Microbialite response to an anthropogenic salinity gradient in Great Salt Lake, Utah

A railroad causeway across Great Salt Lake, Utah (GSL), has restricted water flow since its construction in 1959, resulting in a more saline North Arm (NA; 24%-31% salinity) and a less saline South Arm (SA; 11%-14% salinity). Here, we characterized microbial carbonates collected from the SA and the NA to evaluate the effect of increased salinity on community composition and abundance and to determine whether the communities present in the NA are still actively precipitating carbonate or if they are remnant features from prior to causeway construction. SSU rRNA gene abundances associated with the NA microbialite were three orders of magnitude lower than those associated with the SA microbialite, indicating that the latter community is more productive. SSU rRNA gene sequencing and functional gene microarray analyses indicated that SA and NA microbialite communities are distinct. In particular, abundant sequences affiliated with photoautotrophic taxa including cyanobacteria and diatoms that may drive carbonate precipitation and thus still actively form microbialites were identified in the SA microbialite; sequences affiliated with photoautotrophic taxa were in low abundance in the NA microbialite. SA and NA microbialites comprise smooth prismatic aragonite crystals. However, the SA microbialite also contained micritic aragonite, which can be formed as a result of biological activity. Collectively, these observations suggest that NA microbialites are likely to be remnant features from prior to causeway construction and indicate a strong decrease in the ability of NA microbialite communities to actively precipitate carbonate minerals. Moreover, the results suggest a role for cyanobacteria and diatoms in carbonate precipitation and microbialite formation in the SA of GSL.

Microbial Fe(III) oxide reduction potential in Chocolate Pots hot spring, Yellowstone National Park

Chocolate Pots hot springs (CP) is a unique, circumneutral pH, iron-rich, geothermal feature in Yellowstone National Park. Prior research at CP has focused on photosynthetically driven Fe(II) oxidation as a model for mineralization of microbial mats and deposition of Archean banded iron formations. However, geochemical and stable Fe isotopic data have suggested that dissimilatory microbial iron reduction (DIR) may be active within CP deposits. In this study, the potential for microbial reduction of native CP Fe(III) oxides was investigated, using a combination of cultivation dependent and independent approaches, to assess the potential involvement of DIR in Fe redox cycling and associated stable Fe isotope fractionation in the CP hot springs. Endogenous microbial communities were able to reduce native CP Fe(III) oxides, as documented by most probable number enumerations and enrichment culture studies. Enrichment cultures demonstrated sustained DIR driven by oxidation of acetate, lactate, and H2 . Inhibitor studies and molecular analyses indicate that sulfate reduction did not contribute to observed rates of DIR in the enrichment cultures through abiotic reaction pathways. Enrichment cultures produced isotopically light Fe(II) during DIR relative to the bulk solid-phase Fe(III) oxides. Pyrosequencing of 16S rRNA genes from enrichment cultures showed dominant sequences closely affiliated with Geobacter metallireducens, a mesophilic Fe(III) oxide reducer. Shotgun metagenomic analysis of enrichment cultures confirmed the presence of a dominant G. metallireducens-like population and other less dominant populations from the phylum Ignavibacteriae, which appear to be capable of DIR. Gene (protein) searches revealed the presence of heat-shock proteins that may be involved in increased thermotolerance in the organisms present in the enrichments as well as porin-cytochrome complexes previously shown to be involved in extracellular electron transport. This analysis offers the first detailed insight into how DIR may impact the Fe geochemistry and isotope composition of a Fe-rich, circumneutral pH geothermal environment.

Hydrogenase activity of mineral-associated and suspended populations of Desulfovibrio desulfuricans Essex 6

The interactions between sulfate-reducing microorganisms and iron oxides influence a number of important redox-sensitive biogeochemical processes including the formation of iron sulfides. Enzymes, such as hydrogenase which catalyze the reversible oxidation of molecular hydrogen, are known to mediate electron transfer to metals and may contribute to the formation and speciation of ferrous sulfides formed at the cell-mineral interface. In the present study, we compared the whole cell hydrogenase activity of Desulfovibrio desulfuricans strain Essex 6 growing as biofilms on hematite (hematite-associated) or as suspended populations using different metabolic pathways. Hematite-associated cells exhibited significantly greater hydrogenase activity than suspended populations during sulfate respiration but not during pyruvate fermentation. The enhanced activity of the hematite-associated, sulfate-grown cells appears to be dependent on iron availability rather than a general response to surface attachment since the activity of glass-associated cells did not differ from that of suspended populations. Hydrogenase activity of pyruvate-fermenting cells was stimulated by addition of iron as soluble Fe(II)Cl2 and, in the absence of added iron, both sulfate-reducing and pyruvate-fermenting cells displayed similar rates of hydrogenase activity. These data suggest that iron exerts a stronger influence on whole cell hydrogenase activity than either metabolic pathway or mode of growth. The location of hydrogenase to the cell envelope and the enhanced activity at the hematite surface in sulfate-reducing cells may influence the redox conditions that control the species of iron sulfides on the mineral surface.

Environmental constraints defining the distribution, composition, and evolution of chlorophototrophs in thermal features of Yellowstone National Park

Chlorophotoautotrophy, the use of chlorophylls to convert light energy into chemical energy for carbon dioxide fixation, is the primary metabolic process linking the inorganic and organic carbon pools on Earth. To understand the potential effects of various environmental constraints on the evolution of chlorophototrophy better, we studied the distribution, diversity, and abundance of chlorophylls and genes involved in their synthesis along geothermal gradients in Yellowstone National Park, Wyoming. Genes involved in chlorophyll biosynthesis were constrained to temperatures of less than ~70 °C and were only detected at this elevated temperature when the pH was in the circumneutral to alkaline range. The upper temperature limit for the detection of chlL/bchL(1) and bchY(2) decreased systematically with increasingly acidic pH, an observation likely attributable to sulfide, which upon oxidation, generates acidic spring water and reduces the availability of bicarbonate the preferred source of inorganic carbon for phototrophs. Spring pH was also the best predictor of the phylogenetic diversity of chlL/bchL communities. The phylogenetic similarity of chlL/bchL genes between sites was significantly correlated with that of chlorophylls. The predominance of chlorophyll a and bacteriochlorophyll a among extracted pigments was consistent with predominance of chlL/bchL genes affiliated with the Cyanobacteria and Chloroflexiales, respectively, and might be related to the fact that the majority of these organisms are photoautotrophs. Together, these results suggest that a combination of temperature, pH, and/or sulfide influences the distribution, diversity, and evolution of chlorophotrophs and the chlorophylls that they synthesize.

A late methanogen origin for molybdenum-dependent nitrogenase

Mounting evidence indicates the presence of a near complete biological nitrogen cycle in redox-stratified oceans during the late Archean to early Proterozoic (c. 2.5-2.0 Ga). It has been suggested that the iron (Fe)- or vanadium (V)-dependent nitrogenase rather than molybdenum (Mo)-dependent form was responsible for dinitrogen fixation during this time because oceans were depleted in Mo and rich in Fe. We evaluated this hypothesis by examining the phylogenetic relationships of proteins that are required for the biosynthesis of the active site cofactor of Mo-nitrogenase in relation to structural proteins required for Fe-, V- and Mo-nitrogenase. The results are highly suggestive that among extant nitrogen-fixing organisms for which genomic information exists, Mo-nitrogenase is unlikely to have been associated with the Last Universal Common Ancestor. Rather, the origin of Mo-nitrogenase can be traced to an ancestor of the anaerobic and hydrogenotrophic methanogens with acquisition in the bacterial domain via lateral gene transfer involving an anaerobic member of the Firmicutes. A comparison of substitution rates estimated for proteins required for the biosynthesis of the nitrogenase active site cofactor and for a set of paralogous proteins required for the biosynthesis of bacteriochlorophyll suggests that Nif emerged from a nitrogenase-like ancestor approximately 1.5-2.2 Ga. An origin and ensuing proliferation of Mo-nitrogenase under anoxic conditions would likely have occurred in an environment where anaerobic methanogens and Firmicutes coexisted and where Mo was at least episodically available, such as in a redox-stratified Proterozoic ocean basin.

The M-wave and CNV in the squirrel monkey: generality of cue modality and of reward

Typical M-waves and CNVs were produced in squirrel monkey frontal and rostral parietal cortex by light cues, as well as by tone cues. With light cues, M-waves were about 50% as large as with tone cues, whereas CNVs were of about the same magnitude with either type of cue. Typical M-waves and CNVs were also produced when avoidance of tail-shock was substituted for food as a reinforcer. The magnitude of both ERPs appeared to follow an inverted-U function with increasing shock intensity. The use of tail-shock as a reinforcer generated more complexities both in ERP changes and in behavior than did food. In some animals, at high shock intensities, ERP increased, rather than decreased, as each session progressed. In some animals, a combination of food and avoidance of tail-shock produced a decrease, rathan than an increase, in ERPs and did not improve behavioral performance. Furthermore, animals would not perform effectively on a CNV paradigm with punishment for aborting the trial. It was also demonstrated that increased stress (immobilization of head or tail) resulted in a reduced magnitude of both the M-wave and the CNV. This may account for some of the complexities encountered with avoidance of tail-shock as a reinforcer.

Observations on the M-wave and the CNV in the squirrel monkey

A typical CNV paradigm, with food as reward, evokes in the squirrel monkey's post-arcuate and post-central cortices both M-waves in response to the cues and what appears to be a CNV in the interstimulus interval. Both wave forms appear to be generated locally in cortex and to be more closely related temporally to the cues than to the animal's behavioral response. The CNV, like the M-wave, appears to reflect the animal's level of interest in obtaining the reward. It is smaller when the animal misses the cues than when he responds correctly. It decreases as food pellets are consumed. It varies with the level of interest of the animal in a particular type of food pellet, as judged from the animal's behavior. At some recording sites in some animals it appears to reflect both interest in the reward and interest in manipulating the environment to obtain the reward. It can be maintained for at least 2 sec, but differs from the human CNV in that, with the paradigm used, it is not well resolved by the imperative cue. Averaging evoked activity with the animal's motor response as reference point reveals another negative wave form which just precedes the motor response.

Effects of some psychotropic drugs on M-wave and operant behavior in the squirrel monkey

The effects of low doses of nabilone, chlorpromazine, pentobarbital, meprobamate, diazepam, chlordiazepoxiOde and d-amphetamine on behavioral responding to cues signalling the availability of food rewards, and on the M-wave, an evoked cortical potential previously reported to reflect the conditioned incentive value of the cues were determined in the squirrel monkey. Nabilone and chlorpromazine simultaneously depressed both the M-wave and behavior. Pentobarbital, meprobamate, diazepam and chlordiazepoxide could depress the M-wave without depressing behavior. This effect was most marked with diazepam. The only augmentation of the M-wave observed was the d-amphetamine, and this occurred in only one of five animals. The benzodiazepines were the only drugs to augment behavioral output. However, diazepam occasionally increased the number of cures responded to while concomitantly decreasing both total behavioral output and the amplitude of the M-wave. It is concluded that the M-wave cannot directly reflect the incentive value of the cue, but must rather reflect something that tends to parallel this value.

A cortical evoked potential that reflects the conditioned, positive incentive value of the stimulus. II. Drive motivation (hunger)

It was shown that the late negative wave evoked in squirrel monkey cortex by tone cues signalling the availability of reward was smaller when the animal did not respond to the cue and often disappeared if the animal ignored many cues in succession. It decreased in amplitude as a test session progressed, and the rate of this decrease increased if the rate of reinforcement was increased. In some animals the size of the evoked wave was smaller if the animal was not fasted before a test session. In some animals the average size of a significant positive correlation with the number of cues responded to and a negative correlation with the animal's average reaction time. It is concluded that the evoked wave is probably a reflection of the conditioned incentive value of the cue.

A cortical evoked potential that reflects the conditioned, positive incentive value of the stimulus. III. Manipulative motivation and the value of the reward

Changing animals from a schedule on which they had to bar-press after a tone cue to get a food pellet, to a schedule on which the food pellet was presented automatically after the tone cue and the manipulandum was not available, had no effect on the latency, amplitude, or shape of the M-wave evoked by the tone cue. The amplitude of the evoked wave did, however, decrease more rapidly, as a test session progressed, with the latter schedule than with the former schedule. Failure to fast animals before test sessions markedly reduced the amplitude of the M-wave. Variation in the type of food pellet used caused marked variations in the amplitude of the evoked response. In all cases, there was a high degree of correlation between M-wave amplitude and a behavioral indication of the animal's interest in the reward. It was concluded that the M-wave reflects the conditioned incentive value of the cue, or in other words, the level of interest of the animal in the reward.

A cortical evoked potential that reflects the conditioned, positive incentive value of the stimulus. I. The evoked potential

As squirrel monkeys learned that a tone cue signalled the availability of a food reward there appeared in pre- and post-central cortex a surface negative waveform in response to the cue. This waveform disappeared, concomitantly with the animal's behavioral response, during extinction and pseudoconditioning. It was independent of changes in cue intensity sufficient to markedly alter earlier components of evoked activity and it disappeared when evoked activity was averaged with the behavioral response, rather than the cue, as reference point. It was concluded that this evoked potential reflects cortical activity somewhere in the stimulus-response sequence between the recognition of the cue and the decision to act.

Effects of delta9-tetrahydrocannabinol and pentobarbital on a cortical response evoked during conditioning

A surface-negative wave, evoked by tone cues, appeared in monkey post-arcuate cortex as the monkey learned that the cue signaled the availability of reward. This evoked activity was depressed, concomitantly with changes in the animal's behavioral responding, by doses of delta9-tetrahydrocannabinol (delta9-THC) as low as 0.032 mg/kg and of pentobarbital as low as 4 mg/kg. Pentobarbital tended to increase the latency of the evoked wave, an effect not seen with delta9-THC.