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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 (O_2crit) 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 O_2crit), 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.

SUBMITTER: Montgomery DW

PROVIDER: S-EPMC6805886 | biostudies-literature | 2019 Oct

REPOSITORIES: biostudies-literature

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Rising CO2 enhances hypoxia tolerance in a marine fish.

Montgomery Daniel W DW Simpson Stephen D SD Simpson Stephen D SD Engelhard Georg H GH Birchenough Silvana N R SNR Wilson Rod W RW

Scientific reports 20191022 1

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) ...[more]

PMID: 31641181

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Project description:Rising atmospheric CO 2 and ocean acidification are fundamentally altering conditions for life of all marine organisms, including phytoplankton. Differences in CO 2 related physiology between major phytoplankton taxa lead to differences in their ability to take up and utilize CO 2. These differences may cause predictable shifts in the composition of marine phytoplankton communities in response to rising atmospheric CO 2. We report an experiment in which seven species of marine phytoplankton, belonging to four major taxonomic groups (cyanobacteria, chlorophytes, diatoms, and coccolithophores), were grown at both ambient (500 μatm) and future (1,000 μatm) CO 2 levels. These phytoplankton were grown as individual species, as cultures of pairs of species and as a community assemblage of all seven species in two culture regimes (high-nitrogen batch cultures and lower-nitrogen semicontinuous cultures, although not under nitrogen limitation). All phytoplankton species tested in this study increased their growth rates under elevated CO 2 independent of the culture regime. We also find that, despite species-specific variation in growth response to high CO 2, the identity of major taxonomic groups provides a good prediction of changes in population growth and competitive ability under high CO 2. The CO 2-induced growth response is a good predictor of CO 2-induced changes in competition (R2 > .93) and community composition (R2 > .73). This study suggests that it may be possible to infer how marine phytoplankton communities respond to rising CO 2 levels from the knowledge of the physiology of major taxonomic groups, but that these predictions may require further characterization of these traits across a diversity of growth conditions. These findings must be validated in the context of limitation by other nutrients. Also, in natural communities of phytoplankton, numerous other factors that may all respond to changes in CO2, including nitrogen fixation, grazing, and variation in the limiting resource will likely complicate this prediction.

Project description:Persistent hypoxic or low-oxygen conditions in aquatic systems are becoming more frequent worldwide, causing large-scale mortalities to aquatic fauna. It is poorly understood, however, whether species can acclimate to long-term hypoxic conditions. In two experiments, we exposed juvenile freshwater fish (Murray cod, Maccullochella peelii) to low-oxygen conditions and investigated acclimation effects. Experiment 1 determined how responses could be modified by exposure to different temperatures (20, 24 and 28°C) and oxygen conditions (control 6-8 mgO2 L-1 and low-oxygen 3-4 mgO2 L-1) over 30 days. Experiment 2 determined the acclimation ability of fish exposed to two temperatures (20 and 28°C) and low-oxygen conditions (3-4 mgO2 L-1) for three different acclimation periods (7, 14 and 30 days). Responses were measured by determining critical oxygen tension (P crit), loss of equilibrium and aerobic capacity using resting respirometry. In experiment 1, resting oxygen requirements were negatively affected by long-term low-oxygen exposure except at the highest temperature (28°C). However, long-term acclimation in low-oxygen improved tolerance as measured by loss of equilibrium but not P crit. In experiment 2, fish could tolerate lower oxygen levels before reaching loss of equilibrium after 7 days acclimation, but this declined overtime. Murray cod were most tolerant to low-oxygen at the lowest temperature (20°C) and shortest exposure time (7 days). Extended low-oxygen exposure resulted in reduced aerobic capacity of fish particularly at the lowest temperature. While prior exposure to low-oxygen may allow fish to cope with hypoxic conditions better in the long-term, acclimation time was inversely related to tolerance, suggesting that resistance to hypoxia might decrease as a function of exposure time. Our study fills a much-needed gap in our understanding of how freshwater species acclimate to hypoxia, and in particular, how exposure to prolonged periods of low-oxygen and elevated temperatures affect organisms physiologically.

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Rising CO₂ enhances hypoxia tolerance in a marine fish.

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Rising CO<sub>2</sub> enhances hypoxia tolerance in a marine fish.

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