Rising CO2 enhances hypoxia tolerance in a marine fish

Abstract

Global environmental change is increasing hypoxia in aquatic ecosystems. During hypoxic events, bacterial respiration causes an increase in carbon dioxide (CO2) while oxygen (O2) declines. This is rarely accounted for when assessing hypoxia tolerances of aquatic organisms. We investigated the impact of environmentally realistic increases in CO2 on responses to hypoxia in European sea bass (Dicentrarchus labrax). We conducted a critical oxygen (O2crit) test, a common measure of hypoxia tolerance, using two treatments in which O2 levels were reduced with constant ambient CO2 levels (~530 µatm), or with reciprocal increases in CO2 (rising to ~2,500 µatm). We also assessed blood acid-base chemistry and haemoglobin-O2 binding affinity of sea bass in hypoxic conditions with ambient (~650 μatm) or raised CO2 (~1770 μatm) levels. Sea bass exhibited greater hypoxia tolerance (~20% reduced O2crit), associated with increased haemoglobin-O2 affinity (~32% fall in P50) of red blood cells, when exposed to reciprocal changes in O2 and CO2. This indicates that rising CO2 which accompanies environmental hypoxia facilitates increased O2 uptake by the blood in low O2 conditions, enhancing hypoxia tolerance. We recommend that when impacts of hypoxia on aquatic organisms are assessed, due consideration is given to associated environmental increases in CO2.

Temperature and O2, but not CO2, interact to affect aerobic performance of European sea bass (Dicentrarchus labrax)

“…However, fish have well developed acid-base regulatory mechanisms and blood sampling showed that sea bass in normoxia had fully compensated for the effects of increased environmental CO 2 on blood pH by compensatory accumulation of extra HCO 3 - , resulting in no changes in Hb-O 2 binding affinity (Figure 5 and S1). Additionally, we have recently shown that sea bass are able regulate blood pH when exposed to concurrent progressive hypercapnia during progressive hypoxia over the course of several hours and have higher Hb-O 2 binding affinity in these conditions when compared to fish exposed to progressive hypoxia with no concurrent hypercapnia (Montgomery et al, 2019). In addition, we did not see significant changes in Hb levels or Hct between CO 2 treatments (Figure 7).…”

“…For each treatment all respirometry measurements were conducted in two groups (hereafter referred to as respirometry group), with five fish being measured simultaneously for each group. Following the 12 hour acclimation period we measured ṀO 2 of each sea bass for ∼3-4 hours (from ∼6 am to ∼10 am) before hypoxia tolerance was assessed using a critical O 2 tension (O 2crit ) trial (Figure 6), following protocols set out in Montgomery et al (2019). Carbon dioxide levels in the water were simultaneously increased as O 2 declined during O 2crit trials to reflect the natural rise in CO 2 during hypoxic events in aquatic systems (Melzner et al, 2013; Montgomery et al, 2019).…”

“…It is The copyright holder for this preprint this version posted March 12, 2021. ; https://doi.org/10.1101/2021.03.12.435078 doi: bioRxiv preprint 5.3. Respirometry measurements (SMR,MMR,and O2crit) Rates of oxygen consumption (ṀO2) were made as a proxy of metabolic rate using an intermittent-flow respirometer system, details of which can be found in Montgomery et al (2019), set-up following recommendations by Svendsen et al (2016). Sea bass were starved for 72 hours prior to the start of measurements to ensure that metabolism was not affected by the specific dynamic action of digestion (Chabot et al, 2016).…”

Diel vertical migration into anoxic and high-pCO2 waters: acoustic and net-based krill observations in the Humboldt Current

“…Further, Mayol et al 36 demonstrated that high p CO 2 further challenges respiration in the NHCS. However, recent evidence in fish shows that they are better adapted to tolerate hypoxia under elevated CO 2 conditions as compared to hypoxia alone 64 . Certainly, the effects of p CO 2 and p O 2 cannot be considered independently, as they co-occur in nature and are both involved in respiratory processes.…”

Data_Sheet_2.xlsx

“…Studies of other marine organisms have documented favorable impacts of reduced pH in combination with low oxygen as well. For example, Montgomery et al (2019) found that lower pH increased hypoxia tolerance in a fish, due to increased affinity of red blood cells for oxygen under the lower pH conditions. Invertebrates that lack hemoglobin have also shown positive impacts of reduced pH.…”