Egg and early larval stages of Baltic cod, Gadus morhua, are robust to high levels of ocean acidification

The accumulation of carbon dioxide in the atmosphere will lower the pH in ocean waters, a process termed ocean acidification (OA). Despite its potentially detrimental effects on calcifying organisms, experimental studies on the possible impacts on fish remain scarce. While adults will most likely remain relatively unaffected by changes in seawater pH, early life-history stages are potentially more sensitive, due to the lack of gills with specialized ion-regulatory mechanisms. We tested the effects of OA on growth and development of embryos and larvae of eastern Baltic cod, the commercially most important fish stock in the Baltic Sea. Cod were reared from newly fertilized eggs to early non-feeding larvae in 5 different experiments looking at a range of response variables to OA, as well as the combined effect of CO₂ and temperature. No effect on hatching, survival, development, and otolith size was found at any stage in the development of Baltic cod. Field data show that in the Bornholm Basin, the main spawning site of eastern Baltic cod, in situ levels of pCO₂ are already at levels of 1,100 μatm with a pH of 7.2, mainly due to high eutrophication supporting microbial activity and permanent stratification with little water exchange. Our data show that the eggs and early larval stages of Baltic cod seem to be robust to even high levels of OA (3,200 μatm), indicating an adaptational response to CO₂.

Carbon dioxide encourages risky behaviour in clownfish

Carbon dioxide in the ocean acts like alcohol on fish, leaving them less able to judge risks and prone to losing their senses. The intoxication adds to the threats that global warming and ocean acidification pose to marine ecosystems.

Around 2.3 billion tonnes of human-caused CO₂ emissions dissolve into the world’s oceans every year, turning the water more acidic.

Philip Munday and colleagues at James Cook University in Townsville, Queensland, Australia, have previously found that if you put reef fish into water with more CO₂ than normal in it – similar to the levels expected in oceans by the end of the century – they become bolder and attracted to odours they would normally avoid, including those of predators and unfavourable habitats.

Rising carbon dioxide confuses brain signaling in fish

A new study may explain how rising carbon dioxide concentrations — and the ocean acidification they induce — can cause topsy-turvy changes in the behavior of fish. Like a flipped switch, the normal response of nerve cells can reverse as acidifying seawater perturbs how a fish regulates acids and bases in its body, including the brain.

“This could be a big deal,” says neurobiologist Andrew Dittman of the National Oceanic and Atmospheric Administration’s Northwest Fisheries Science Center in Seattle. Dittman, who was not affiliated with the study, says the new findings could go a long way toward explaining curious sensory changes observed in fish exposed to acidifying waters. The scary scent of predators, for example, can suddenly become alluring.

For the new study, published online January 15 in Nature Climate Change, Göran Nilsson of the University of Oslo and his colleagues homed in on brain chemistry.

Carbon dioxide is “driving fish crazy”

Rising human carbon dioxide emissions may be affecting the brains and central nervous system of sea fishes with serious consequences for their survival, an international scientific team has found.

Carbon dioxide concentrations predicted to occur in the ocean by the end of this century will interfere with fishes’ ability to hear, smell, turn and evade predators, says Professor Phillip Munday of the ARC Centre of Excellence for Coral Reef Studies and James Cook University.

“For several years our team have been testing the performance of baby coral fishes in sea water containing higher levels of dissolved CO2 – and it is now pretty clear that they sustain significant disruption to their central nervous system, which is likely to impair their chances of survival,” Prof. Munday says.

Near-future carbon dioxide levels alter fish behaviour by interfering with neurotransmitter function

Predicted future CO₂ levels have been found to alter sensory responses and behaviour of marine fishes. Changes include increased boldness and activity, loss of behavioural lateralization, altered auditory preferences and impaired olfactory function. Impaired olfactory function makes larval fish attracted to odours they normally avoid, including ones from predators and unfavourable habitats. These behavioural alterations have significant effects on mortality that may have far-reaching implications for population replenishment, community structure and ecosystem function. However, the underlying mechanism linking high CO₂ to these diverse responses has been unknown. Here we show that abnormal olfactory preferences and loss of behavioural lateralization exhibited by two species of larval coral reef fish exposed to high CO₂ can be rapidly and effectively reversed by treatment with an antagonist of the GABA-A receptor. GABA-A is a major neurotransmitter receptor in the vertebrate brain. Thus, our results indicate that high CO₂ interferes with neurotransmitter function, a hitherto unrecognized threat to marine populations and ecosystems. Given the ubiquity and conserved function of GABA-A receptors, we predict that rising CO₂ levels could cause sensory and behavioural impairment in a wide range of marine species, especially those that tightly control their acid–base balance through regulatory changes in HCO₃⁻ and Cl⁻ levels.

Algal evolution in relation to atmospheric CO2: carboxylases, carbon-concentrating mechanisms and carbon oxidation cycles

Oxygenic photosynthesis evolved at least 2.4 Ga; all oxygenic organisms use the ribulose bisphosphate carboxylase-oxygenase (Rubisco)–photosynthetic carbon reduction cycle (PCRC) rather than one of the five other known pathways of autotrophic CO₂ assimilation. The high CO₂ and (initially) O₂-free conditions permitted the use of a Rubisco with a high maximum specific reaction rate. As CO₂ decreased and O₂ increased, Rubisco oxygenase activity increased and 2-phosphoglycolate was produced, with the evolution of pathways recycling this inhibitory product to sugar phosphates. Changed atmospheric composition also selected for Rubiscos with higher CO₂ affinity and CO₂/O₂ selectivity correlated with decreased CO₂-saturated catalytic capacity and/or for CO₂-concentrating mechanisms (CCMs). These changes increase the energy, nitrogen, phosphorus, iron, zinc and manganese cost of producing and operating Rubisco–PCRC, while biosphere oxygenation decreased the availability of nitrogen, phosphorus and iron. The majority of algae today have CCMs; the timing of their origins is unclear. If CCMs evolved in a low-CO₂ episode followed by one or more lengthy high-CO₂ episodes, CCM retention could involve a combination of environmental factors known to favour CCM retention in extant organisms that also occur in a warmer high-CO₂ ocean. More investigations, including studies of genetic adaptation, are needed.

The effects of ocean acidification and upwelling conditions on the growth and calcification of the red abalone (Haliotis rufescens)

Upwelling events along the California coast expose invertebrates to low dissolved oxygen simultaneously with high pCO2 levels that are progressively increasing as a result of rising atmospheric CO2. These multiple stressors could potentially impact the growth and calcification of economically valuable molluscs, such as abalone. To evaluate this threat, juvenile red abalone were maintained over a 4-week period in seawater undersaturated with respect to aragonite and containing 85% dissolved oxygen, which simulated an upwelling event. Seawater conditions were then returned to ambient levels for 3 weeks to determine the ability of the abalone to recover from the potential effects of low oxygen and high pCO2 conditions. Abalone exposed to the treatment had lower shell weights and calcium content per shell than abalone in the ambient group. Shells also appeared much lighter in color following the acidification period. After both groups were returned to ambient conditions, shells of the abalone in the treatment group still weighed less and had lower calcium content than the shells of the ambient group. The amount of weight gained by the abalone during the 3-week ambient period, however, was the same for both groups, suggesting an ability to recover a normal rate of weight gain after exposure. These findings suggest that juvenile red abalone experienced decreased net calcification following exposure to high CO2 and decreased DO. Though abalone were able to recover to normal growth rates, they were not able to accelerate their net calcification to catch up to the shells weights and calcium content of the ambient group, suggesting that they may have thinner or less dense shells following each upwelling event

How will ocean acidification affect marine fishes? (video)