowledge there are no earlier measurements on the relative importance of resource and predation limitation for micoorganisms in the pelagic environment.

Structural changes in an aquatic microbial food web caused by inorganic nutrient addition

Temporal and spatial variations of pelagic microorganisms in the northern Baltic Sea were studied, as well as factors influencing their abundance and growth rates. Three main questions were asked 1) How does increased productivity influence the structure of the microbial food web? 2) Does predation limitation vary between trophic levels? 3) What is the relative importance of resource and predation limitation at different trophic levels?A field study in the northern Baltic Sea showed that dominating protozoa, flagellates and ciliates, increased with increasing primary productivity from north to south. Furthermore, relatively small protozoan cells dominated in the low productive north, while larger cells became more dominant in the south. The relationship between plankton size structure and productivity was further studied in an experimental system. In agreement with present theories regarding nutrient status of pelagic food webs, increased productivity caused a lengthening of the food chain as well as a change in plankton size structure. While microplankton dominated in nutrient rich treatments pico- and nanoplankton dominated during nutrient poor treament. The flagellate community was dominated by a potentially mixotroph, Chrysochromulina sp., at low nutrient concentrations. To our knowledge this is the first experimental study showing that Chrysochromulina sp. in resemblance with other mixotrophs is favoured by nutrient poor conditions compared to strict autotrophs and heterotrophs.During a stratified summer period autotrophic microorganisms in the northern Baltic Sea did not respond to removal of potential predators, indicating that they were primarily limited by inorganic nutrients. An exception was small eucaryotic picoplankton that showed a large response to predator removal. Among the heterotrophic microorganisms direct effect of predation seemed to increase from ciliates, heterotrophic bacteria, small heterotrophic flagellates, medium flagellates to large flagellates. No quick indirect effect was observed, but after four days trophic cascades were detected.The relative importance of resource and predation limitation was studied among heterotrophic bacteria, flagellates and ciliates in the northern Baltic Sea. For all these groups, resource limitation seemed to prevail during the summer period. The results also indicated that the relative importance of predation increased with the productivity of the system. To our knowledge there are no earlier measurements on the relative importance of resource and predation limitation for micoorganisms in the pelagic environment.

The role of C, N and P in dissolved and particulate organic matter as a nutrient source for phytoplankton growth, including toxic species

Phytoplankton have traditionally been regarded as strictly phototrophic, with a well defined position at the base of pelagic food webs. However, recently we have learned that the nutritional demands of a growing number of phytoplankton species can be met, at least partially, or under specific environmental conditions, through heterotrophy. Mixotrophy is the ability of an organism to be both phototrophic and heterotrophic, in the latter case utilizing either organic particles (phagotrophy) or dissolved organic substances (osmotrophy). This finding has direct implications for our view on algal survival strategies, particularly for harmful species, and energy- and nutrient flow in pelagic food webs. Mixotrophic species may outcompete strict autotrophs, e.g. in waters poor in inorganic nutrients or under low light. In the traditional view of the ‘microbial loop’ DOC is thought to be channeled from algal photosynthesis to bacteria and then up the food chain through heterotrophic flagellates, ciliates and mesozooplankton. Are mixotrophic phytoplankton that feed on bacteria also significantly contributing to this transport of photosynthetic carbon up the food chain? How can we estimate the fluxes of carbon and nutrients between different trophic levels in the plankton food web involving phagotrophic algae? These questions largely remain unanswered. In this review we treat evidence for both osmotrophy and phagotrophy in phytoplankton, especially toxic marine species, and some ecological implications of mixotrophy.

Microbial oxidation of mixtures of methylmercaptan and hydrogen sulfide

Refinery spent-sulfidic caustic, containing only inorganic sulfides, has previously been shown to be amenable to biotreatment with Thiobacillus denitrificans strain F with complete oxidation of sulfides to sulfate. However, many spent caustics contain mercaptans that cannot be metabolized by this strict autotroph. An aerobic enrichment culture was developed from mixed Thiobacilli and activated sludge that was capable of simultaneous oxidation of inorganic sulfide and mercaptans using hydrogen sulfide (H2S) and methylmercaptan (MeSH) gas feeds used to simulate the inorganic and organic sulfur of a spent-sulfidic caustic. The enrichment culture was also capable of biotreatment of an actual mercaptan-containing, spent-sulfidic caustic but at lower rates than predicted by operation on MeSH and H2S fed to the culture in the gas phase, indicating that the caustic contained other inhibitory components.

Bioenergetics parameters and transport in obligate acidophiles

Obligate acidophiles, which grow optimally in the pH range of 2-4, are an intriguing group of organisms. They span the taxonomic and physiological groupings from archaebacteria (e.g., Sulfolobus acidocaldarius) to eukaryotes (e.g., Cyanidium caldarius) and strict autotrophs to strict heterotrophs. The acidic environments they inhabit abound: vast areas of low-lying soils, mining regions, diverse aquatic environments, and sulfur rich geothermal areas [1]. The acidophilic bacteria have considerable economic importance in bioremediation of environmental acid pollution [2], biodesulfurization of fossil fuels [3], and biomining [3,4]. The latter technologies depend on biological activities possible only at very low pH values. The most striking bioenergetic characteristic of acidophiles is their ability to maintain a large pH gradient, even under highly stressful conditions of pH and nutrition. Thus, these organisms possess to an exquisite degree a vital property of all living cells, namely the ability to maintain a different ionic milieu compared to the external environment. This magnified perspective makes it likely that the relevant mechanisms would be more easily amenable to study and dissection in these bacteria. The acidophilic way of life, since it depends on the existence of pH gradients, is also of interest in the context of evolution of chemiosmotic mechanisms.

Multiple enzymic lesions in obligate methanotrophic bacteria

Most methane-utilizing bacteria are obligate methylotrophs [1-4]. Possible reasons for their obligate dependence on Cl-SUbstrates, as in case of strict autotrophs, are the metabolic blocks in energetic and biosynthetic pathways [1-8]. For instance, Type I obligate methanotrophs, with the ribulosemonophosphate pathway of formaldehyde assimilation, possess an incomplete Krebs cycle, lacking a-ketoglutarate dehydrogenase. As a result they are unable to oxidize and grow upon polycarbon compounds. In contrast, Type II obligate methanotrophs, with the serine:pathway, have a complete set of the Krebs cycle enzymes but lack hexokinase and 6-phosphogluconate dehydrogenase. However, the absence of a-ketoglutarate dehydrogenase in Type I methanotrophs appears to be an insufficient reason for explaining their failure to grow on multicarbon compounds, since this enzyme may be completely repressed in typical heterotrophic bacteria, e.g., Escherichia coli growing anaerobically on glucose [6,7]. The cause for obligateness in Type II methanotrophs is also not clear, since the lack of hexokinase and 6phosphogluconate dehydrogenase only impairs growth of these bacteria on sugars. In order to explore this problem of obligateness further, we set out to analyze possible metabolic routes of pyruvate and acetate in both types of obligate methanotrophs. Preliminary results of this research have been reported [3,9]. 2.1. Organisms and growth conditions

Effect of supplementary aeration on the growth of Thiobacillus thiooxidans in shaken cultures.

The growth rate of shaken cultures of the strict autotroph, Thiobacillus thiooxidans, usually very slow, has been increased about threefold by increasing the level of aeration. The cell yield has also been improved somewhat by this procedure. CO2 enriched air does not enhance growth further.

Regulation of citrate synthase activity in strict and facultatively autotrophic thiobacilli

The citrate synthases of Thiobacillus denitrificans and T. neapolitanus are inhibited by ∝-ketoglutarate but not by NADH. These characteristics are in accord with a purely biosynthetic role for the tricarboxylic acid (TCA) cycle in strict autotrophs. In contrast, the citrate synthases from two facultatively autotrophic thiobacilli, T. novellus and Thiobacillus A2, are not inhibited by ∝-ketoglutarate. The enzyme from Thiobacillus A2 resembles those of other Gram-negative bacteria in being inhibited by NADH whereas that from T. novellus is not. This may account, to some degree, for the better heterotrophic potential of Thiobacillus A2 compared with T. novellus .

Assimilation and metabolism of exogenous organic compounds by the strict autotrophs Thiobacillus thioparus and Thiobacillus neapolitanus.

The assimilation and utilization of the individual carbon atoms of pyruvate and acetate by cells of Thiobacillus thioparus and T. neapolitanus, in the presence and absence of an energy source, were studied by use of radioactive substrates. Both organisms produced (14)CO(2) from (14)C-labeled pyruvate, but more came from carbon 1 than from carbons 2 or 3. The conversion of the carbons of acetate to CO(2) by both organisms was much less than that from any of the pyruvate carbons. When labeled pyruvate and acetate were incubated with these organisms, small amounts of radioactivity were found in the tricholoacetic acid-soluble material, nucleic acids, and lipids, and larger amounts were found in the protein fraction. The composition of the incubation medium affected the amount of utilization and incorporation of labeled substrates by both organisms. The presence of an exogenous energy source (Na(2)S(2)O(3)) suppressed incorporation of the labeled substrates into various cellular components by T. thioparus, but enhanced incorporation by T. neapolitanus. When (14)C-pyruvate was used as a substrate, as many as 12 radioactive compounds were found in the water-soluble fraction in the experiments with T. neapolitanus, whereas no more than three radioactive compounds were detected in this fraction in the experiments with T. thioparus. Of the total (14)C activity found in the water-soluble fractions, malic acid contained the highest percentage. These findings are discussed in light of the overall metabolism of these two sulfur-oxidizing obligate chemoautotrophs, as well as in relation to the biochemical basis of chemoautotrophy.

Effect of adenosine monophosphate, adenosine diphosphate, and reduced nicotinamide adenine dinucleotide on adenosine triphosphate-dependent carbon dioxide fixation in the autotroph Thiobacillus neapolitanus.

The observation that adenosine triphosphate (ATP)-dependent CO(2) fixation in extracts of chemosynthetic and photosynthetic autotrophs may be regulated in part by adenosine monophosphate (AMP) was extended to the strict autotroph Thiobacillus neapolitanus (X). In addition, this report presents data which include adenosine diphosphate (ADP) in the regulatory role. When the primary CO(2) acceptor, ribose-5-phosphate, was replaced by ribulose-1,5-diphosphate, no inhibition of CO(2) fixation occurred unless the Mg(++) concentration was limiting. A molar ratio of 5:1 AMP or ADP to ATP reduced the specific activity (micromoles of CO(2) fixed per milligram of protein per minute) of the extracts from 0.22 to 0.12 and 0.11, respectively. The reported stimulation of the carboxylative phase of ATP-dependent CO(2) fixation by reduced nicotinamide adenine dinucleotide (NADH(2)) was investigated. Adding NADH(2) to the extracts did not stimulate CO(2) fixation, even at carbonate levels from 0.05 to 30 mumoles, except in the absence of ribose-5-phosphate. Slight increases in CO(2) fixation were noted when the assay system was incubated in air instead of the usual helium atmosphere.