The influence of haemoglobin on behavioural thermoregulation and oxygen consumption in Daphnia carinata

Oyster larvae used for ecosystem restoration benefit from increased thermal fluctuation

A bottleneck in restoring self-sustaining beds of the European oyster (Ostrea edulis) is the successful development and settlement of larvae to bottom habitats. These processes are largely governed by temperature but a mechanistic understanding of larval performance across ecologically relevant temperatures is lacking. We reared larvae at low (20-21 °C) and high (20-24 °C) fluctuating temperatures and applied short-term exposures of larvae to temperatures between 16 and 33 °C to assess vital rates and thermal coping ranges. Larval thermal preference was between 25 and 30 °C for both rearing treatments which corresponded with optimum temperatures for oxygen consumption rates and locomotion. Larvae had 5.5-fold higher settling success, however, when reared at the high compared to the low fluctuating temperatures. Higher mean and periods of increased temperature, as projected in a future climate, may therefore enhance recruitment success of O. edulis in northern European habitats.

A miniaturized electrothermal array for rapid analysis of temperature preference behaviors in ecology and ecotoxicology

Due to technical limitations, there have been minimal studies performed on thermal preferences and thermotactic behaviors of aquatic ectotherm species commonly used in ecotoxicity testing. In this work, we demonstrate an innovative, purpose-built and miniaturized electrothermal array for rapid thermal preference behavioral tests. We applied the novel platform to define thermal preferences in multiple invertebrate and vertebrate species. Specifically, Dugesia notogaea (freshwater planarians), Chironomus tepperi (nonbiting midge larvae), Ostracoda (seed shrimp), Artemia franciscana (brine shrimp), Daphnia carinata (water flea), Austrochiltonia subtenuis (freshwater amphipod), Physa acuta (freshwater snail), Potamopyrgus antipodarum (New Zealand mud snail) and larval stage of Danio rerio (zebrafish) were tested. The Australian freshwater water fleas, amphipods, snail Physa acuta as well as zebrafish exhibited the most consistent preference to cool zones and clear avoidance of zones >27 °C out of nine species tested. Our results indicate the larval stage of zebrafish as the most responsive species highly suitable for prospective development of multidimensional behavioral test batteries. We also showcase preliminary data that environmentally relevant concentrations of pharmaceutical pollutants such as non-steroidal anti-inflammatory drug (NSAID) ibuprofen (9800 ng/L) and insecticide imidacloprid (4600 ng/L) but not anti-depressant venlafaxine (2200 ng/L) and (iv) anticonvulsant medications gabapentin (400 ng/L) can perturb thermal preference behavior of larval zebrafish. Collectively our results demonstrate the utility of simple and inexpensive thermoelectric technology in rapid exploration of thermal preference in diverse species of aquatic animals. We postulate that more broadly such technologies can also have added value in ecotoxicity testing of emerging contaminants.

Editorial: Untangling the oxygen transport cascade: a tribute to Peter Frappell (Frapps)

This collection of research articles was put together in honour of respiratory physiologist Professor Peter Frappell's (Frapps's) academic achievements. It encompasses various topics relating to the oxygen transport cascade, which was central to Frapps' career as a comparative physiologist. This issue highlights the diversity and outreach of his influence on the field and his pioneering spirit; promoting novel perspectives, methodologies and research techniques. This issue also demonstrates how Frapps' knowledge and scientific findings answered some of the fundamental questions within the field of respiratory physiology while creating and fostering a rather unique work atmosphere in the laboratories he led. We thank Frapps for the contributions he has made and the friendships he has nurtured over his career. Cheers, Frapps - we love you mate!

Breaking free from thermodynamic constraints: thermal acclimation and metabolic compensation in a freshwater zooplankton species

Respiration rates of ectothermic organisms are affected by environmental temperatures, and sustainable metabolism at high temperatures sometimes limits heat tolerance. Organisms are hypothesized to exhibit acclimatory metabolic compensation effects, decelerating their metabolic processes below Arrhenius expectations based on temperature alone. We tested the hypothesis that either heritable or plastic heat tolerance differences can be explained by metabolic compensation in the eurythermal freshwater zooplankton crustacean Daphnia magna We measured respiration rates in a ramp-up experiment over a range of assay temperatures (5-37°C) in eight genotypes of D. magna representing a range of previously reported acute heat tolerances and, at a narrower range of temperatures (10-35°C), in D. magna with different acclimation history (either 10 or 25°C). We discovered no difference in temperature-specific respiration rates between heat-tolerant and heat-sensitive genotypes. In contrast, we observed acclimation-specific compensatory differences in respiration rates at both extremes of the temperature range studied. Notably, there was a deceleration of oxygen consumption at higher temperature in 25°C-acclimated D. magna relative to their 10°C-acclimated counterparts, observed in active animals, a pattern corroborated by similar changes in filtering rate and, partly, by changes in mitochondrial membrane potential. A recovery experiment indicated that the reduction of respiration was not caused by irreversible damage during exposure to a sublethal temperature. Response time necessary to acquire the respiratory adjustment to high temperature was lower than for low temperature, indicating that metabolic compensation at lower temperatures requires slower, possibly structural changes.

Thermoregulation as a disease tolerance defense strategy

Physiological responses that occur during infection are most often thought of in terms of effectors of microbial destruction through the execution of resistance mechanisms, due to a direct action of the microbe, or are maladaptive consequences of host-pathogen interplay. However, an examination of the cellular and organ-level consequences of one such response, thermoregulation that leads to fever or hypothermia, reveals that these actions cannot be readily explained within the traditional paradigms of microbial killing or maladaptive consequences of host-pathogen interactions. In this review, the concept of disease tolerance is applied to thermoregulation during infection, inflammation and trauma, and we discuss the physiological consequences of thermoregulation during disease including tissue susceptibility to damage, inflammation, behavior and toxin neutralization.

Combined effects of hypoxia and ammonia to Daphnia similis estimated with life-history traits

The degradation of cyanobacterial blooms often causes hypoxia and elevated concentrations of ammonia, which can aggravate the adverse effects of blooms on aquatic organisms. However, it is not clear how one stressor would work in the presence of other coexistent stressors. We studied the toxic effects of elevated ammonia under hypoxia using a common yet important cladoceran species Daphnia similis isolated from heavily eutrophicated Lake Taihu. A 3 × 2 factorial experimental design was conducted with animals exposed to three un-ionized ammonia levels under two dissolved oxygen levels. Experiments lasted for 14 days and we recorded the life-history traits such as survival, molt, maturation, and fecundity. Results showed that hypoxia significantly decreased survival time and the number of molts of D. similis, whereas ammonia had no effect on them. Elevated ammonia significantly delayed development to maturity in tested animals and decreased their body sizes at maturity. Both ammonia and hypoxia were significantly detrimental to the number of broods, the number of offspring per female, and the number of total offspring per female, and significantly synergistic interactions were detected. Our data clearly demonstrate that elevated ammonia and hypoxia derived from cyanobacterial blooms synergistically affect the cladoceran D. similis.

The influence of natural stressors on the toxicity of nickel to Daphnia magna

Global warming has become a source of awareness regarding the potential deleterious effects of extreme abiotic factors (e.g., temperature, dissolved oxygen (DO) levels) and also their influence on chemicals toxicity. In this work, we studied the combined effects of nickel and temperature (low and high levels) and nickel and low levels of DO to Daphnia magna, and concentration addition and independent action concepts as well as their deviations for synergism/antagonism, dose ratio and dose level dependency, were applied to survival and feeding rate data. Nickel single exposure showed an LC(50) value for 48 h of 7.36 mg l(-1) and an EC(50) value for feeding impairment at 2.41 mg l(-1). In the acute exposures to high and low temperatures, 50% of mortality was observed, respectively, at 30.7 degrees C and 4.2 degrees C whereas 50% reduction on the feeding activity was recorded at 22.6 degrees C and 16.0 degrees C. Relatively to low DO levels, a LC(50) value for 48 h of 0.5 mg l(-1) was obtained; feeding activity EC(50) value was 2 mg l(-1). On acute combined experiments, antagonism was observed for the combination of nickel and extreme temperatures, whereas a synergistic behaviour was observed in the combined exposure of nickel and low DO levels. At sublethal levels, nickel showed to be the main inducer of toxicity at high and low temperatures but not at low levels of dissolved oxygen. Toxicokinetics and toxicodynamics modelling studies should be made in the future to understand the toxicological pathways involved on complex combinations of stressors and to validate any conclusions.

Toxicity prediction of binary combinations of cadmium, carbendazim and low dissolved oxygen on Daphnia magna

Environmental contamination is often characterised by a combination of stress factors of various sources (biological, physical and chemical). The predictability of their joint effects is an important stage in environmental risk assessment procedures. In this study, the two main conceptual models for mixture evaluation based on the effect of individual compounds, concentration addition (CA) and independent action (IA) and deviations to synergism/antagonism, "dose ratio" and "dose level" dependency were used. The single and combined effects of cadmium, carbendazim and low dissolved oxygen levels were assayed for life-cycle parameters (survival and feeding) of the water flea Daphnia magna Straus. The results of single exposures revealed an increase of acute and chronic toxicity as concentrations of cadmium and carbendazim increases. At low dissolved oxygen levels both survival and feeding parameters were significantly affected (P< or =0.05). In the acute mixture exposure of cadmium and carbendazim a "dose ratio" dependency was observed with a higher toxicity when cadmium was dominant whereas at high concentrations of carbendazim a lower effect on survival was observed. At chronic exposures an antagonistic deviation from IA model was observed for this mixture. The IA model showed to be adequate for toxicity prediction on acute exposure combinations with low DO levels where a synergistic behaviour was observed. However, at sublethal exposures IA and CA models failed by underestimation. Validation from toxicokinetic and toxicodynamic modelling studies should be made in the future as a way to understand toxicological pathways involved in complex mixture/combination exposures.

Metabolic, respiratory and cardiovascular responses to acute and chronic hypoxic exposure in tadpole shrimp Triops longicaudatus

Hypoxic exposure experienced during sensitive developmental periods can shape adult physiological capabilities and define regulatory limits. Tadpole shrimp were reared under normoxic (19-21 kPa O(2)), moderate (10-13 kPa O(2)) or severe (1-3 kPa O(2)) hypoxic conditions to investigate the influence of developmental oxygen partial pressure (P(O(2))) on adult metabolic, respiratory and cardiovascular physiology. Developmental P(O(2)) had no effect on metabolic rate or metabolic response to hypoxic exposure in adults. All rearing groups decreased O(2) consumption as water P(O(2)) decreased. Heart rate, stroke volume and cardiac output were independent of P(O(2)) down to 5 kPa O(2) in all rearing groups. Below this, cardiac output was maintained only in tadpole shrimp reared under severe hypoxic conditions. The enhanced ability to maintain cardiac output was attributed to an increase in hemoglobin concentration and O(2)-binding affinity in those animals. Oxygen-delivery potential was also significantly higher in the group reared under severe hypoxic conditions (1,336 microl O(2) min(-1)) when compared with the group reared under normoxic conditions (274 microl O(2) min(-1)). Differences among the rearing groups that were dependent on hemoglobin were not considered developmental effects because hemoglobin concentration could be increased within seven days of hypoxic exposure independent of developmental P(O(2)). Hypoxia-induced hemoglobin synthesis may be a compensatory mechanism that allows tadpole shrimp to regulate O(2) uptake and transport in euryoxic (O(2) variable) environments. The results of this study indicate that increased hemoglobin concentration, increased O(2)-binding affinity and transient decreases in metabolic demand may account for tadpole shrimp hypoxic tolerance.

Thermal behaviour of crustaceans

Specific thermoreceptors or putative multimodal thermoreceptors are not known in Crustacea. However, behavioural studies on thermal avoidance and preference and on the effects of temperature on motor activity indicate that the thermosensitivity of crustaceans may be in the range 0.2-2 degrees C. Work on planktonic crustaceans suggests that they respond particularly to changes in temperature by klinokinesis and orthokinesis. The thermal behaviour of crustaceans is modified by thermal acclimation among other factors. The acclimation of the critical maximum temperature is an example of resistance acclimation, while the acclimation of preference behaviour may be classified as capacity acclimation of some other function. In crustaceans, the use of the concepts stenothermy and eurythermy at the species level is questionable, and it is not possible to divide crustacean species into thermal guilds as suggested for fishes. Thermal preference behaviour contributes to fitness in different ways in different species, often by maximising the aerobic metabolic scope for activity. In crustaceans the peripheral nervous system seems to have retained the capacity for thermosensitivity and thermal acclimation independently of the central nervous system control of behaviour.

Effects of hypoxia acclimation on morpho-physiological traits over three generations of Daphnia magna

The mechanisms, dynamics and effects of hypoxia acclimation were studied in the water flea Daphnia magna over three successive generations (parental, first and second filial generation: P, F1 and F2). The P generation was raised under normoxic conditions at 20 degrees C and became exposed to environmental hypoxia (10-19% air saturation) at maturity. Their progenies (F1 and F2) experienced hypoxia from birth onwards. Controls were kept under normoxic conditions. Individuals were successively sampled in a 3-day interval from each acclimation group to determine morpho-physiological parameters relevant in oxygen transport and regulation. Hypoxia acclimation induced adjustments at the haemoglobin (Hb) and metabolic level (within 3 days) but none at the systemic level. The convective performance and oxygen-sensitive control of the ventilatory and circulatory systems were the same in both acclimation groups. The Hb concentration and oxygen affinity increased by 266% and 32%, respectively. The 22% decrease in mass-specific oxygen consumption rate reduced the energy allocation to somatic growth without greatly affecting reproduction. The onset and duration of hypoxic exposure during ontogenesis have had a significant influence on Hb oxygen affinity and body size. Transgenerational effects of hypoxia acclimation could not be observed. The adjustments at the Hb and metabolic levels in combination with the smaller body size, which is advantageous to diffusive oxygen transport, reduced the critical ambient oxygen tension by approximately 50%.

Oxygen Preference ofDaphnia magnaIs Influenced by Po2Acclimation and Biotic Interactions

Oxygen preference of the planktonic crustacean Daphnia magna was observed within an oxygen gradient. Long-term acclimation to specific oxygen conditions determined the preferred oxygen tension. Animals from hypoxic acclimation stayed in oxygen-poor water at first. They chose oxygen tensions above the critical partial pressure, thus maintaining aerobic energy supply, but they avoided higher oxygen concentrations. However, they gradually took more oxygen-rich positions within 2 d. Animals from severe hypoxic and hyperoxic acclimation showed stenoxic behavior. Specimens raised in moderately hypoxic or normoxic medium spread out more widely in the oxygen gradient (euryoxic behavior). The possible role of hemoglobin and antioxidant defense capacity limiting the animals' distribution at low or high oxygen concentrations, respectively, is discussed. In tests with mixed groups of D. magna from opposing oxygen acclimation conditions, the animals clustered at intermediate oxygen conditions. However, the acclimation condition of the major group was found to determine the position taken within the gradient to a greater extent. Selected oxygen tensions were closer to the preference values of the larger group, yet the influence of the minority prevented the majority in heterogeneous groups from taking their originally determined preference position. Thus, aggregation behavior interfered with the acclimation-dependent oxygen preference behavior.

Oxygen consumption of the chicken embryo: interaction between temperature and oxygenation

We measured the effects of hypoxia and changes in ambient temperature (T) on the oxygen consumption (VO2) of chicken embryos at embryonic days 11, 16 and 20 (E11, E16 and E20, respectively), and post-hatching day 1 (H1). Between 30 and 39 degrees C, at E11 and E16, VO2 changed linearly with T, as in ectothermic animals, with a Q10 of about 2.1. At E20, VO2 did not significantly change with T, indicating the onset of endothermy. At H1, a drop in T increased VO2, a clear thermogenic response. Hypoxia (11% O2 for 30 min) decreased VO2, by an amount that varied with T and age. At H1, hypoxia lowered VO2 especially at low T. At E20, hypoxic hypometabolism was similar at all T. At E11 and E16, hypoxia lowered VO2 only at the higher T. In fact, at E11, with T=39 degrees C even a modest hypoxia (15-18% O2) decreased VO2. Upon return to normoxia after 40 min of 11% O2, VO2 did not rise above the pre-hypoxic level, indicating that the hypometabolism during hypoxia did not generate an O2 debt. At E11, during modest hypoxia (16% O2) at 36 degrees C, the drop in VO2 was lifted by raising the T to 39 degrees C, suggesting that the hypoxic hypometabolism at 36 degrees C was not due to O2-supply limitation. In conclusion, the hypometabolic effects of hypoxia on the chicken embryo's VO2 depend on the development of the thermogenic ability, occurring predominantly at high T during the early (ectothermic phase) and at low T during the late (endothermic) phase. At E11, both low T and low oxygen force VO2 to drop. However, at a near-normal T, modest hypoxia promotes a hypometabolic response with the characteristics of regulated O2 conformism.

Control of oxygen transport in the microcrustacean Daphnia: regulation of haemoglobin expression as central mechanism of adaptation to different oxygen and temperature conditions

The pathway for oxygen, the control of oxygen transport and the role of haemoglobin expression for the physiological adaptation to different oxygen and temperature conditions were studied in the ecological model organism Daphnia magna. Ventilation of the inner walls of the carapace as the main gas exchange area as well as of the embryos in the brood pouch are controlled, oxygen-dependent processes. The P(O2)-dependent increase of heart rate as well as perfusion rate during short-term, progressive hypoxia improves the circulatory oxygen transport within the body. The regulation of haemoglobin (Hb) expression is the central mechanism for a medium-term adaptation to hypoxia. Genetic control elements and oxygen conditions near the two Hb synthesis sites (fat cells, epipodite epithelial cells) determine, which types of Hb subunits and, accordingly, hetero-multimeric Hb macromolecules are produced. One synthesis site may respond mainly to internal, the other one to external oxygen conditions. Depending on environmental condition, either higher quantities of macromolecules of unchanged functionality (P50) or increasing amounts of macromolecules with higher oxygen affinity are synthesized. The Hb subunit DmHbA is probably of considerable importance for this functional change. The physiological benefits of haemoglobin in Daphnia are discussed. Physiological adaptation of Daphnia to different temperatures is also related to the control of oxygen transport processes with the regulation of haemoglobin expression again as a central mechanism.

A swimming activity assay shows that the thermal tolerance ofDaphnia magnais influenced by temperature acclimation

Daphnia magna Straus, 1820 is a widespread zooplanktic organism enduring considerable changes in oxygen concentration and temperature within its natural habitat. The thermal tolerance window of D. magna was analyzed using the animals' swimming activity as a test parameter in a photometrical assay. Acclimation to different temperatures (10, 20, 30 °C) resulted in a shift of the thermal optimum corresponding to acclimation conditions. Acclimation to warm temperatures also increased the upper thermal tolerance limit in acute thermal tolerance tests. However, the magnitude of the resulting shift in the acute thermal tolerance (LT50) was much smaller. An increase in acclimation temperature by 10 °C changed the thermal optimum by approximately this value, whereas the LT50was enhanced only by 1–2 °C. The time course of the acclimation process was followed by surveying temperature-dependent swimming activity upon the transfer of animals raised in a medium at 20 °C to a medium at 30 °C. Maximum swimming intensity at 20 °C was lost within 3 days. The swimming behavior resembled that of animals acclimated to 30 °C after 2 weeks, indicating that acclimation to the elevated temperature was achieved.

Behavioural response of the cladoceran Daphnia magna STRAUS to sublethal Copper stress--validation by image analysis

In this study changes in the locomotory behaviour of the freshwater cladoceran Daphnia magna STRAUS were used as sublethal indicators of toxic copper (Cu) stress. The behavioural responses were determined by a real time image analysis, using a video camera and a Pentium-PC equipped with a standard low cost frame grabber. For a sequence of 50 images per group, where 10 daphnids were moving simultaneously, the trajectories have been reconstructed in binary image sequences. As biological endpoints, we defined the average swimming velocity and the average duration of swimming activity and inactivity. The behavioural responses of the daphnids were analysed under normal conditions (without Cu stress) and after application of sublethal Cu stress of following concentrations (in microg l(-1) (ppb)): C1=1, C2=5, C3=10, C4=20, C5=30. The test organisms were exposed to the Cu concentration for 24 h under static conditions. Already after 9 h of Cu-exposure a significant (*P<0.05) decrease of the average swimming velocity could be observed at the group of the highest Cu concentration (30 ppb). After 13 h of Cu-contamination the swimming velocity was significantly (*P<0.05) reduced at group of 20 ppb Cu-treatment and after 14 h a significant (*P<0.05) decrease of the average swimming velocity could be measured at the group of 10 ppb Cu-treatment. No significant decrease of the swimming velocity could be observed in the 1 and 5 ppb Cu-treatment.

Mechanisms of acute and chronic waterborne nickel toxicity in the freshwater cladoceran, Daphnia magna

We present evidence that Mg2+ antagonism is one mechanism for acute toxicity of waterborne Ni to Daphnia magna. Acutely, adult D. magna were exposed to either control or 694 microg Ni L(-1) as NiSO4 in moderately soft water (45 mg L(-1) as CaCO3; background Ni approximately 1 microg Ni L(-1)) for 48 h without feeding. Chronically, adults were exposed to either control or 131 microg Ni L(-1) for 14 days (fed exposure). These concentrations were approximately 65% and 12%, respectively, of the measured 48-h LC50 (1068 microg Ni L(-1)) for daphnid neonates in this water quality. The clearest effect of Ni exposure was on Mg2+ homeostasis, as whole-body [Mg2+] was significantly decreased both acutely and chronically by 18%. Additionally, unidirectional Mg2+ uptake rate (measured with the stable isotope 26Mg) was significantly decreased both acutely and chronically by 49 and 47%, respectively, strongly suggesting that Ni is toxic to D. magna due at least in part to Mg2+ antagonism. No impact was observed on the whole-body concentrations or unidirectional uptake rates of Ca2+ during either acute or chronic Ni exposure, while only minor effects were seen on Na+ and Cl- balance. No acute toxic effect was seen on respiratory parameters, as both oxygen consumption rate (MO2) and whole-body hemoglobin concentration ([Hb]) were conserved. Chronically, however, Ni impaired respiratory function, as both MO2 and [Hb] were significantly reduced by 31 and 68%, respectively. Acutely, Ni accumulation was substantial, rising to a plateau between 24 and 48 h of approximately 15 microg g(-1) wet weight--an increase of approximately 25-fold over control concentrations. Mechanisms of acute toxicity of Ni in D. magna differ from those in fish; it is likely that such mechanistic differences also exist for other metals.

Temperature acclimation influences temperature-related behaviour as well as oxygen-transport physiology and biochemistry in the water fleaDaphnia magna

To evaluate the role of temperature in oxygen transport in the water flea Daphnia magna, temperature-related behaviour as well as oxygen-transport physiology and biochemistry were investigated in animals long-term acclimated to moderate temperatures: 10, 20, or 30°C (normoxia) or 20°C (hypoxia). Study of the behaviour of animal groups within a normoxic thermal gradient showed their preferred temperatures and distribution patterns to be connected with acclimation conditions; for example, cold-acclimated individuals, with their relatively high metabolic rates and low oxygen-transport capacities, crowded at low temperatures where the oxygen concentration was high. One key to explaining these data is to assume a strategy of avoiding oxygen shortage. Both temperature and hypoxia acclimation also modified oxygen-transport variables such as oxygen consumption, ventilation and circulation rates, or the concentration and oxygen affinity of haemoglobin. Characteristic features of cold-acclimated D. magna were relatively high oxygen-consumption and heart rates and a low concentration and oxygen affinity of haemoglobin. Hypoxia-acclimated D. magna showed relatively low convective rates (frequency of thoracic-limb movements, heart rate) but a high concentration and oxygen affinity of haemoglobin. Additional experiments showed that independently of each other, temperature and ambient oxygen concentration modify haemoglobin quantity and quality, the former possibly acting via a temperature-induced hypoxia. The varying oxygen affinity is probably due to alterations of haemoglobin subunit composition, with the particular participation of subunits A, E, and G as revealed by two-dimensional gel electrophoresis.

Behavioural thermoregulation in Daphnia carinata from different depths of a natural water body: influence of environmental oxygen levels and temperature

The influence of environmental parameters (water temperature and dissolved oxygen content) on the haemoglobin content of a naturally occurring population of Daphnia carinata was studied in a population resident in an intermittently flowing, shallow body of water. It was found that the Hb content of the animals was influenced by a combination of both the water temperature and dissolved oxygen profiles of the water body. In Summer, stratification, as indicated by sampling at three depths, constrained thermoregulatory behaviour; Hb-poor animals from the upper levels of the water column chose a lower temperature in hypoxia than their Hb-rich counterparts from greater depth. In Winter, the water column was mixed and remained moderately hypoxic at all times. Due to the low temperature of the water, Hb-contents in winter were much lower than those found in Summer. Overall, it was found that thermoregulatory behaviour in D. carinata is dependent upon Hb-content, which is strongly influenced by the animals' environment.

Nonvertebrate hemoglobins: functions and molecular adaptations

Hemoglobin (Hb) occurs in all the kingdoms of living organisms. Its distribution is episodic among the nonvertebrate groups in contrast to vertebrates. Nonvertebrate Hbs range from single-chain globins found in bacteria, algae, protozoa, and plants to large, multisubunit, multidomain Hbs found in nematodes, molluscs and crustaceans, and the giant annelid and vestimentiferan Hbs comprised of globin and nonglobin subunits. Chimeric hemoglobins have been found recently in bacteria and fungi. Hb occurs intracellularly in specific tissues and in circulating red blood cells (RBCs) and freely dissolved in various body fluids. In addition to transporting and storing O(2) and facilitating its diffusion, several novel Hb functions have emerged, including control of nitric oxide (NO) levels in microorganisms, use of NO to control the level of O(2) in nematodes, binding and transport of sulfide in endosymbiont-harboring species and protection against sulfide, scavenging of O(2 )in symbiotic leguminous plants, O(2 )sensing in bacteria and archaebacteria, and dehaloperoxidase activity useful in detoxification of chlorinated materials. This review focuses on the extensive variation in the functional properties of nonvertebrate Hbs, their O(2 )binding affinities, their homotropic interactions (cooperativity), and the sensitivities of these parameters to temperature and heterotropic effectors such as protons and cations. Whenever possible, it attempts to relate the ligand binding properties to the known molecular structures. The divergent and convergent evolutionary trends evident in the structures and functions of nonvertebrate Hbs appear to be adaptive in extending the inhabitable environment available to Hb-containing organisms.