A methodological evaluation of the determination of critical oxygen threshold in an estuarine teleost

Abstract

One measure of hypoxia tolerance is the critical oxygen threshold, Pcrit, which is the point where standard metabolism can no longer be maintained through aerobic processes. Traditionally, Pcrit was determined using closed respirometry, whereby the fish's respiration naturally lowered O2. More recently, intermittent flow techniques have been adopted, where N2 is used to displace O2, which ostensibly reduces end-product build-up. This study used a paired design on the marine teleost, red drum. Pcrit is comparable between closed (4.6±0.2 kPa; mean±s.e.m.) and intermittent flow (4.4±0.2 kPa; mean±s.e.m.) respirometry. pCO2, ammonia and pH changes within the chamber were measured prior to the onset of Pcrit and at the end of a typical Pcrit trial and revealed changes in water chemistry in both closed and intermittent flow. Pcrit values were similar in both methods of hypoxia induction regardless of subsequent water chemistry changes that occurred in both methods.

Coronary blood flow influences tolerance to environmental extremes in fish

“…Measurements of cardiorespiratory variables in normoxia commenced in the morning and were recorded for a minimum of 2 h before the start of the hypoxia challenge. Once steady state heart rate values had been obtained during the initial recording period, the hypoxia challenge commenced and was performed using closed circuit respirometry following previously described protocols (Negrete and Esbaugh, 2019;Reemeyer and Rees, 2019). Briefly, by turning off the inflow of normoxic water (Pw O2 ∼21 kPa at 100% air saturation) into the respirometers, the O 2 consumption of the fish caused a gradual decrease in Pw O2 to hypoxic levels until the fish reached P LOE (Nilsson and Östlund-Nilsson, 2004), which was defined as the Pw O2 where the fish lost equilibrium for longer than 10 s. Cardiac measurements were recorded at P LOE for one more minute, after which the individual respirometer was reperfused with aerated water to bring the Pw O2 inside the respirometer back to normoxic conditions.…”

“…The specific mechanisms dictating hypoxia tolerance are multifaceted and can be broadly related to various combinations of a fish's capacity to: (i) maintain aerobic metabolism during hypoxia, (ii) depress overall metabolism and/or (iii) sustain anaerobic metabolism (for review, see Mandic and Regan, 2018;Richards, 2011). Hypoxia tolerance is often quantified by the critical Pw O2 (P crit ), defined as the Pw O2 below which the fish cannot sustain the O 2 consumption rate (M ̇O2 ; a proxy for aerobic metabolic rate) needed for aerobic standard metabolic rate (SMR) in hypoxia (Negrete and Esbaugh, 2019;Ultsch and Regan, 2019). When the animal cannot sustain SMR below P crit , a mismatch between tissue ATP supply and demand develops and ultimately leads to loss of equilibrium at a certain Pw O2 (i.e.…”

“…When the animal cannot sustain SMR below P crit , a mismatch between tissue ATP supply and demand develops and ultimately leads to loss of equilibrium at a certain Pw O2 (i.e. P LOE ; Claireaux and Chabot, 2016;Negrete and Esbaugh, 2019;Rogers et al, 2016). There are several cardiovascular adjustments thought to promote O 2 extraction, transport and delivery to tissues in hypoxia (Gamperl and Driedzic, 2009;Sandblom and Axelsson, 2011;Stecyk, 2017).…”

Oxygen supply capacity breathes new life into critical oxygen partial pressure (Pcrit)

“…However, SMR estimates may vary with respirometry or statistical methodology (Chabot et al, 2021) and other variables, which will influence P crit determination (e.g. Negrete and Esbaugh, 2019;Regan and Richards, 2017). What constitutes SMR is also unclear for the majority of species and many measurements to date may be more appropriately described as routine metabolic rate (RMR), for which activity and feeding history are unknown.…”

Hypoxia Performance Curve: Assess a Whole-Organism Metabolic Shift from a Maximum Aerobic Capacity towards a Glycolytic Capacity in Fish

“…In contrast, SMR is a less ambiguous baseline that can be reliably estimated using established testing apparatus, protocols and analytical tools provided Ṁ O 2 in undisturbed fish is continuously measured over at least 48 h (e.g., [ 41 , 42 , 43 ]). Thus, a recent consensus among fish biologists is that a SMR-derived O 2crit represents a robust analytical approach with a stable baseline that increases the repeatability of O 2crit measurements and the utility of O 2crit for comparisons [ 44 , 45 , 46 , 47 ]. Whether or not SMR has ecological relevance will be an ongoing debate.…”