Ecology: biology of interactions. 5.16. Water balance of organisms

English (latest version) / Russian (update discontinued) 5.15. Greenhouse Effect D. Shabanov, M. Kravchenko. Ecology: the biology of interaction Chapter 5. Autecology and fundamentals of environmental science 5.17. Adaptive Biorhythms 5.16. Water Balance of Organisms In the most general sense, a living organism can be described as an aqueous solution enclosed in a membrane—the body surface. Knut Schmidt-Nielsen Regardless of the external environment in which an organism finds itself, the concentration of aqueous solutions in its internal milieu is vital. To preserve life, this concentration must be kept constant within fairly narrow limits. The organism’s membranes are permeable both to water and to certain dissolved substances. The concentrations of solutions in the internal and external media virtually never coincide. Thus, regulation of water balance is one of the most important conditions for organismal survival. Of course, the specifics of this balance differ among organisms, for example those coping with heat and dryness in a wind‑eroded desert or maintaining the necessary salt concentration in the meltwater of a mountain stream. Processes influencing the water balance of terrestrial animals are shown in Table 5.16.1. Table 5.16.1. Components of the water balance of terrestrial animals Water loss Water acquisition Evaporation from body surfaces Evaporation from respiratory organs Evaporation of sweat and saliva Excretion with urine Excretion with feces Other forms of excretion (e.g., with milk) Drinking Absorption through the body surface Intake with food Metabolic water production All animals can tolerate certain deviations from the normal body water content (usually about 10%). To avoid life‑threatening deviations, many must develop appropriate adaptations. Several examples: — Frogs possess special tubular glands on their ventral side that serve not for excretion but for absorbing water from moist soil; — The substances through which nitrogenous waste products are eliminated differ among animal groups (Fig. 5.16.1); those that must conserve water the most excrete nitrogen as uric acid; — The skin of some arboreal caecilian amphibians (Gymnophiona) is covered with a waxy layer that prevents water loss, and these amphibians excrete nitrogen not as urea solution but as a thick paste of uric acid; — As snake and lizard eggs develop, their mass increases because they acquire additional water from the environment needed for development; — Some insects can absorb water directly from humid air through their tracheae; — If shipwreck survivors drink seawater, they only exacerbate dehydration, because the salt concentration in seawater exceeds that in urine; marine turtles, unlike humans, can drink seawater thanks to salt glands located in the corners of their eyes that secrete a concentrated saline solution; albatrosses and other tube‑nosed birds perform a similar function with tubular glands at the base of the beak; — The high milk production of cetaceans and pinnipeds, which can float on the water surface like a soufflé, reduces water loss by the nursing mother. [IMG_1] Fig. 5.16.1. Decomposition products of major food‑component groups and three principal nitrogen‑excretion pathways Seawater contains about 3.5 % dissolved salts, of which roughly 90 % is the well‑known sodium chloride—common salt. Overall, seawater composition is relatively constant, although it depends on many conditions. For example, the salinity of the Mediterranean Sea exceeds the norm (up to 4 %) because water evaporation is not balanced by river inflow. Surface‑water salinity in the Black Sea is considerably lower than the norm (1.8 %), and the Azov Sea’s is even lower, ranging from 1 to 1.3 %. This reduction is due to dilution of seawater by a strong influx of fresh water. Fresh water is compositionally far more diverse than seawater. Even rainwater contains a small amount of salts; its source is salty dust formed from sea‑spray droplets over the ocean. After precipitation reaches land, its composition changes. If it runs over granite substrate, it acquires virtually no dissolved substances and remains soft (low in calcium and magnesium salts). If it percolates through limestone, many salts (primarily calcium and magnesium) dissolve, and the water becomes hard. Mixing with seawater renders fresh water brackish. This is especially characteristic of estuaries—the zones where rivers discharge into seas and their waters mix. Why is seawater salinity so important for organisms? Most of their coverings are semipermeable: they allow some substances to pass while retaining others. Typically, body coverings (and cellular membranes) permit water and dissolved gases to pass but retain ions and relatively large dissolved‑molecule solutes. Examine Fig. 5.16.2, noting not only the concentration of dissolved substances but also the “concentration” of water itself in the solution. The “concentration” of water on the right side of the membrane is lower. Consequently, by ordinary diffusion, water molecules will move through the semipermeable membrane from left to right! [IMG_2] Fig. 5.16.2. Where solutions of differing concentrations contact each other across a semipermeable membrane, osmosis arises—diffusion of water toward the more concentrated solution The movement of water (or another solvent) molecules from a zone of low solute concentration to a zone of higher solute concentration across a semipermeable membrane is called osmosis. In the illustrated case, when will osmosis cease? Either when solute concentrations on both sides of the membrane equalize, or when pressure in the more concentrated solution rises enough to impede water movement across the membrane. The pressure required to stop osmosis is termed osmotic pressure. Naturally, it depends on the difference in solute concentrations between the two solutions. By comparing a solution with pure water, we can determine that solution’s osmotic pressure—the higher the amount of dissolved substances, the higher the pressure. Moreover, different solutes (e.g., depending on their degree of dissociation into ions and how those ions interact with water molecules) affect osmotic pressure to varying extents. A crucial consequence for living organisms derived from these principles is that the direction of osmosis between them and the external environment depends on the ratio of their osmotic pressures. As with many other environmental factors, a mismatch between the internal and external osmotic pressures can be either overcome or endured, leading to internal‑environment changes via osmosis. Organisms that allow the salinity of their internal milieu to vary are called osmoconformers. Examples include cnidarians, polychaete worms, many mollusks, echinoderms, and other aquatic animals. Osmoconformers differ in the range of internal‑environment concentration changes they can tolerate. For instance, oysters withstand far greater salinity fluctuations than sea stars. Those that actively maintain a constant internal salinity are called osmoregulators. Bony fishes belong to the osmoregulators. Consider the challenges they may face (Fig. 5.16.3). [IMG_3] Fig. 5.16.3. For bony fishes, life in marine and freshwater environments requires completely different physiological processes to maintain water balance Marine fishes are hypo‑osmotic: their internal salt concentration is lower than that of the surrounding water. Osmotic processes draw water out of their bodies, dehydrating them. Can fishes block this process? The skin of vertebrates can be nearly impermeable to water, but impermeable coverings would impede gas exchange. At least in the gills, the internal milieu must contact the external environment through a thin semipermeable membrane (permeable not only to gases but also to water!). Freshwater fishes face the opposite situation. Their bodies are hyper‑osmotic and “suck” water from the surrounding environment. How do fishes solve these problems? Marine bony fishes must drink seawater and excrete the excess salts via feces and across the gill surface. Freshwater fishes excrete copious dilute urine, and the resulting salt deficit is compensated by uptake across the gill surface. Now imagine the complex regulatory tasks confronting salmonid fishes migrating from the sea to rivers for spawning, or estuarine inhabitants whose habitat salinity may change depending, for example, on wind direction! Cartilaginous fishes have eased the task of maintaining a constant internal composition by accumulating urea, thereby raising their internal osmotic pressure. Salt concentration in shark blood is roughly the same as in bony fishes, but urea levels are about 100 times higher than in our blood! Two consequences follow. First, people who consume shark and ray meat may notice a slight urine‑like odor and may need to remove excess urea (e.g., by soaking in fresh water). Second, cartilaginous fishes, because of difficulties in osmoregulation, have largely failed to colonize freshwater, unlike bony fishes. The reduced energetic cost of osmoregulation has led this group to lose the ability to inhabit many potential habitats. Only a few shark and ray species freely enter fresh water, and only one truly freshwater species is known: the Amazonian river stingray. Its blood contains no excess urea. Additional materials: Teaching model: Plant water balance 5.15. Greenhouse Effect D. Shabanov, M. Kravchenko. Ecology: the biology of interaction Chapter 5. Autecology and fundamentals of environmental science 5.17. Adaptive Biorhythms