Thermal effects on the blood respiratory properties of southern bluefin tuna, Thunnus maccoyii

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Abstract

Thermal effects on the blood respiratory properties of southern bluefin tuna (Thunnus maccoyii) at 10, 23 and 36 °C, and at 0.5 and 1.5% CO2 were investigated. A reversed temperature effect occurred as the oxygen partial pressure required for 50% haemoglobin saturation (P50) at 0.5% CO2 decreased from 2.9 kPa at 10 °C to 1.7 kPa at 23 °C (apparent heat of oxygenation, ΔH°,  = + 27 kJ mol 1). However, oxygen binding was essentially independent of temperature at warmer temperatures (P50 = 1.7–2.0 kPa from 23–36 °C at 0.5% CO2; ΔH° =  6.5 kJ mol 1). Hill's coefficient (nH) ranged from 1.3 to 1.6, and there was a large effect of temperature on the Bohr factor (ΔlogP50/ΔpH =  1.6 at 10 °C and − 0.9 at 36 °C). This is the first study of whole blood to demonstrate the thermal dependence of ΔH° itself, whereby the oxygen equilibrium curve is more sensitive to temperature in the lowest thermal range examined. We suggest that the functional basis for these observations lies in the necessity to ensure a sufficient oxygen supply to all tissues, including the heart and liver, without suffering from premature or excessive oxygen unloading around the heat exchanger prior to delivery of oxygen to organs and tissues that lie efferent to the exchanger.

Introduction

Tunas are powerful and highly athletic predatory fishes that are widespread in tropical and temperate seas. Their high metabolic rates are linked with a degree of endothermy that is unusual among fishes (Carey et al., 1971, Brill, 1987, Bernal et al., 2001, Fitzgibbon et al., 2008-this issue). Vascular heat exchangers (retia mirabilia) harness metabolically produced heat in specific regions of the body including the eye, brain, viscera and swimming musculature, thus providing thermodynamic advantage to selected organs and tissues (Carey, 1973, Graham and Dickson, 2004). The retia mirabilia are comprised of dense networks of arterioles and venules that run countercurrently in close enough proximity to exchange heat (Carey, 1973). The warm venous blood leaving active tissues transfers heat to the cooler arterial blood entering them, and heat is conserved in the tissues and body core rather than being lost via peripheral blood vessels and gills that remain at ambient water temperature. There are several types of retia in tunas. The aerobic swimming musculature is supported by heat exchangers located centrally in the red muscle distributed adjacent to the vertebral column in smaller species (e.g. Katsuwonus pelamis) or along the lateral line in larger species (e.g. Thunnus obesus, Thunnus maccoyii) (Graham and Dickson, 2004). Also in larger species (e.g. T. thynnus), there are several discrete retia that exchange heat from the warm visceral organs that they individually serve to the cool blood derived from the coeliac artery (Carey et al., 1984, Fudge and Stevens, 1996). This creates a situation where blood may change by 20 °C during its transit between the gills and the tissues and organs (Carey and Gibson, 1983, Dewar et al., 1994). Furthermore, tunas may undergo extensive vertical migration in the water column and thus encounter progressively colder water at the gill exchange surface as the animal descends to greater depths (Block et al., 2001). No other animal experiences such extreme and sudden regional changes in blood temperature, so it is intuitive to suggest that tunas may possess unique blood respiratory properties to maintain oxygen transport in the face of these large and abrupt thermal challenges.

Oxygen carrying capacity in tuna blood is enhanced by high haemoglobin (Hb) concentration achieved through high mean cell haemoglobin concentration (MCHC) rather than by increased numbers of erythrocytes that might compromise blood viscosity (Barrett and Connor, 1962, Klawe et al., 1963, Alexander et al., 1980, Brill and Jones, 1994). Nevertheless, further increases in haemoglobin concentration ([Hb]) may occur through splenic contraction and release of erythrocytes during vigorous exercise (Wells et al., 1986). Oxygen carrying capacity alone, however, is not sufficient to ensure efficient transfer of oxygen from the gills to metabolically active tissues, and so blood oxygen affinity is a critical parameter. This poses a potential problem for tunas, because haemoglobin-oxygen binding is an exothermic process (Weber, 2000, Weber and Fago, 2004) and thus as the animal encounters different water temperatures, blood affinity in the gills is expected to change relative to that of blood in the regionally warm tissues.

Rossi-Fanelli and Antonini (1960) were the first to examine the effect of temperature on oxygen binding properties of tuna haemoglobin. They discovered the unusual phenomenon that oxygen binding in purified haemoglobin from northern bluefin tuna, T. thynnus, was essentially independent of temperature from 5–35 °C. Thermal insensitivity, and even reversed temperature effects (endothermic oxygen binding, where temperature increases affinity), have subsequently been reported for isolated tuna Hb (Carey and Gibson, 1977, Ikeda-Saito et al., 1983). The thermodynamics of oxygen binding in isolated, purified Hb are modified however, in the intact erythrocyte where the binding reactions of Hb to protons (Bohr and Root effects) and red cell inorganic phosphates (allosteric modulation) may modify the overall thermal relations. For example, endothermic processes that counter the normal exothermic nature of Hb-oxygen binding include the oxygen-linked release of protons and anions, and conformational changes associated with oxygenation (Weber and Fago, 2004). Nonetheless, whole blood studies generally confirm the independent or reversed temperature sensitivity of tuna blood (Cech et al., 1984, Jones et al., 1986, Brill and Bushnell, 1991, Lowe et al., 2000, Brill and Bushnell, 2006).

Two contrasting hypotheses have been advanced to explain the adaptive significance of the reduced temperature effect on Hb-oxygen binding in tuna blood. Rossi-Fanelli and Antonini (1960) proposed that temperature insensitive oxygen binding enabled tunas to exploit waters of very different temperatures without compromising oxygen uptake at the gills. This idea seems sensible in terms of both the latitudinal differences in temperature encountered by tuna during their migrations, and also in terms of the sharp drop in temperature below the thermocline when tuna make regular vertical feeding excursions (Block et al., 2001, Davis and Stanley, 2002). There do not appear to be any studies of haemoglobin entropy in equivalent ecotypes such as the ectothermic scombrid fishes. This hypothesis, however, does not address the observation that thermally independent oxygen binding is typically associated with regional endothermy, nor does it address the reversed temperature effect seen in the haemoglobins of some tuna species. An alternative hypothesis is that independent or reversed temperature effects evolved in tunas to reduce the problem of premature oxygen transfer from the arteries to the veins as blood perfuses the countercurrent heat exchangers en route to the tissues (Graham, 1973). Several investigators have modelled this hypothesis (Graham, 1973, Carey and Gibson, 1983, Cech et al., 1984), despite the conclusion of Stevens et al. (1974) that “the vessels in the central heat exchanger of skipjack tuna are an order of magnitude larger than those in a typical swim-bladder, thus permitting heat transfer but preventing gas exchange”.

Most investigations to date have focussed on relatively small tropical species with low thermal inertia that exploit water surface temperatures in the range 20–30 °C (Cech et al., 1984, Jones et al., 1986, Brill and Bushnell, 1991). The bluefin tunas (e.g. T. thynnus, T. orientalis and T. maccoyii) however, stand apart from all other scombrids by virtue of their extremely large body mass with more regions served by countercurrent heat exchangers, and maintenance of a greater environment-to-organism temperature differential than seen in tropical species (Barrett and Hester, 1964, Carey and Teal, 1969, Stevens and Carey, 1981, Korsmeyer and Dewar, 2001, Marcinek et al., 2001). Furthermore, bluefins are characterised by swimming speeds up to 70 km h 1, extensive migratory movements from the tropics to cool temperate waters, and depth range to below 2500 m where water temperatures are below 5 °C (Carey and Lawson, 1973, Young et al., 1997, Gunn and Young, 1999, Block et al., 2001, Davis and Stanley, 2002, Blank et al., 2004, Farley et al., 2007).

The present study examines the respiratory properties of whole blood from juvenile southern bluefin tuna (T. maccoyii). These fish migrate through the Southern Ocean across the Great Australian Bight between November and March each year, where they are likely to encounter surface water temperatures as low as 10 °C (Gunn and Young, 1999). Field observations also confirm that T. maccoyii dives to considerable depths during feeding excursions (Young et al., 1997, Gunn and Young, 1999, Abello et al., 2002). The present communication is the first study of a bluefin species to examine the thermal properties of the blood–oxygen transport system over the likely temperature range experienced by blood of fish in the wild. A specific aim was to expand on previous studies of tuna whole blood (e.g. Brill and Bushnell, 1991, Lowe et al., 2000, Brill and Bushnell, 2006) to examine the thermal dependence of Hb oxygenation within high and low temperature ranges. In light of these new data, we review previously published reports for other tuna species, and discuss unusual thermal effects on Hb-oxygen binding.

Section snippets

Fish capture and blood sampling

Southern bluefin tuna, T. maccoyii (Castelnau), were caught using purse seine nets in the Great Australian Bight in January 2006, and transported to commercial sea cages (40 m diameter, 12 m depth) at Port Lincoln, South Australia, where they were fed daily on Australian sardines (Sardinops sagax). Sea surface temperature was approximately 18 °C. Fish were held for approximately four weeks in captivity to recover from capture stress and transport. Five juvenile fish (body mass 16–19 kg) were

Haematology and oxygen carrying capacity

Haematological values for T. maccoyii remained stable throughout the experimental period and are summarised in Table 1. On the basis that 1 g Hb combines with 1.34 mL oxygen, 15.3 g Hb dL 1 results in a theoretical oxygen carrying capacity of 20.5 mL dL 1. Equilibration with a gas mixture containing CO2 causes maximum oxygen content to drop progressively from this value, demonstrating a strong Root effect. Increasing the PCO2 from 0.49 kPa to 1.48 48 kPa at 23 °C (pH from 7.846 to 7.721)

Discussion

Our estimate of Hb concentration confirms the view of the tuna as having an exceptionally high oxygen carrying capacity (Hall and Gray, 1929, Klawe et al., 1963, Stevens, 1972, Cech et al., 1984, Jones et al., 1986, Brill and Bushnell, 1991), with values approaching those seen in mammals (cf. Prosser, 1973). Our calculated theoretical maximum oxygen content of T. maccoyii blood, based on saturation of all the haeme groups, was only marginally higher than values obtained by direct measurement of

Acknowledgements

This project was funded by grants from the Fisheries Research and Development Corporation through the Aquafin Cooperative Research Centre, the Australian Research Council, and the University of Adelaide. Our appreciation is extended to Brian Taylor and Mick Kent of La Trobe University for constructing the Pwee50 spectrophotometer. Jeffrey Buchanan (South Australian Research and Development Institute) is thanked for collecting blood samples and arranging their transport to the laboratory. The

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    Published as part of the Russell V. Baudinette Memorial Symposium held in Adelaide, Australia, 1-2 October 2005.

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