Adaptations to Salinity

Salt affects organisms through alteration of the water balance of cells and through salt toxicity. Since osmotic pressure pushes water toward tissues with higher solute concentrations, and seawater is often more concentrated than the cells of organisms, water tends to flow out of cells in aquatic organisms in the presence of seawater. Similarly, water uptake into organisms and cells is impaired when in saline solutions, resulting in water limitation similar to what occurs in arid systems. Salt toxicity results when salt affects enzymatic activities and energy processes, such as reductions in photosynthesis and respiration. Organisms living in saline environments have two general adaptive strategies - tolerating and avoiding salt stress. Organisms adapted to live in saline environments are called halophiles; in particular, such plants are called halophytes (Figure 1).

Stress tolerance is generally achieved through metabolic or physiological adaptations. Both plants and animals have a diversity of adaptations to various levels of salinity in the environment, as either hyper- or hypo-salinity can cause physiological stress. Osmoconformers are organisms that keep their internal fluids isotonic to their environment, that is, they maintain an internal salinity similar to their ambient conditions (e.g., most marine invertebrates, seagrass). Osmoregulators, conversely, maintain a constant osmotic pressure within their bodies by balancing water uptake and loss through the controlled movement of solutes across membranes between internal fluids and the external environment (e.g., most aquatic vertebrates, some marine invertebrates such as fiddler crabs). Organisms may have selective cellular uptake of particular salts, for example, preventing sodium but allowing potassium uptake (salt grass, cordgrass). Plants

Figure 1 Halophytic plants, such as the pickleweed (Salicornia bigelovii) depicted here, have adaptations to allow them to deal with the excess salts in their environment. Pickleweed actively transports excess salt to the tips of the terminal jointed leaves, which are then shed. Photo credit: D. Talley.

Figure 1 Halophytic plants, such as the pickleweed (Salicornia bigelovii) depicted here, have adaptations to allow them to deal with the excess salts in their environment. Pickleweed actively transports excess salt to the tips of the terminal jointed leaves, which are then shed. Photo credit: D. Talley.

Figure 2 This species of side-blotched lizard, Uta tumidarostra, lives on islands in the Gulf of California where in situ terrestrial production is quite low, and thus feeds extensively on intertidal invertebrates. This species has evolved large nasal salt glands that allow it to excrete the excess salts consumed with its prey. Reproduced by permission of L. Grismer.

Figure 2 This species of side-blotched lizard, Uta tumidarostra, lives on islands in the Gulf of California where in situ terrestrial production is quite low, and thus feeds extensively on intertidal invertebrates. This species has evolved large nasal salt glands that allow it to excrete the excess salts consumed with its prey. Reproduced by permission of L. Grismer.

and animals may take saline water into their tissues but then accumulate organic compounds to increase cell osmotic potential to prevent cellular explosion (e.g., cordgrass). Other organisms have glands through which salt is excreted (e.g., gulls, some salt marsh plants), tissues through which they take up salts to maintain osmotic balance (e.g., freshwater fishes), or move salt to cells which are eventually lost, such as the specialized hairs or leaf tips of halophytes (e.g., salt marsh plants such as pickleweed and black grass) (Figure 2). Succulent plants dilute salts by taking up more water, but still have to regulate salts by sequestering them in cell vacuoles, isolated from the cytoplasm and organelles of the cells.

Stress avoidance includes regulation or direct avoidance of salt, either through structural adaptations or behavioral responses. In halophytes, for example, structural adaptations may include specialized root cells that may filter out salt and result in the sap consisting of pure water. Other organisms have behavioral adaptations, such as timing of reproduction, emergence, or dispersal to avoid undesirable conditions (e.g., insects, crab larvae). Mobile organisms, such as fish or crustaceans, may be able to avoid hypo- or hypersaline conditions by moving out of an area.

All of these adaptations come at a cost - the energy used to perform these functions is thus unavailable for other physiological demands, such as growth and reproduction. Therefore, organisms dealing with salt stress, like other physical stresses, are usually faced with a tradeoff between coping with salt and facing competition or predation in less saline areas. Most vascular plants, for example, are salt tolerant but would perform better in fresher conditions ifnot for competition with taller brackish and freshwater plants.

Many of these factors broadly apply to terrestrial organisms as well. Here, osmoregulation is often handled through specialized organs (e.g., kidneys, Malpighian tubules), and a common physiological stress is a lack of salt, as opposed to an overabundance. Nonetheless, similar issues of adaptation and tolerance for high or low salinity environments apply.

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