Direct measurement of cruising and burst swimming speeds of the shortfin mako shark (Isurus oxyrinchus) with estimates of field metabolic rate

The shortfin mako shark is a large-bodied pursuit predator thought to be capable of the highest swimming speeds of any elasmobranch and potentially one of the highest energetic demands of any marine fish. Nonetheless, few direct speed measurements have been reported for this species. Here, animal-borne bio-loggers attached to two mako sharks were used to provide direct measurements of swimming speeds, kinematics and thermal physiology. Mean sustained (cruising) speed was 0.90 m s−1 (±0.07 s.d.) with a mean tail-beat frequency (TBF) of 0.51 Hz (±0.16 s.d.). The maximum burst speed recorded was 5.02 m s−1 (TBFmax = 3.65 Hz) from a 2 m long female. Burst swimming was sustained for 14 s (mean speed = 2.38 m s−1), leading to a 0.24°C increase in white muscle temperature in the 12.5 min after the burst. Routine field metabolic rate was estimated at 185.2 mg O2 kg−1 h−1 (at 18°C ambient temperature). Gliding behaviour (zero TBF) was more frequently observed after periods of high activity, especially after capture when internal (white muscle) temperature approached 21°C (ambient temperature: 18.3°C), indicating gliding probably functions as an energy recovery mechanism and limits further metabolic heat production. The results show shortfin mako sharks generally cruise at speeds similar to other endothermic fish – but faster than ectothermic sharks – with the maximum recorded burst speed being among the highest so far directly measured among sharks, tunas and billfishes. This newly recorded high-oxygen-demand performance of mako sharks suggests it may be particularly vulnerable to habitat loss due to climate-driven ocean deoxygenatio

On the curvature in logarithmic plots of rate coefficients for chemical reactions

In terms of the reduced potential energy barrier ζ = ΔuTS/kT, the rate coefficients for chemical reactions are usually expressed as proportional to e-ζ. The coupling between vibrational modes of the medium to the reaction coordinate leads to a proportionality of the regularized gamma function of Euler Q(a,ζ) = Γ(a,ζ)/Γ(a), with a being the number of modes coupled to the reaction coordinate. In this work, the experimental rate coefficients at various temperatures for several chemical reactions were fitted to the theoretical expression in terms of Q(a,ζ) to determine the extent of its validity and generality. The new expression affords lower deviations from the experimental points in 29 cases out of 38 and it accounts for the curvature in the logarithmic plots of rate coefficients versus inverse temperature. In the absence of tunneling, conventional theories predict the curvature of these plots to be identically zero

The Evolution of Compact Binary Star Systems

We review the formation and evolution of compact binary stars consisting of white dwarfs (WDs), neutron stars (NSs), and black holes (BHs). Binary NSs and BHs are thought to be the primary astrophysical sources of gravitational waves (GWs) within the frequency band of ground-based detectors, while compact binaries of WDs are important sources of GWs at lower frequencies to be covered by space interferometers (LISA). Major uncertainties in the current understanding of properties of NSs and BHs most relevant to the GW studies are discussed, including the treatment of the natal kicks which compact stellar remnants acquire during the core collapse of massive stars and the common envelope phase of binary evolution. We discuss the coalescence rates of binary NSs and BHs and prospects for their detections, the formation and evolution of binary WDs and their observational manifestations. Special attention is given to AM CVn-stars -- compact binaries in which the Roche lobe is filled by another WD or a low-mass partially degenerate helium-star, as these stars are thought to be the best LISA verification binary GW sources.Comment: 105 pages, 18 figure

Evidence that a ‘dynamic knockout’ in Escherichia coli dihydrofolate reductase does not affect the chemical step of catalysis

The question of whether protein motions play a role in the chemical step of enzymatic catalysis has generated much controversy in recent years. Debate has recently reignited over possible dynamic contributions to catalysis in dihydrofolate reductase, following conflicting conclusions from studies of the N23PP/S148A variant of the Escherichia coli enzyme. By investigating the temperature dependence of kinetic isotope effects, we present evidence that the reduction in the hydride transfer rate constants in this variant is not a direct result of impairment of conformational fluctuations. Instead, the conformational state of the enzyme immediately before hydride transfer, which determines the electrostatic environment of the active site, affects the rate constant for the reaction. Although protein motions are clearly important for binding and release of substrates and products, there appears to be no detectable dynamic coupling of protein motions to the hydride transfer step itself

Protein motions, dynamic effects and thermal stability in dihydrofolate reductase from the hyperthermophile thermotoga maritima

Dihydrofolate reductase (DHFR) has long been used as a model system in studies of the relationship between enzyme structure and catalysis. DHFR from the hyperthermophilic bacterium Thermotoga maritima (TmDHFR) is substantially different to other chromosomal DHFRs. It is dimeric where most others are monomeric, it lacks the conformational behaviour of monomeric DHFRs, and the kinetics of the catalysed reaction are significantly different. Experimental and computational studies of TmDHFR and comparison to other DHFRs have yielded deep insights into the role of enzyme motions and dynamics in catalysis. Mutational studies and formation of hybrids between TmDHFR and a monomeric homologue have demonstrated that dimerisation is required for extreme thermostability, but also leads to an inability to adequately close the active site with detrimental effects for the speed of the catalysed reaction. However, in common with other DHFRs there is no involvement of large-scale enzyme motions in the chemical reaction itself and dynamic coupling to the reaction coordinate is efficiently minimised. Studies of DHFRs from hyperthermophilic organisms and comparisons to their mesophilic counterparts remain a rich source of information on the fundamental nature of enzyme catalysis

The vulnerability of sharks, skates, and rays to ocean deoxygenation: Physiological mechanisms, behavioral responses, and ecological impacts

Levels of dissolved oxygen in open ocean and coastal waters are decreasing (ocean deoxygenation), with poorly understood effects on marine megafauna. All of the more than 1000 species of elasmobranchs (sharks, skates, and rays) are obligate water breathers, with a variety of life-history strategies and oxygen requirements. This review demonstrates that although many elasmobranchs typically avoid hypoxic water, they also appear capable of withstanding mild to moderate hypoxia with changes in activity, ventilatory responses, alterations to circulatory and hematological parameters, and morphological alterations to gill structures. However, such strategies may be insufficient to withstand severe, progressive, or prolonged hypoxia or anoxia where anaerobic metabolic pathways may be used for limited periods. As water temperatures increase with climate warming, ectothermic elasmobranchs will exhibit elevated metabolic rates and are likely to be less able to tolerate the effects of even mild hypoxia associated with deoxygenation. As a result, sustained hypoxic conditions in warmer coastal or surface-pelagic waters are likely to lead to shifts in elasmobranch distributions. Mass mortalities of elasmobranchs linked directly to deoxygenation have only rarely been observed but are likely underreported. One key concern is how reductions in habitat volume as a result of expanding hypoxia resulting from deoxygenation will influence interactions between elasmobranchs and industrial fisheries. Catch per unit of effort of threatened pelagic sharks by longline fisheries, for instance, has been shown to be higher above oxygen minimum zones compared to adjacent, normoxic regions, and attributed to vertical habitat compression of sharks overlapping with increased fishing effort. How a compound stressor such as marine heatwaves alters vulnerability to deoxygenation remains an open question. With over a third of elasmobranch species listed as endangered, a priority for conservation and management now lies in understanding and mitigating ocean deoxygenation effects in addition to population declines already occurring from overfishing

Measuring deoxygenation effects on marine predators: A new animal-attached archival tag recording in situ dissolved oxygen, temperature, fine-scale movements and behaviour

1. Global climate-driven ocean warming has decreased dissolved oxygen (DO) levels (ocean deoxygenation) leading to expansions of hypoxic zones, which will affect the movements, behaviour, physiology and distributions of marine animals. However, the precise responses of animals to low DO remains poorly understood because movements and activity levels are seldom recorded alongside instantaneous DO in situ. 2. We describe a new animal-attached (dissolved oxygen measuring, DOME) archival tag with an optical oxygen sensor for recording DO, in addition to sensors for temperature and depth, a triaxial accelerometer for fine-scale movements and activity, and a GPS for tag recovery. All sensors were integrated on a single electronic board. 3. Calibration tests demonstrated small mean difference between DOME tag and factory-calibrated DO sensors (mean relative error of 5%). No temporal drift occurred over a test period three times longer than the maximum deployment time. Deployments on four blue sharks (Prionace glauca) in the central North Atlantic Ocean showed regular vertical oscillations from the surface to a maximum of 404 m. Profiles from diving sharks recorded DO concentrations ranging from 217 to 272 μmol L−1, temperatures between 13°C and 23°C, and identified an oxygen maximum at ~45 m depth, all of which were consistent with ship-based measurements. Interestingly, the percentage of time sharks spent burst swimming was greater in the top 85 m compared to deeper depths, potentially because of higher prey availability in the surface layer. 4. The DOME tag described blue shark fine-scale movements and activity levels in relation to accurately measured in situ DO and temperature, with the potential to offer new insights of animal performance in low oxygen environments. Development of a tag with physico-chemical and movement sensors on a single electronic board is a first step towards satellite relay of these data over broader spatiotemporal scales (months over thousands of kilometres) to determine direct and indirect responses of marine animals to heatwave and deoxygenation event

Reduced Susceptibility of Moritella profunda Dihydrofolate Reductase to Trimethoprim is Not Due to Glutamate 28

The E28D variant of dihydrofolate reductase from Moritella profunda was generated and found to have the same K i (within error) for the competitive inhibitor trimethoprim as the wild type enzyme. Contrary to a previous claim in the literature, Glu 28 is therefore not the cause of the reduced affinity for trimethoprim relative to dihydrofolate reductase from Escherichia coli