Reverse translation: effects of acclimation temperature and acute temperature challenges on oxygen consumption, diffusive water flux, net sodium loss rates, Q10 values and mass scaling coefficients in the rainbow trout (Oncorhynchus mykiss)

The effects of dissolved organic carbon and model compounds (DOC analogues) on diffusive water flux, oxygen consumption, nitrogenous waste excretion rates and gill transepithelial potential in Pacific sanddab (Citharichthys sordidus) at two salinities

Many flatfish species are partially euryhaline, such as the Pacific sanddab which spawn and feed in highly dynamic estuaries ranging from seawater to near freshwater. With the rapid increase in saltwater invasion of freshwater habitats, it is very likely that in these estuaries, flatfish will be exposed to increasing levels of dissolved organic carbon (DOC) of freshwater origin at a range of salinities. As salinity fluctuations often coincide with changes in DOC concentration, two natural freshwater DOCs [Luther Marsh (LM, allochthonous) and Lake Ontario (LO, autochthonous) were investigated at salinities of 30 and 7.5 ppt. Optical characterization of the two natural DOC sources indicate salinity-dependent differences in their physicochemistry. LO and LM DOCs, as well as three model compounds [tannic acid (TA), sodium dodecyl sulfate (SDS) and bovine serum albumin (BSA)] representing key chemical moieties of DOC, were used to evaluate physiological effects on sanddabs. In the absence of added DOC, an acute decrease in salinity resulted in an increase in diffusive water flux (a proxy for transcellular water permeability), ammonia excretion and a change in TEP from positive (inside) to negative (inside). The effects of DOC (10 mg C L-1) were salinity and source-dependent, with generally more pronounced effects at 30 than 7.5 ppt, and greater potency of LM relative to LO. Both LM DOC and SDS increased diffusive water flux at 30 ppt but only SDS had an effect at 7.5 ppt. TA decreased ammonia excretion at 7.5 ppt. LO DOC decreased urea-N excretion at both salinities whereas the stimulatory effect of BSA occurred only at 30 ppt. Likewise, the effects of LM DOC and BSA to reduce TEP were present at 30 ppt but not 7.5 ppt. None of the treatments affected oxygen consumption rates. Our results demonstrate that DOCs and salinity interact to alter key physiological processes in marine flatfish, reflecting changes in both gill function and the physicochemistry of DOCs between 30 and 7.5 ppt.

The first direct measurements of ventilatory flow and oxygen utilization after exhaustive exercise and voluntary feeding in a teleost fish, Oncorhynchus mykiss

A new "less invasive" device incorporating an ultrasonic flow probe and a divided chamber, but no stitching of membranes to the fish, was employed to make the first direct measurements of ventilatory flow rate (V̇w) and % O2 utilization (%U) in juvenile rainbow trout (37 g, 8ºC) after exhaustive exercise (10-min chasing) and voluntary feeding (2.72% body mass ration). Under resting conditions, the allometrically scaled V̇w (300 ml kg-1 min-1 for a 37-g trout = 147 ml kg-1 min-1 for a 236-g trout exhibiting the same mass-specific O2 consumption rate, ṀO2) and the convection requirement for O2 (CR = 4.13 L mmol-1) were considerably lower, and the %U (67%) was considerably higher than in previous studies using surgically attached masks or the Fick principle. After exhaustive exercise, V̇w and ṀO2 approximately doubled whereas frequency (fr) and %U barely changed, so increased ventilatory stroke volume (Vsv) was the most important contributor to increased ṀO2. CR declined slightly. Values gradually returned to control conditions after 2-3 h. After voluntary feeding, short-term increases in V̇w, Vsv and ṀO2 were comparable to those after exercise, and fr again did not change. However, %U increased so CR declined even more. The initial peaks in V̇w, Vsv and ṀO2, similar to those after exercise, were likely influenced by the excitement and exercise component of voluntary feeding. However, in contrast to post-exercise fish, post-prandial fish exhibited second peaks in these same parameters at 1-3 h after feeding, and %U increased further, surpassing 85%, reflecting the true "specific dynamic action" response. We conclude that respiration in trout is much more efficient than previously believed.

Interindividual plasticity in metabolic and thermal tolerance traits from populations subjected to recent anthropogenic heating

To better understand temperature's role in the interaction between local evolutionary adaptation and physiological plasticity, we investigated acclimation effects on metabolic performance and thermal tolerance among natural Fundulus heteroclitus (small estuarine fish) populations from different thermal environments. Fundulus heteroclitus populations experience large daily and seasonal temperature variations, as well as local mean temperature differences across their large geographical cline. In this study, we use three populations: one locally heated (32°C) by thermal effluence (TE) from the Oyster Creek Nuclear Generating Station, NJ, and two nearby reference populations that do not experience local heating (28°C). After acclimation to 12 or 28°C, we quantified whole-animal metabolic (WAM) rate, critical thermal maximum (CTmax) and substrate-specific cardiac metabolic rate (CaM, substrates: glucose, fatty acids, lactate plus ketones plus ethanol, and endogenous (i.e. no added substrates)) in approximately 160 individuals from these three populations. Populations showed few significant differences due to large interindividual variation within populations. In general, for WAM and CTmax, the interindividual variation in acclimation response (log2 ratio 28/12°C) was a function of performance at 12°C and order of acclimation (12-28°C versus 28-12°C). CTmax and WAM were greater at 28°C than 12°C, although WAM had a small change (2.32-fold) compared with the expectation for a 16°C increase in temperature (expect 3- to 4.4-fold). By contrast, for CaM, the rates when acclimatized and assayed at 12 or 28°C were nearly identical. The small differences in CaM between 12 and 28°C temperature were partially explained by cardiac remodeling where individuals acclimatized to 12°C had larger hearts than individuals acclimatized to 28°C. Correlation among physiological traits was dependent on acclimation temperature. For example, WAM was negatively correlated with CTmax at 12°C but positively correlated at 28°C. Additionally, glucose substrate supported higher CaM than fatty acid, and fatty acid supported higher CaM than lactate, ketones and alcohol (LKA) or endogenous. However, these responses were highly variable with some individuals using much more FA than glucose. These findings suggest interindividual variation in physiological responses to temperature acclimation and indicate that additional research investigating interindividual may be relevant for global climate change responses in many species.

Effect of temperature on phenanthrene accumulation from hydraulic fracturing flowback and produced water in rainbow trout (Oncorhynchus mykiss)

Hydraulic fracturing has become widely used in recent years to access vast global unconventional sources of oil and gas. This process involves the injection of proprietary mixtures of water and chemicals to fracture shale formations and extract the hydrocarbons trapped within. These injection fluids, along with minerals, hydrocarbons, and saline waters present within the formations being drilled into, return to the surface as flowback and produced water (FPW). FPW is a highly complex mixture, containing metals, salts and clay, as well as many organic chemicals, including polycyclic aromatic hydrocarbons such as phenanthrene. The present study sought to determine the effects of temperature on the accumulation of phenanthrene in rainbow trout (Oncorhynchus mykiss). This model organism resides in rivers overlapping the Montney and Duvernay formations, both highly developed formations for hydraulic fracturing. Rainbow trout acclimated to temperatures of 4, 13 and 17 °C were exposed to either 5% or 20% FPW, as well as saline mixtures representing the exact ionic content of FPW to determine the accumulation of radiolabelled ¹⁴C phenanthrene within the gill, gut, liver and gallbladder. FPW exposure reduced the overall accumulation of phenanthrene in a manner most often similar to high salinity exposure, indicating that the high ionic strength of FPW is the primary factor affecting accumulation. Accumulation was different at the temperature extremes (4 and 17 °C), although no consistent relationship was observed between temperature and accumulation across the observed tissues. These results indicate that several physiological responses occur as a result of FPW exposure and water temperature change which dictate phenanthrene uptake, particularly in the gills. Temperature (and seasonality) alone cannot be used to model the potential accumulation of polycyclic aromatic hydrocarbons after FPW spills.

Interactive effects of temperature and hypoxia on diffusive water flux and oxygen uptake rate in the tidepool sculpin, Oligocottus maculosus

The osmorespiratory compromise hypothesis posits that respiratory epithelial characteristics and physiological regulatory mechanisms which promote gas permeability also increase permeability to ions and water. The hypothesis therefore predicts that physiological responses which increase effective gas permeability will result in increased effective ion and water permeabilities. Though analyses of water and gas effective permeabilities using high temperature have generally supported the hypothesis, water permeability responses to hypoxia remain equivocal and the combination of high temperature and hypoxia untested. We measured diffusive water flux (DWF) and oxygen uptake rate (Ṁo₂) in response to acute temperature change, hypoxia, and the combination of high temperature and hypoxia in a hypoxia-tolerant intertidal fish, the tidepool sculpin (Oligocottus maculosus). In support of the osmorespiratory compromise hypothesis, Ṁo₂ and DWF increased with temperature. In contrast, DWF decreased with hypoxia at a constant temperature, a result consistent with previously observed decoupling of water and gas effective permeabilities during hypoxia exposure in some hypoxia tolerant fishes. However, DWF levels during simultaneous high temperature and hypoxia exposure were not different from fish exposed to high temperature in normoxia, possibly suggesting a failure of the mechanism responsible for down-regulating DWF in hypoxia. These results, together with time-course analysis of hypoxia exposure and normoxic recovery, suggest that tidepool sculpins actively downregulate effective water permeability in hypoxia but the mechanism fails with multi-stressor exposure. Future investigations of the mechanistic basis of the regulation of gill permeability will be key to understanding the role of this regulatory ability in the persistence of this species in the dynamic intertidal environment.