Paradoxical anaerobism in desert pupfish

Responses of animals and plants to physiological doses of ethanol: a molecular messenger of hypoxia?

Our viewpoint is that ethanol could act as a molecular messenger in animal and plant organisms under conditions of hypoxia or other stresses and could elicit physiological responses to such conditions. There is evidence that both animal and plant organisms have endogenous levels of ethanol, but reports on the changes induced by this alcohol at physiological levels are sparse. Studies have shown that ethanol has different effects on cell metabolism at low and high concentrations, resembling a hormetic response. Further studies have addressed the potential cellular and molecular mechanisms used by organisms to sense changes in physiological concentrations of ethanol. This article summarizes the possible mechanisms by which ethanol may be sensed, particularly at the cell membrane level. Our analysis shows that current knowledge on this subject is limited. More research is required on the effects of ethanol at very low doses, in plants and animals at both molecular and physiological levels. We believe that further research on this topic could lead to new discoveries in physiology and may even help us understand metabolic adjustments related to climate change. As temperatures rise more frequently, dissolved oxygen levels drop, leading to hypoxic conditions and consequently, an increase in cellular ethanol levels.

Thermal tolerance in an extremophile fish from Mexico is not affected by environmental hypoxia

The thermal ecology of ectotherm animals has gained considerable attention in the face of human-induced climate change. Particularly in aquatic species, the experimental assessment of critical thermal limits (CTmin and CTmax) may help to predict possible effects of global warming on habitat suitability and ultimately species survival. Here we present data on the thermal limits of two endemic and endangered extremophile fish species, inhabiting a geothermally heated and sulfur-rich spring system in southern Mexico: The sulfur molly (Poecilia sulphuraria) and the widemouth gambusia (Gambusia eurystoma). Besides physiological challenges induced by toxic hydrogen sulfide and related severe hypoxia during the day, water temperatures have been previously reported to exceed those of nearby clearwater streams. We now present temperature data for various locations and years in the sulfur spring complex and conducted laboratory thermal tolerance tests (CTmin and CTmax) both under normoxic and severe hypoxic conditions in both species. Average CTmax limits did not differ between species when dissolved oxygen was present. However, critical temperature (CTmax=43.2°C) in P. sulphuraria did not change when tested under hypoxic conditions, while G. eurystoma on average had a lower CTmax when oxygen was absent. Based on this data we calculated both species' thermal safety margins and used a TDT (thermal death time) model framework to relate our experimental data to observed temperatures in the natural habitat. Our findings suggest that both species live near their thermal limits during the annual dry season and are locally already exposed to temperatures above their critical thermal limits. We discuss these findings in the light of possible physiological adaptions of the sulfur-adapted fish species and the anthropogenic threats for this unique system.

Complete mitochondrial genome of the Salt Creek pupfish, Cyprinodon salinus salinus: characterization and identification of single nucleotide polymorphisms (SNPs)

The Salt Creek pupfish, Cyprinodon salinus salinus Miller, 1943 is endemic to Death Valley, California, USA, and resides as a single population within one of the most extreme inland aquatic environments capable of supporting fish. Here we report the sequencing of complete 16,499 base pair (bp) mitochondrion genomes from four C. salinus salinus individuals. The mitochondrial genome of C. salinus salinus comprises 13 protein-coding regions, 12S and 16S rRNAs, 22 tRNAs, and an 832 bp D-loop region. The first reported single nucleotide polymorphisms (SNPs) were identified within the mtDNA of C. salinus salinus, with the four mitogenomes exhibiting only 0.0485% nucleotide sequence divergence indicative of low intraspecific variation. These complete mitogenomes will facilitate future genetic analyses of intraspecific diversity between the two described subspecies of C. salinus as well as other Cyprinodon pupfishes in southwestern North America.

Reduced Oxygen as an Environmental Pressure in the Evolution of the Blind Mexican Cavefish

Extreme environmental features can drive the evolution of extreme phenotypes. Over the course of evolution, certain environmental changes may be so drastic that they lead to extinction. Conversely, if an organism adapts to harsh environmental changes, the adaptations may permit expansion of a novel niche. The interaction between environmental stressors and adaptive changes is well-illustrated by the blind Mexican cavefish, Astyanaxmexicanus, which has recurrently adapted to the stark subterranean environment. The transition from terrestrial rivers and streams (occupied by extant surface morphs of the same species) to the cave has been accompanied by the resorption of eyes, diminished pigmentation and reduced metabolism in cave-dwelling morphs. The principal features of caves most often associated with evolution of these common cave features are the absence of light and limited nutrition. However, a putatively essential cave feature that has received less attention is the frequently low concentration of oxygen within natural karst environments. Here, we review the potential role of limited oxygen as a critical environmental feature of caves in the Sierra de El Abra. Additionally, we review evidence that Astyanax cavefish may have evolved adaptive features enabling them to thrive in lower oxygen compared to their surface-dwelling counterparts.

The gut microbiome and its potential role in paradoxical anaerobism in pupfishes of the Mojave Desert

BACKGROUND

Pupfishes frequently enter paradoxical anaerobism in response to endogenously produced or exogenously supplied ethanol in a dose-dependent manner. To decipher the role of the gut microbiota in ethanol-associated paradoxical anaerobism, gut microbial communities were depleted using a cocktail of antibiotics and profiled using 16S rRNA gene sequencing.

RESULTS

Compared to the control group (n = 12), microbiota-depleted fish (n = 12) spent more time in paradoxical anaerobism. Our analysis indicated that the bacterial phyla Proteobacteria, Fusobacteria, Bacteroidetes, Firmicutes, Actinobacteria, Patescibacteria, and Dependentiae dominated the pupfish gut, which is consistent with other fish gut microbiota. Although the gut microbial communities with and without antibiotic treatment were similarly diverse, they were distinct and the greatest contribution to the dissimilarity (27.38%) was the common fish commensal Cetobacterium.

CONCLUSIONS

This study reports the first characterization of gut microbial communities of pupfish and suggests the microbiome may play a critical role in regulating metabolic strategies that are critical for survival in extremes of temperature and oxygen concentration. We speculate that Cetobacterium, a primary fermenter, also consumes ethanol through secondary fermentation via an alcohol dehydrogenase and therefore regulates the transition from paradoxical anaerobism to aerobic respiration in fish. Given the wide distribution and abundance of Cetobacterium in warm-water fishes, this process may be of broad importance, and suggests that the microbiome be carefully considered for both conservation and aquaculture.

Warming waters beget smaller fish: evidence for reduced size and altered morphology in a desert fish following anthropogenic temperature change

Poikilothermic organisms are predicted to show reduced body sizes as they experience warming environments under a changing global climate. Such a shrinking of size is expected under scenarios where rising temperatures increase cellular reaction rates and basal metabolic energy demands, therein requiring limited energy to be shifted from growth. Here, we provide evidence that the ecological changes associated with warming may not only lead to shrinking body size but also trigger shifts in morphology. We documented 33.4 and 39.0% declines in body mass and 7.2 and 7.6% reductions in length for males and females, respectively, in a wild population of Amargosa pupfish, Cyprinodon nevadensis amargosae, following an abrupt anthropogenically driven temperature increase. That reduction in size was accompanied by the partial or complete loss of paired pelvic fins in approximately 34% of the population, a morphological change concomitant with altered body dimensions including head size and body depth. These observations confirm that increasing temperatures can reduce body size under some ecological scenarios and highlight how human-induced environmental warming may also trigger morphological changes with potential relevance for fitness.

Physiological impacts and bioaccumulation of dietary Cu and Cd in a model teleost: The Amazonian tambaqui (Colossoma macropomum)

Increasing anthropogenic activities in the Amazon have led to elevated metals in the aquatic environment. Since fish are the main source of animal protein for the Amazonian population, understanding metal bioaccumulation patterns and physiological impacts is of critical importance. Juvenile tambaqui, a local model species, were exposed to chronic dietary Cu (essential, 500 μg Cu/g food) and Cd (non-essential, 500 μg Cd/g food). Fish were sampled at 10-14, 18-20 and 33-36 days of exposure and the following parameters were analyzed: growth, voluntary food consumption, conversion efficiency, tissue-specific metal bioaccumulation, ammonia and urea-N excretion, O₂ consumption, P_crit, hypoxia tolerance, nitrogen quotient, major blood plasma ions and metabolites, gill and gut enzyme activities, and in vitro gut fluid transport. The results indicate no ionoregulatory impacts of either of the metal-contaminated diets at gill, gut, or plasma levels, and no differences in plasma cortisol or lactate. The Cd diet appeared to have suppressed feeding, though overall tank growth was not affected. Bioaccumulation of both metals was observed. Distinct tissue-specific and time-specific patterns were seen. Metal burdens in the edible white muscle remained low. Overall, physiological impacts of the Cu diet were minimal. However dietary Cd increased hypoxia tolerance, as evidenced by decreased P_crit, increased time to loss of equilibrium, a lack of plasma glucose elevation, decreased plasma ethanol, and decreased NQ during hypoxia. Blood O₂ transport characteristics (P₅₀, Bohr coefficient, hemoglobin, hematocrit) were unaffected, suggesting that tissue level changes in metabolism accounted for the greater hypoxia tolerance in tambaqui fed with a Cd-contaminated diet.

The Gut Microbiome, Energy Homeostasis, and Implications for Hypertension

PURPOSE OF REVIEW

The influence of gut bacteria upon host physiology is increasingly recognized, but mechanistic links are lacking. Diseases of energetic imbalance such as obesity and diabetes represent major risk factors for cardiovascular diseases such as hypertension. Thus, here, we review current mechanistic contributions of the gut microbiota to host energetics.

RECENT FINDINGS

Gut bacteria generate a multitude of small molecules which can signal to host tissues within and beyond the gastrointestinal tract to influence host physiology, and gut bacteria can also influence host digestive efficiency by altering the bioavailability of polysaccharides, yet the quantitative energetic effects of these processes remain unclear. Recently, our team has demonstrated that gut bacteria constitute a major anaerobic thermogenic biomass, which can quantitatively account for obesity. Quantitative understanding of the mechanisms by which gut bacteria influence energy homeostasis may ultimately inform the relationship between gut bacteria and cardiovascular dysfunction.

Differential survivorship of congeneric ornamental fishes under forecasted climate changes are related to anaerobic potential

Two Amazonian closely related tetras – cardinal Paracheirodon axelrodi and green neon P. simulans – were artificially acclimatized to environmental chambers mimicking future climate change scenarios (mild, moderate and extreme), using a microcosm facility. P. simulans survived (100%) to all scenarios after 30 days exposure, while P. axelrodi presented decreasing survival percentages according to environmental severity. These differences may be the reflection of distinct natural acclimatization to microhabitats between the species, which differ in thermal conditions. Survival responses might be related to differences in relative gene expression of lactate dehydrogenase (Ldh), suggesting that P. axelrodi anaerobic potential is lower or non-existent compared to P. simulans, not tolerating long-term thermal challenges. Accordingly, increases in temperature and in CO₂ levels caused increases in energy demand and resulted in activation of the anaerobic pathway, as demonstrated by the higher enzyme levels measured in head and tail portions of both species. Sustained anaerobic glycolysis is possible when fish live in challenging environments (low oxygen or high temperature). Our results clearly show that P. simulans has a larger scope for survival to higher energy demands due to its increased anaerobic potential compared to P. axelrodi.

Ethanol metabolism varies with hypoxia tolerance in ten cyprinid species

During periods of severe hypoxia or anoxia, Carassius spp. are known for their ability to produce ethanol as their anaerobic end product, which diffuses into the environment thereby reducing the osmotic and acidotic load associated with "anaerobic" glycolysis. However, the relationship between alcohol dehydrogenase (ADH) and acetaldehyde dehydrogenase (ALDH) activities, key ethanol metabolizing enzymes, and hypoxia tolerance among Carassius spp. and their closely related non-ethanol-producing cyprinids remains unclear. To address this, we quantified the activity levels of key anaerobic enzymes in liver and muscle in species of cyprinids over 48 h of severe hypoxia exposure (0.7 kPa). As predicted, muscle ADH activity was highest in the two most hypoxia-tolerant species (Carassius spp.), with very low levels present in the other species examined. However, liver ADH activities showed an inverse relationship with hypoxia tolerance, with the most hypoxia-tolerant fish having the lowest ADH activity. There was no correlation between hypoxia tolerance and ALDH and LDH activities in muscle or liver. All species produced lactate, reaching their highest levels after 8 h, but returning to near-baseline levels by 48 h of sustained exposure to hypoxia, suggesting lactate oxidation or depressed ATP demand. Liver glycogen content was not affected by 48 h hypoxia exposure in the most hypoxia-tolerant species, whereas the least tolerant species consumed the majority of the liver glycogen stores, which is probably due to the greater relative hypoxia exposure experienced by these species. Our findings that liver ADH activities were inversely related to hypoxia tolerance suggests that in all but Carassius spp., the ethanol metabolizing pathways in cyprinids is largely similar to that observed in other vertebrates and plays a role in the detoxification of ethanol. Furthermore, conservation of glycogen stores may be the result of metabolic-depressing pathways in the more tolerant species, regardless of the ability to produce ethanol, or adaptations that improve oxygen uptake to reduce metabolic demands due to hypoxia.

Comprehensive Assessments of Energy Balance in Mice

Increasing evidence supports a major role for the renin-angiotensin system (RAS) in energy balance physiology. The RAS exists as a circulating system but also as a local paracrine/autocrine signaling mechanism in target tissues including the gastrointestinal tract, the brain, the kidney, and distinct adipose beds. Through activation of various receptors in these target tissues, the RAS contributes to the control of food intake behavior, digestive efficiency, spontaneous physical activity, and aerobic and anaerobic resting metabolism. Although the assortment of methodologies available to assess the various aspects of energy balance can be daunting for an investigator new to this area, a relatively straightforward array of entry-level and advanced methodologies can be employed to comprehensively and quantitatively dissect the effects of experimental manipulations on energy homeostasis. Such methodologies and a simple initial workflow for the use of these methods are described in this chapter, including the use of metabolic caging systems, bomb calorimetry, body composition analyzers, respirometry systems, and direct calorimetry systems. Finally, a brief discussion of the statistical analyses of metabolic data is included.

Endocrine and metabolic impacts of warming aquatic habitats: differential responses between recently isolated populations of a eurythermal desert pupfish

Temperatures of inland aquatic habitats are increasing with climate change, and understanding how fishes respond physiologically to thermal stress will be crucial for identifying species most susceptible to these changes. Desert fishes may be particularly vulnerable to rising temperatures because many species occupy only a fraction of their historical range and occur in habitats with already high temperatures. Here, we examined endocrine and metabolic responses to elevated temperature in Amargosa pupfish, Cyprinodon nevadensis amargosae. We studied C. n. amargosae from two habitats with distinct thermal conditions: the Amargosa River, which experiences diurnally and seasonally variable temperatures (0.2-40°C); and Tecopa Bore, a spring and marsh fed by hot groundwater (47.5°C) from an artesian borehole. These allopatric populations differ in morphology, and prior evidence suggests that temperature might contribute to these differences via altered thyroid hormone (TH) regulation of morphological development. Here, we document variation in hepatic iodothyronine deiodinase type 2 (dio2) and type 3 (dio3) and TH receptor β (trβ) gene transcript abundance between the Amargosa River and Tecopa Bore wild populations. Fish from these populations acclimated to 24 or 34°C retained differences in hepatic dio2, dio3 and trβ mRNAs and also varied in transcripts encoding the TH membrane transporters monocarboxylate transporter 8 (mct8) and organic anion-transporting protein 1c1 (oatp1c1). Tecopa Bore pupfish also exhibited higher dio2 and trβ mRNA levels in skeletal muscle relative to Amargosa River fish. Muscle citrate synthase activity was lower at 34°C for both populations, whereas lactate dehydrogenase activity and lactate dehydrogenase A-chain (ldhA) transcripts were both higher and 3,5,3'-triiodothryonine responsive in Tecopa Bore pupfish only. These findings reveal that local population variation and thermal experience interact to shape how pupfish respond to elevated temperatures, and point to the need to consider such interactions in management actions for desert fishes under a changing climate.

Oxygen Consumption is Limited at an Ecologically Relevant Rearing Temperature in Pupfish Eggs

The habitat of the critically endangered Devils Hole Pupfish, Cyprinodon diabolis is marked by constant high temperatures and low oxygen availability. In order to explore the effects of these conditions on development and recruitment of eggs in Devils Hole, we tested the effects of two ecologically relevant temperatures on the development, hatch success, and oxygen consumption of eggs from a refuge population of pupfish derived from C. diabolis and eggs from its close sister species, Cyprinodon nevadensis mionectes. We developed a simple method to measure oxygen consumption in a single egg. Parent acclimation temperature, rather than incubation temperature, was the most important factor influencing hatch success. Eggs incubated at 33°C hatched more quickly compared to those incubated at 28°C. Despite this accelerated development, larvae from both temperatures were of similar size at hatch. Unexpectedly, eggs incubated at 33°C experience lower than expected oxygen consumption rates compared to those incubated at 28°C. Oxygen consumption rates would be limited at PO₂ values that are much higher than environmental oxygen tensions. Oxygen consumption increased dramatically upon hatch, indicating that low oxygen conditions such as those present in Devils Hole may limit developing eggs.