Understanding Animal Physiology
Scientists have discovered, deep down into the Mediterranean seafloor, the first animals (multicellular for that matter) that are capable of living and surviving in an environment that is entirely oxygen-free. Certain types of single-celled organisms and bacteria are capable of surviving without oxygen, but nothing that is as complex as three species of Loricifera (a group of aquatic sediment-dwellers that inhabit one of the planet’s little known and extreme environments). The discovery of these forms of life has led to the opening of new perspectives for studies of metazoan life in habitats that lack molecular oxygen (Danovaro et al. 30).
Just like other Loricifera, the discovered species are also sub-millimeter long, have Lovecraftian tangles of tentacles and shell. Their closest taxonomical relatives are found among penis worms and mud dragons. They, however, do not possess the mitochondria that are found almost in every other animal cell that convert oxygen and nutrients into chemical energy. The new Loricifera species possess structures called hydrogenosomes that are present in single-celled organisms and require no oxygen to convert nutrients into chemical energy (Guppy and Withers 35).Scientific findings allow for conclusions that these anoxic animals live under anoxic situations and conditions using anaerobic metabolism that is almost similar to the anaerobic metabolism that is demonstrated so far only for unicellular eukaryotes.
Adaptations Related to Anoxic Animals
Most of the current environments on earth, especially the areas that are more accessible to human investigations, are always fully oxygenated. They are ever in diffusion equilibrium with the concentrations of free atmospheric molecular oxygen. Such ecosystems are usually inhabited by organisms which do possess aerobic mitochondria (considered as the main locations of generation of biological energy in creatures that are aerobic in nature).
Mitochondria which are aerobic use pyruvate dehydrogenase (PDH) in oxidative decarboxylation of pyruvate into acetyl-CoA. The formed acetyl-CoA is then completely oxidized into Carbon IV oxide using the Krebs cycle, thereby leading to the generation of reducing equivalents that take the form of the reduced coenzymes FADH2 and NADH. The electrons which are generated from FADH2 and NADH are then transferred to the electron transport chain (ETC) that pumps protons. This chain consists of a series of membrane proteins that are considered integral or distinguished by four groups: I–IV and located within the membrane found in the inner layers of the mitochondrion. Each of these steps of the chain releases a small quantity of energy. This energy is used in proton-pumping from the suspension of the mitochondria to the space between the membranes, thereby converting the covalent bonds’ free chemical energy into an electrochemical gradient.
The proton-motive force (PMF) that is created as a result of this drives (moves) protons down the gradient across the membrane located on the inner layer, through the proton channel of a different integral membrane protein referred to as ATP synthase (Storey 81). The mechanical energy that is generated propels Adenosine Tri-Phosphate (ATP) synthesis from inorganic phosphate and Adenosine Di-Phosphate (ADP). The coenzymes NADH and FADH2 that are reduced are then re-oxidized by donation of the electrons which are gained to the electron transport chain coenzymes and, consecutively, to a terminal acceptor which his excreted as metabolic waste in its reduced form. Research shows that in aerobic mitochondria, the final protein complex in the electron transport chain of the so-termed ‘oxidative phosphorylation’ process is a Cytochrome oxidase. This cytochrome exploits oxygen as a terminal acceptor of electrons and leads to the generation of water.
Organisms that dwell strictly in habitats that are oxygen-free (anoxic) or oxygen-poor (hypoxic) are strict anaerobes or microaerophilic. Among the latter, some of them bear mitochondria that are anaerobic, in which ATP is produced by use of nitrate as the main terminal electron acceptor. Here the end product of the process is nitrite but not water.
These organisms use a reductase that is nitrate in form instead of using the cytochrome oxidase that is found in aerobic mitochondria that do result into lower yield of ATP. This arises due to the lower reduction potential of nitrate as opposed to molecular oxygen. Meaning that compared to aerobic respiration, anaerobic respiration is less efficient energetically. This kind of respiration (nitrate respiration) has been discovered to exist for ciliates of the Loxodesgenus, yeasts, and foraminifera (Storey andStorey 221).
Fermentations are far more widespread in anoxic organelles: this involves metabolic pathways whereby donation of electrons to terminal acceptors is represented by intermediate metabolism products. Acceptors considered “common terminal” do include fumarate which is reduced to succinate, acetyl-CoA which is reduced to fatty acids,pyruvate, acetaldehyde which is reduced to ethanol, and protons which are reduced to molecular hydrogen. Scientific research dictates that fermentations are always characterized by disproportionate reactions. In such reactions, the organic substrate generates a further reduced into a further oxidized end product.
Given the multiple forms of metabolic injury that can arise because of oxygen limitation in oxygen-sensitive organisms, it is clear that anoxic organisms must address a variety of issues so as to survive periods of oxygen deprivation. The overriding strategy for this kind of lifestyle is not compensation but conservation. In addition to the above survival methods under anoxic conditions, these organisms may sometimes have to use strategies to minimize their ATP use, optimize the time that fixed internal fuel reserves can fuel metabolism, and limit the disruption of cellular homeostasis.
five main categories of biochemical adaptation
1. Fuel supply: Anoxic animals maintain large reserves of glycogen in their tissues and marine invertebrates also maintain substantial pools of fermentable amino acids (e.g., aspartate, glutamate).
2. Enhance ATP yield: Anaerobic ATP production by the basic glycolytic pathway can be supplemented with other reactions that increase the ATP yield per glucose catabolized. For example, many marine molluscs catabolize glucose to succinate and propionate, with additional substrate level phosphorylation reactions increasing the yield to 4 or 6 ATP per glucose, compared with 2 ATP per glucose converted to lactate.
3. Minimize cytotoxicity: Cells generating ATP from anaerobic glycolysis ending in lactate production soon undergo significant acidification as well as major end-product accumulation. Solutions include enhanced buffering capacity (they release calcium carbonate from their shell and bone to buffer acid build-up and also move huge amounts of lactate into their shells for storage), making less acidic end-products (e.g., synthesis of succinate or propionate generates a much lower proton load than does lactate output), or making products that can be excreted easily (e.g., they may catabolize lactate to ethanol + CO2 and then excrete both).
4. MRD: A coordinated and strong reduction in the rates of ATP consumption by multiple cell functions reduces ATP demand into line with ATP output from fermentative pathways and extends the time that fixed internal reserves of fermentable fuels can sustain anaerobic survival.
5. Antioxidant defense: Well-developed enzymatic and metabolite antioxidant defenses minimize oxidative stress during the transition from anaerobiosis back to aerobic life. For example, anoxia tolerant freshwater organisms show the highest constitutive antioxidant defenses among ectotherms (closely comparable to mammalian levels of defense) and anoxic or ischemic stresses frequently induce the synthesis of antioxidants in species that encounter low oxygen stress less frequently.
Adaptations of anoxic animals to such hostile ecosystems
• Development of special organs – development of modifications of specialized regions of the body for gaseous exchange.
• Improving oxygen conditions – mechanisms to improve the oxygen gradient across the diffusible membrane.
• Internal structural changes – such as increased vascularization.
• Physiological adaptations – this includes shifts in metabolic pathways
• Behavioral activities – such as decreased locomotive activity or closing a cell during these low oxygen stress.
In adaptation to anoxia, mitochondria have been converted into a spectrum of mitochondrion-related organelles with diverse function. These conversions have occurred independently in a wide range of eukaryotic lineages.
Adaptation of the mitochondrial functions in different organisms
The mitochondrial functions believed to be almost universally retained by these organelles include Fe-S cluster biosynthesis and those functions required for maintenance of the organelle and its environment in this pathway, namely protein import and substrate exchange. Convergent evolution is also apparent in the newly acquired functions of the MROs of distinct eukaryotic lineages (Hochachkaand Lutz 341). Organisms adapting to strictly anoxic are no longer under selection pressure to maintain oxidative phosphorylation and so many MROs have independently lost some of all the components of the TCA cycle, the electron transport complexes and the F-F ATP synthesis synthase (Ride). In addition, anaerobic lineages have independently acquired the means to generate ATP under anoxic conditions and the pathways involved share a number of common features such as the conversion of pyruvates into acetyl-CoA, subsequent formation of acetate in the course of ATP synthesis and the involvement of Fe-Fe hydrogenase as a means of removing reducing equivalents. However, the independent nature of these adaptations is also apparent in their diversity. The newly acquired pyruvate metabolism may take place in the MRO, in the cytosol, or even in the chloroplast.
Some of the organisms living in oxygen-free environments (anoxic) are facultative anaerobes in nature. In the presence of oxygen, they are capable of phosphorylation that is oxidative and in anoxic conditions they solely rely on fermentation (Rider 67). Other organelles living in such oxygen-poor habitats are microaerophilic (strict anaerobes). They bare anaerobic mitochondria, whereby ATP is generated using nitrate and not oxygen as an electron acceptor terminal whereby the final product of this process is a nitrite and not water.
These organisms apply a reductase in nitrate form instead of the cytochrome oxidase that is always present in aerobic mitochondria. This results in a lower yield of ATP that is caused due to the lower potential of reduction of nitrate as opposed to molecular oxygen. This, in turn, means that anaerobically is less efficient (energetically) than aerobic respiration. Scientific discoveries have reported that this type of nitrate respiration does exist for ciliates of the genus, Foraminifera, and Loxodesamongst others. In special organelles that don’t have a mitochondria-like cristae and a genome are always characterized by the existence of hydrogenosomes that produce ATP strictly via substrate-level phosphorylation. They lack electron transport chain that is membrane-associated.