Michael's hypoxia lectures Flashcards
Oxygen in aquatic environments
-Less plentiful, there is 33x less than in air, and moves 300,000x slower than air
-It is influenced by oceanography and is balanced by supply and consumption
-It is variable across space and time.
Oxygen variation
-Daily = daily cycles of low oxygen and re-oxygenation (intertidal), 13%
-Seasonal (longer-term) = stratification in summer estuaries and fjords, all hypoxia below the warmer surface area since there is no mixing (benthic hypoxia), 28%
-Permanent = almost permanent hypoxia in deep sea oxygen minimum zones (OMZ), typically at intermediate depths within ocean systems. Since they are far from the surface but have a high biomass, it can last for years <2%
Anthropogenic oxygen change
-Coastal regions = eutrophication, when the fertilisers and nutrients run off into the sea. This leads to phytoplankton and algal blooms, they then decay and then the microbes come to decompose, they then use up all the oxygen in that area.
-Shallow coasts to the deep sea = climate change, increase in hypoxic areas due to warming waters which hold less oxygen. Temperature can also mess up currents which are there to refresh the ocean.
-Hypoxia severity- it is on the rise with an increased intensity, duration and frequency.
Hypoxia definition
Any oxygen level <2 mg O2 L-1, or any O2 lower than normoxia.
-Based on the threshold were a fishery would collapse
-Once hypoxia begins you lose the mobile species first (anything active)
-Scientists find most things to be absent at that oxygen level
Physiologists suggest hypoxia is described by any partial pressure of oxygen less than normoxia (the norm), but there is a problem with this since not everyone shares the same norm.
Normoxia definition
“normal” oxygen: 6-12 mg O2 L-1 / 100% a.s. / 20kPa
Hyperoxia definition
O2 higher than normoxia: > 6-12mg O2 L-1 / >100% a.s. / > 20kPa
Different taxa = different sensitivity to hypoxia
-Meta analysis of >200 species, where many experienced lethal and sublethal effects at oxygen levels far above <2.
-Shows that there are a lot of species that are more sensitive to hypoxia.
Intensity + duration of hypoxia
Severity/intensity of hypoxia:
-Mild 50%, moderate vs severe 20-30% hypoxia
Duration:
-Acute = typically talking several hours - several days
-Chronic = several days – weeks - months - years
Anaerobic metabolisms: Loricifera
-They are the only known multicellular animals that live alone on an anaerobic metabolism, they live without any oxygen
-Lives in the deep sea, in anoxic and hypersaline sediments
-They can do this since they don’t have mitochondria
Cellular respiration: electron transport chain
-Liberate energy from food through reactions to make ATP
-Without oxygen, around 2 ATP are produced
-With oxygen, around 30 ATP are produces
-Aerobic organisms have a terminal electron accepter in the mitochondrial electron transport chain.
How does the oxygen get there?
-Through diffusion from the environment to the mitochondria which drives oxygen uptake and delivery
-Hypoxia could reduce the difference which could create a problem with diffusion
Metabolism under hypoxia: oxyformers/regulators
-Oxyregulators- maintain the rate of oxygen uptake over a wide range of environmental PO2. The critical oxygen tension (Pc) is where oxygen uptake can’t be regulated anymore and declines. This leads to an increase in anaerobic metabolism and/or hypometabolism. But they don’t really exist because past a certain point of decline they switch to oxyconforming.
-Oxyconformers is when oxygen uptake declines linearly with a decrease in ambient pO2.
These are both metabolic responses that do exist. But there aren’t many true conformers Most organisms display a degree of regulation.
Pcrit = proxy for hypoxia tolerance
-Shows good regulatory abilities in species from low oxygen environments
-Low Pcrit = likely to avoid the transition to anaerobic pathways
-Animals from hypoxic environemtns tend to have a lower Pcrit, those from envrionemnts with normal or high oxygen have a higher Pcrit
-Might suggest those used to lower oxygen levels could survive more extreme conditions of hypoxia.
Regulation of metabolism
-Adjust circulation
-Increase ventilation rate to get more water in, to help load oxygen at the surface. Can also hyperventilate for this reason.
Alter aspects of perfusion
-Pumps; heart could increase the perfusion of respiratory surfaces and tissues
-Complex; cardiac output a combination of heart rate and stroke volume
-Circulatory; could change circulation, for example, shunting the blood from viscera to limbs in crabs.
Oxygen transport by haemolymph
-Oxygen transport can be shown via a binding curve (by looking at the saturation of pigment (haemoglobin/hemocyanin) across PO2.
-Sigmoid is due to the cooperative binding of pigments
-Stronger pigments = better metabolic regulation
-Higher oxygen affinity with haemoglobin is associated with stronger regulation of metabolism under hypoxia.
Respiratory alkalosis
When hyperventilation blows off carbon dioxide which raises the pH.
-Organic modulators are lactate, urate and certain ions like Ca, Mg + bicarbonates.
Prolonged survival in hypoxia
Try to decrease ATP demand if you can’t depress your metabolism (which is very difficult).
-Then you can start to open anaerobic pathways to start anaerobic ATP production (anaerobic metabolism).
-Then you have to deal with cellular damage.
Prolonged survival in vertebrates
When ATP levels fall it causes a change in ion regulation which leads to a loss of membrane integrity and an influx of calcium ions triggering cell death.
Quiescent
What animals become when there is a reduction in ventilation and circulation rates. On a cellular level, it’s when organisms switch off non-essential processes.
-For example, fish switch off genes associated with protein synthesis and locomotion.
Strategies
-Increase ATP supply from anaerobic pathway, not as efficient but better than nothing
-There are diverse anaerobic pathways p
Lactate pathway + goldfish
Anaerobic glycolysis -> produces 2 ATP
-Uses carbohydrates as substrate (like glycogen)
-Anaerobic glycolysis typically results in lactate (which is toxic at high levels)
Goldfish are very tolerant of anoxia; they have large glycogen stores and have ways to reduce lactate. Their muscular tissues can catabolise lactate to ethanol, which is easier to excrete across gills.
Dealing with cellular damage
-Metabolic depression = cutting costs and switching off cellular processes and anaerobiosis
-Proteins like heat-shock proteins can be upregulated in some organisms
-May be adaptive for entry into hypometabolic state, below Pcrit for proteome preservation
Long-term
-Animals can survive longer term under moderate hypoxia
-Acute hyperventilation and circulation are costly, may cause a decline in and long-term adjustments to oxygen supply over time
-Sometimes leads to long-term increase in recruitment of anaerobic metabolism, and a reduction in metabolic costs in some organisms.
Increasing respiratory SA
-Crucian carp + other fish remodel their gills under hypoxia
-There is also controlled apoptosis of interlamellar cell masses under hypoxia, where lamellae protrude
-There is a reversible trade off with osmoregulation + exposure to parasites.
Increasing oxygen supply in blood or haemolymph
-They synthesise more respiratory pigment so haemoglobin/ hemocyanin carry more oxygen
-Rainbow trout increase haemoglobin as hypoxic exposure increases
-The hormone erythropoietin (EPO) synthesises red blood cells, to increase the amount
Increasing the oxygen carriage of blood or haemolymph
-Produce respiratory pigment isoforms which have a higher oxygen affinity, and subunit switching
-Red drum (Hba-3.1) is a predominant isoform, but after 3 weeks of 30% a.s., there was an increased production of different isoforms (Hba-2 and Hba-3.2). They are associated with increased oxygen affinity (reduced P50, bottom)
Health consequence = halved other processes like reproduction growth + feeding rates.
Moderate hypoxia may reduce aerobic scope + amphipods
-Pcrit typically measured for resting metabolism
-Models suggest that maximum metabolic rate (MMR) will become limited first at Pcrit-max, aerobic scope declines below Pcrit-max
-Reduced aerobic energy with implications for fitness
Effects in amphipods = reduced MMR, so also reduced AS. Shift to anaerobic metabolism and have lactate in tissues. Cellular stress so eat shock proteins get upregulated. Next generation has less hypoxia tolerance.
summary of hypoxia challenges + pathways
-Hypoxia poses a potential challenge to metabolism and organism performance
-A range of mechanisms can be elicited to deal with hypoxia whether that be to increase oxygen supply, reduce oxygen or ATP demand, and/or recruit anaerobic pathways.
-Organism responses highly dependent upon severity and duration
-Acutely, regulation of metabolism typically relies on increasing aspects of ventilation, circulation and/or modulation of oxygen binding by respiratory pigments. Under severe hypoxia, often a transition towards anaerobic metabolism/hypometabolism
-Chronically, longer term adjustments e.g. alterations of aspects of respiratory pigment synthesis (concentration/isoform switching), plasticity of respiratory surfaces. May be fitness consequences of even moderate hypoxia.
