Intracellular pH, Buffering Substances and Proteins: Imidazole Protonation and the Conservation of Protein Structure and Function. In: Gilles R, Gilles-baillien M, editors. Transport Processes, Iono- and Osmoregulation. Berlin: Springer Berlin Heidelberg; 1985. pp. 454-68. [DOI: 10.1007/978-3-642-70613-4_38]

Fluctuating seawater pH/pCO2 regimes are more energetically expensive than static pH/pCO2 levels in the mussel Mytilus edulis

Ocean acidification (OA) studies typically use stable open-ocean pH or CO2 values. However, species living within dynamic coastal environments can naturally experience wide fluctuations in abiotic factors, suggesting their responses to stable pH conditions may not be reflective of either present or near-future conditions. Here we investigate the physiological responses of the mussel Mytilus edulis to variable seawater pH conditions over short- (6 h) and medium-term (2 weeks) exposures under both current and near-future OA scenarios. Mussel haemolymph pH closely mirrored that of seawater pH over short-term changes of 1 pH unit with acidosis or recovery accordingly, highlighting a limited capacity for acid-base regulation. After 2 weeks, mussels under variable pH conditions had significantly higher metabolic rates, antioxidant enzyme activities and lipid peroxidation than those exposed to static pH under both current and near-future OA scenarios. Static near-future pH conditions induced significant acid-base disturbances and lipid peroxidation compared with the static present-day conditions but did not affect the metabolic rate. These results clearly demonstrate that living in naturally variable environments is energetically more expensive than living in static seawater conditions, which has consequences for how we extrapolate future OA responses in coastal species.

Spontaneous daily torpor and fasting-induced torpor in Djungarian hamsters are characterized by distinct patterns of metabolic rate

The Djungarian hamster is a rodent species that expresses both spontaneous daily torpor (SDT) when acclimated to winter conditions as well as fasting-induced torpor (FIT) during summer. In an earlier report we argued that these two thermoregulatory phenomena differ in several parameters. In the present study, we further complete this comparison by showing that metabolic rate patterns differ between both SDT and FIT. SDT bouts were significantly longer and deeper compared to FIT bouts. Additionally, respiratory quotient measures support the view that SDT is entered from a state of energetic balance while FIT appears to be an emergency shutdown of energy demanding thermogenesis due to a shortage of energy sources. In a second experiment, we also confirm that brief periods of food restriction during the hamsters' torpor season increase the frequency of SDT, but do not affect its depth or duration. Although winter-acclimated animals could flexibly alter torpor frequency in order to stay in energetic balance, we also found evidence for torpor expression patterns that resembled FIT, rather than SDT. Consequently, if energetic challenges cannot be compensated with increased SDT expression any longer, the hamsters seem to be driven in a negative energy balance resulting in FIT as a last resort.

Larval and post-larval stages of Pacific oyster (Crassostrea gigas) are resistant to elevated CO2

The average pH of surface oceans has decreased by 0.1 unit since industrialization and is expected to decrease by another 0.3-0.7 units before the year 2300 due to the absorption of anthropogenic CO2. This human-caused pH change is posing serious threats and challenges to the Pacific oyster (Crassostrea gigas), especially to their larval stages. Our knowledge of the effect of reduced pH on C. gigas larvae presently relies presumptively on four short-term (<4 days) survival and growth studies. Using multiple physiological measurements and life stages, the effects of long-term (40 days) exposure to pH 8.1, 7.7 and 7.4 on larval shell growth, metamorphosis, respiration and filtration rates at the time of metamorphosis, along with the juvenile shell growth and structure of the C. gigas, were examined in this study. The mean survival and growth rates were not affected by pH. The metabolic, feeding and metamorphosis rates of pediveliger larvae were similar, between pH 8.1 and 7.7. The pediveligers at pH 7.4 showed reduced weight-specific metabolic and filtration rates, yet were able to sustain a more rapid post-settlement growth rate. However, no evidence suggested that low pH treatments resulted in alterations to the shell ultrastructures (SEM images) or elemental compositions (i.e., Mg/Ca and Sr/Ca ratios). Thus, larval and post-larval forms of the C. gigas in the Yellow Sea are probably resistant to elevated CO2 and decreased near-future pH scenarios. The pre-adapted ability to resist a wide range of decreased pH may provide C. gigas with the necessary tolerance to withstand rapid pH changes over the coming century.

Increased feeding and nutrient excretion of adult Antarctic krill, Euphausia superba, exposed to enhanced carbon dioxide (CO₂)

Ocean acidification has a wide-ranging potential for impacting the physiology and metabolism of zooplankton. Sufficiently elevated CO₂ concentrations can alter internal acid-base balance, compromising homeostatic regulation and disrupting internal systems ranging from oxygen transport to ion balance. We assessed feeding and nutrient excretion rates in natural populations of the keystone species Euphausia superba (Antarctic krill) by conducting a CO₂ perturbation experiment at ambient and elevated atmospheric CO₂ levels in January 2011 along the West Antarctic Peninsula (WAP). Under elevated CO₂ conditions (∼672 ppm), ingestion rates of krill averaged 78 µg C individual⁻¹ d⁻¹ and were 3.5 times higher than krill ingestion rates at ambient, present day CO₂ concentrations. Additionally, rates of ammonium, phosphate, and dissolved organic carbon (DOC) excretion by krill were 1.5, 1.5, and 3.0 times higher, respectively, in the high CO₂ treatment than at ambient CO₂ concentrations. Excretion of urea, however, was ∼17% lower in the high CO₂ treatment, suggesting differences in catabolic processes of krill between treatments. Activities of key metabolic enzymes, malate dehydrogenase (MDH) and lactate dehydrogenase (LDH), were consistently higher in the high CO₂ treatment. The observed shifts in metabolism are consistent with increased physiological costs associated with regulating internal acid-base equilibria. This represents an additional stress that may hamper growth and reproduction, which would negatively impact an already declining krill population along the WAP.

Biological impacts of deep-sea carbon dioxide injection inferred from indices of physiological performance

A recent proposal to store anthropogenic carbon dioxide in the deep ocean is assessed here with regard to the impacts on deep-living fauna. The stability of the deep-sea has allowed the evolution of species ill-equipped to withstand rapid environmental changes. Low metabolic rates of most deep-sea species are correlated with low capacities for pH buffering and low concentrations of ion-transport proteins. Changes in seawater carbon dioxide partial pressure (P(CO(2))) may thus lead to large cellular P(CO(2)) and pH changes. Oxygen transport proteins of deep-sea animals are also highly sensitive to changes in pH. Acidosis leads to metabolic suppression, reduced protein synthesis, respiratory stress, reduced metabolic scope and, ultimately, death. Deep-sea CO(2) injection as a means of controlling atmospheric CO(2) levels should be assessed with careful consideration of potential biological impacts. In order to properly evaluate the risks within a relevant timeframe, a much more aggressive approach to research is warranted.