In the brink of extinction - many freshwater fish

Tiistai 12.12.2023 klo 17.23 - Mikko Nikinmaa

When I did my Ph.D. thesis 40-45 years ago, the topic: effects of temperature and hypoxia on respiration of rainbow trout, was hardly noticed by the general public. In 1976 we, biology students arranged a theme evening about the pollution of the Baltic  Sea and invited media. Nobody came, and when I asked a newspaper reporter why that was the case, he answered that the topic had no general interest. I wish the situation were the same today: fish would not suffer from pollution, increased temperature and decreased oxygen level. Unfortunately that is wishful thinking.

IUCN (International Union for the Conservation of Nature) has recently updated its Red List with very worrying information of fish, particularly freshwater fish. A quarter of the freshwater species is in immediate danger of extinction. The most important proximate cause is pollution, but toxicant effects cannot be separated from temperature increases and eutrophication, which causes oxygen lack. All of the previous problems increase parasite loads and cause fish diseases. In addition, overfishing and building of waterways, which has destroyed spawning sites or made it impossible to reach them, decreases fish populations.

Below I give a couple of examples of how all of the above cause the disappearance of specific species. First, eels are critically endangered species, which are characterized by their catadromous way of life and long spawning migrations. They grow and reach maturity in freshwater. Their migration from sea to freshwater feeding sites is critically dependent on smell sensing, which is dramatically disturbed by pesticides and metals. Consequently, those types of pollution may be an important cause of declining eel populations. Second, burbot is a coldwater fish. It spawns in the middle of winter, and it is ice-fished in January-February to get the fish and its eggs for soup. Now that the temperature is increasing, burbot is already living at the high end of its temperature tolerance, and may soon become extinct. Another group of coldwater fish is salmon and its relatives. In addition, it requires clear water with high oxygen content. Since both an increase in temperature and eutrophication decrease the oxygen level, salmon and its relatives may become extinct. Lampreys have succeeded in temperate waters for 500 000 000 years. Many of the species live in sea as adults, but spawn in rivers. Because of building of waterways, e.g. hydroelectric power plants, their long saga may be coming to the end. Finally, aquarium hobby is very popular throughout the world. Many of the ornamental fish do not reproduce in captivity, and are thus fished wild. This has made many species endangered because of overfishing.

 

Environmental Pollution Accentuates Problems of Hypoxic Fish: Why?

Maanantai 20.9.2021 klo 14.55 - Mikko Nikinmaa

Hypoxia, i.e., low oxygen concentration, has increased in aquatic environments throughout the world. Hypoxia is mainly caused by eutrophication of waters whereby the oxygen consumption of organisms (but also oxidation of dead materials) increases. The occurrence of hypoxia is either continuous or diurnal. Diurnal changes in oxygen levels occur, if the eutrophication is mainly resulting from the increased biomass of photosynthetising organisms: during the day, when light is available, the increased photosynthesis can cause the environment to become hyperoxic, while at night even those organisms only respire, whereby the oxygen level drops. Continuous hypoxia occurs, when the oxygen demand always exceeds its diffusion (especially from the air) and production.

Fish take up the oxygen they need via the gills. The gills are also the major site of acid-base and ion regulation. This dual function has generated the “osmorespiratory compromise”:   high functional surface area and low diffusion distance favour oxygen uptake, whereas low functional surface area and high diffusion distance favour ion regulation. Aspects of this have been reviewed by Wood and Eom in Comparative Biochemistry and Physiology A-Molecular & Integrative Physiology 2021 Vol. 254 (DOI: 10.1016/j.cbpa.2021.110895). From the environmental pollution point of view, it is important to note that many if not most pollutants have important effects on gills.

Because of the effects of pollutants on gills, it can be expected that the hypoxia responses of fish are affected by pollutants. This is all the more worrisome, as the same sources are the cause of both eutrophicating nutrients and many toxic wastes, i.e., water that has gone through wastewater treatment plants. Lau et al. have studied, how hypoxia responses of fish vary in clean water and in effluent from a modern wastewater treatment plant (Lau et al. Environmental Pollution 2021 Vol. 284, DOI: 10.1016/j.envpol.2021.117373). They observed that the hypoxia tolerance of fish was markedly decreased. This was associated with a reduction in the decrease of the so called intralamellar cell mass. Recently, it has been found that fish increase the functional area of gills in hypoxia largely by decreasing the intralamellar cell mass. If this cannot be done, hypoxia tolerance is impaired. It is not known, why the intralamellar cell mass could not be reduced in the effluent-treated fish. However, the results clearly show that the osmorespiratory compromise of the gills is an important factor to be taken into account when the success of fish in polluted, hypoxic environments is studied.

Hypoxia acts together with pollutants to decrease animal success in aquatic environments

Torstai 13.2.2020 klo 13.55 - Mikko Nikinmaa

Hypoxia is recognized as one of the most important problems for aquatic organisms. The ocean deoxygenation problem is the subject of a recent IUCN report, downloadable from https://www.iucn.org/resources/publications. Deoxygenation is not acting alone. Many anthropogenic pollutants cause oxidative stress. In recent years, measuring parameters associated with oxidative stress has become one of the favourite areas among aquatic toxicologists. Thus, one would have expected that investigations about interactions between hypoxia and pollutants causing oxidative stress would be common. However, this is not so, even though the interactions may cause increased mortality either directly or by affecting energy metabolism, or  decreased reproductive success either directly or by decreasing energy available for reproduction.

The reason for this is that hypoxia responses normally take place in or are initiated by reducing environment – hypoxia in absence of pollutants is such an environment. Already in 2004 we published a paper (Nikinmaa et al., Journal of Cell Science 117, 3201-3206) concluding: “As redox reactions play a pronounced role in the stability, DNA binding and phosphorylation of HIF-1a protein in salmonid cells, it is likely that environmental disturbances involving oxidative stress, such as metal or organic pollutant contamination and increased UV radiation, may influence HIF-1a protein function and consequent gene expression.” Already then the importance of   HIF (Hypoxia-Inducible Protein) was clear, but its importance in regulating hypoxia responses was emphasized by the Nobel prize, which was given to scientists (Semenza, Ratcliffe and Kaelin), who have clarified its regulation. We showed in 2004 (Vuori et al. Aquatic Toxicology 68, 301-313) that the reproductive disturbance   of Baltic salmon, which may be associated with oxidative stress, is also associated with HIF dysfunction. If hypoxic conditions occur together with oxidative stress, the combination may be fatal, because the hypoxia response pathway is disturbed.

While the above scenario is hypothetical, it clearly shows the possibility of hypoxia-oxidative stress interactions, and the reason and rationale for studies investigating them. As a third component for such studies is temperature increase, occurring together with the spreading of hypoxic areas.

Australian bushfires are over, but not the harm to the animals

Tiistai 11.2.2020 klo 16.00 - Mikko Nikinmaa

During the worst bushfires ever in Australia, millions of terrestrial animals have died, burned alive. The newscasts throughout the world have shown koalas with bad burns being rescued, only few out of the many which died in the fires. Among the species which have suffered most, koala is probably the first, since the habitat of koala is the most likely to suffer from fires, and the animals are not fast, and therefore cannot escape the fires.

The fires finally have ended now, because very heavy rain has fallen in Eastern Australia. In New South Wales the rain has been heaviest in thirty years. It has rained in 24 hours (February 8-10, 2020) as much as in one and a half months in a normal year this time. The heavy rain has brought record floods, which have followed record drought. Unfortunately this is exactly what the climate researchers have predicted; as a result of climate change droughts and heavy rains both occur, the weather becomes very unpredictable and extreme phenomena occur. This is also seen in Europe: in Central Norway one measured +19^oCn early January and it snowed 20 cm in Madrid in the late January while grass is green in Southern Finland.

The bushfires of Australia have been extinguished by the heavy rain, and firemen and terrestrial animals can suck of relief. Now it is the turn of aquatic animals to suffer. The rains take the ash and any chemicals that were used in firefighting to the rivers. One has already seen the consequence of this in tens of thousands of dead fish all over the place. Since Australia has very little inland water, the aquatic fauna is quite vulnerable and can easily become extinct. There are two major reasons for fish deaths. First, many of the chemicals used in extinguishing the fires are quite toxic. For example, chemicals of extinguishing foam contain fluorocarbons and aluminium sulphate, which are quite toxic. Aluminium ion was considered to be the major reason behind fish deaths during the acid rain period in Europe and North America in late 20^th century. However, in the leachate from bushfires, the chemical load is probably of only very minor importance. The ash reaching the rivers consumes the oxygen, and the fish living in running water usually require well-oxygenated water. When the ash, which contains organic carbon, uses up the oxygen, and renders the water hypoxic, the fish die of oxygen lack. What is worse, there is nothing that can be done to prevent fish deaths: they are the ultimate consequence of bushfires and heavy rain. The only thing we can do is to wait and hope that an adequate number of fish survive for the populations to recover within 5-10 years.

Rhythmic functions are disturbed by environmental changes

Sunnuntai 2.2.2020 klo 19.39 - Mikko Nikinmaa

Virtually all physiological functions show circadian and seasonal rhythms, with light (day and night, long daylength in summer and short light period in winter) as the major synchronising factor. Light-temperature interactions have been used as reliable indicators by animals for timing of reproduction, winter sleep or hibernation, changes of protective colouring and so on. The temperature increase associated with climate change has made light rhythm unreliable predictor of suitable time to carry out the function that has evolved to use the light rhythm as a reliable cue. As an example, short daylength has indicated to hares that it is time to change to white winter colouring. Since the temperature has increased so that there is no snow in latitudes, which earlier had it, the light cue results in white hares in dark background. Foxes certainly like easy hunting. There are various temperature-dependent responses of fish, which are signalled by light rhythm. Because of light-temperature interactions have rapidly changed, it is, for example, possible that hatching of fish takes place at a time that the larvae have no food after the yolk sac has been used up. In addition to the increase of temperature the oceans are suffering from decreased oxygen level (hypoxia). Hypoxia disturbs the generation of rhythms. Also, several environmental pollutants found in the aquatic environment affect rhythms. Consequently, temperature changes, hypoxia, and environmental contaminants affect aquatic life by affecting rhythmic functions. Because of this and because of interactions between temperature, hypoxia and contaminants, effects on sea life can occur after much smaller changes of any of the environmental conditions than hitherto supposed.

Deoxygenation of oceans is an increasing problem with effets on sealife

Perjantai 13.12.2019 klo 18.01 - Mikko Nikinmaa

The deoxygenation of the seas has increased markedly during the last 100 years. The areas with reduced oxygen have increased ten times between 1900 and 2000. There have always been oxygen-minimum zones in oceans, but their volume has increased markedly in the recent past, because of decreased ocean circulation and as a result of increased respiration following elevated temperature. In addition to the climate change-caused increase in hypoxic seas, the eutrophication of coastal areas because of human actions have caused pronounced low-oxygen areas especially in the traditionally industrialized western countries.

Spreading hypoxia is a major problem, as it decreases the populations of fish and other organisms. It further affects the species distributions with more preferred species decreasing and decreases biodiversity. The effects of reduced oxygen level as such are aggravated by an increased water temperature, i.e. climate change,  because the oxygen consumption of fish and other poikilothermic animals increases with temperature increase. Simultaneously, the oxygen solubility in water decreases. Even this isn’t enough, but the oxygen binding by haemoglobin is reduced at a given oxygen tension with increased temperature. This reduces the capability of fish and other animals to survive in hypoxic conditions. This makes it more difficult of animals to tolerate increased temperature.

So, climate change and the pollution of the seas together cause deoxygenation. The pollution further decreases  the capability of microscopic algae to produce oxygen by photosynthesis. To combat the deoxygenation problem we need to stop eutrophication, and sea pollution by wastewater cleaning. Further, we need to combat climate change much more effectively than we have hitherto done. We need healthy seas to be able to feed the world, and the current increase in ocean deoxygenation is not doing that.

The ocean deoxygenation problem is the subject of an IUCN report, downloadable from https://www.iucn.org/resources/publications. (p.s. I have been studying hypoxia responses in fish from1980).

Nobel Prize for Oxygen Sensing and Hypoxia: the Environmental Relevance of Phenomena

Tiistai 8.10.2019 klo 9.37 - Mikko Nikinmaa

One of the most conspicuous changes that occur in the aquatic environment is the increasing occurrence of hypoxic areas. The Nobel Prize in Physiology and Medicine is this year given to three scientists, Kaelin, Ratcliffe and Semenza, who have studied and discovered the mechanism of how oxygen deficiency controls gene expression in man. Compared to air-breathers, fish and other aquatic animals must get by with 1/30^th of the oxygen concentration. They are further faced with marked variations in oxygen level both daily and seasonally (or unknown periods of time). Further, since fish are poikilothermic, temperature changes affect their oxygen requirements conspicuously.

The oxygen sensing and transport system of fish must therefore be more versatile than that of mammals. We have studied the oxygen sensing and hypoxia-inducible factor (HIF) system, i.e. the phenomena now awarded Nobel Prize, since the late 1990’s. First, we observed that hypoxia-inducible factor was present in cells already in normal venous oxygen tension, although it increased in hypoxia (in humans and laboratory rodents it is only found in hypoxic conditions). Second, we observed that although the hypoxia-inducible factor level was controlled posttranscriptionally, also gene transcription could be modified. The HIF transcription depended on the number of hypoxic bouts experienced by the animal (in humans the control of HIF level occurs posttranscriptionally). Finally, we observed that HIF level was affected by temperature (something that is irrelevant for us homeotherms). These facts, together with the observations of interactions between HIF and circadian rhythms and environmental pollutants show that the system given the Nobel Prize for is more versatile in poikilothermic water breathers than humans.

Given that oxygen is a limiting factor in aquatic environment, it is no surprise that HIF system in fish has evolved differently in different fish groups depending on their oxygen requirements. In continuation, the possibilities of fish to adapt to climate change and environmental pollution are markedly affected by what their HIF system is. Thus, the Nobel Prize winning studies have a significant environmental angle. This has been reviewed to some extent in Nikinmaa, M. and Rees, B.B. (2005) Oxygen-dependent gene expression in fishes. Am. J. Physiol. - Regul. Integr. Comp. Physiol. 288, 1079-1090 and in Prokkola, JM and Nikinmaa M (2018) Circadian rhythms and environmental disturbances - underexplored interactions. J. Exp. Biol. 221, jeb179267. The evolution of HIF system in animals was explored in Rytkönen KT et al (2011) Molecular Evolution of the Metazoan PHD–HIF Oxygen-Sensing System. Mol. Biol. Evol. 28: 1913-1926.

Phenotypic plasticity- a key to environmental responses

Maanantai 23.9.2019 klo 16.54 - Mikko Nikinmaa

We have argued that individual variability is very important in responses to toxicants, not just unwanted noise (Nikinmaa and Anttila 2019. Individual variation in aquatic toxicology: Not only unwanted noise. Aquatic Toxicology 207, 29-33; the article is open access, so everyone is free to read it). Reading all the work nowadays with the outset that plasticity is integral to environmental responses convinces me more and more that plasticity of the responses of animals and individual variability are very much overlooked features in how animals can tolerate environmental changes. Just recently, scientists from Israel (Segev et al 2019. Phenotypic plasticity and local adaptations to dissolved oxygen in larvae fire salamander (Salamandra infraimmaculata). Oecologia 190, 737-746) exposed salamander larvae from stream and pond environment to normoxia and hypoxia. The hypothesis was naturally that, since stream larvae never get exposed to hypoxia whereas pond larvae do, pond larvae would respond more to hypoxic conditions than stream larvae. A simple measure of responding, a change in gill size was taken. However, the results show that regardless of the origin of the larvae, the gill size change between normoxia and hypoxia was the same. This is possible only if the animals can respond plastically to oxygen level. If plasticity is great, and is maintained, genetic changes are not required for environmental responses. In fact, genetic changes are a worse alternative than large plasticity within a genotype, since variation achieved by genetic heterogeneity will decrease as soon as conditions are such that some genotypes of the population will not tolerate them. That will not happen, if the variance is a property of one plastic genotype.

Hydrogen sulphide - both toxicant and a cell signalling molecule

Perjantai 13.10.2017 klo 11.53 - Mikko Nikinmaa

With increased eutrophication many aquatic bodies have anoxic bottom sediments. They are characterized by high concentration of hydrogen sulphide, smelling of rotten eggs. Hydrogen sulphide is considered to be a highly toxic substance, and together with the lack of oxygen contributing to the death of organisms in the hypoxic areas. This may be so, but the initial effects disturbing the functions of organisms may not be caused by the traditionally considered effect of sulphide disturbing aerobic respiration.

It has recently become clear that hydrogen sulphide is an important cellular signalling molecule, in various cases the "oxygen sensor" of the cells. Thus, variations in its cellular concentrations fine-tune oxygen-dependent effects tp occur appropriately. Consequently, any disturbances in the level of hydrogen sulphide can disturb cellular signalling, and harmful effects can take place because of disturbances of cell signalling even when the concentration would not be adequate to cause breakdown of aerobic respiration and death because of that.