7.2 Stress Tolerance Overview

As previously discussed, many environments can be stressful. Indeed, many environments are inherently variable, constantly challenging the homeostasis of organisms and their cells. For example, temperature fluctuates on both a daily and seasonal basis. Water availability in terrestrial (land) environments also changes over time with alternation between rainy and dry days and seasons. Estuaries (where rivers meet oceans) exhibit daily fluctuations in salinity as the tide comes in and out. Some environments are not particularly variable, but always challenging – for example ocean water near the poles is always cold. Because most environments are stressful in some way, most organisms need some kind of stress tolerance.

Definitions of Stress Tolerance

Just as there are different ways to think about stress, there are different ways to think about stress tolerance. In its simplest form, stress tolerance is the ability to recover normal function following exposure to a stressor. We can think about this in the same context as in Chapter 7.1, in which a stress-tolerant organism is able to recover homeostasis during or after exposure to a stressor via an effective stress response (Figure 7.5A). However, there are other ways an organism can tolerate stressful conditions, especially when we consider that stress can cause damage to cells – a more extreme situation than temporary deviations from homeostasis. If an organism is damaged by exposure to a stressor, that organism could exhibit stress tolerance by being able to repair that damage and eventually restore homeostasis (Figure 7.5B). Alternatively, an organism might be stress-tolerant by pre-emptively decreasing/preventing deviations from homeostasis and damage (Figure 7.5C) or by altering its homeostatic state. For example, mammals and birds that hibernate can survive winter partly because they intentionally drop their body temperature during hibernation (alter their homeostatic state), along with multiple other adjustments that prevent damage from occurring while they are cold.

Variation in Stress Tolerance Among Species

While many organisms exhibit some level of stress tolerance, that stress tolerance varies among species. In general, the extent to which a particular species can tolerate stress depends on the environments they are adapted to. Organisms that are adapted to living in freshwater (e.g., lakes and ponds) don’t experience high concentrations of salt often, and are unlikely to have the appropriate adaptations to tolerate high concentrations of salt. Organisms that live in marine environments (oceans, seas, 3% salt) are better adapted to tolerate high concentrations of salt, and organisms that live in hypersaline lakes (e.g., Lake Tyrrell in Australia, 20% salt) are adapted to tolerate very high concentrations of salt.

Some organisms are known as extremophiles because they tolerate extreme stress. Many of the most impressive extremophiles are single-celled organisms, such Archaea, but there are eukaryotic examples as well. For example, some plants – like the moss pictured in Figure 7.6 – can survive months or years after losing up to 95% of their body water, tolerating extreme dehydration stress. That amount of dehydration would kill most other organisms (because our cells need water!), so most other organisms are less tolerant of dehydration stress than resurrection plants. Tardigrades (also called water bears) are an animal example of extreme stress tolerance – they can also survive intense dehydration, freezing, and the vacuum of space! The single-celled alga Dunaliella salina lives in hypersaline lakes and can tolerate very high concentrations of salt. These extremophiles (and others) provide a fascinating window into how cells can survive under challenging conditions, teaching us about the limits of life itself.

Variation in Stress Tolerance Over Time

In addition to varying among species, stress tolerance can also vary within a single organism over time. For example, some life history stages are more stress-tolerant than others. Many plant seeds are much better at surviving dehydration and low temperatures than fully-grown plants, especially seeds that must survive long cold winters. Similar patterns can be seen in animals. As an example, the spruce budworm (an insect that eats spruce trees) normally overwinters as a larva (similar to a caterpillar). These larvae are much better at surviving low temperatures than other stages (e.g., eggs, pupae, adults) because those other life stages don’t normally experience extreme low temperatures.

In some species, this variation in stress tolerance over time is genetically programmed into the organism, e.g., based on the life stage. In many species, variation in stress tolerance is also based on the environmental conditions that an organism experiences. For example, as temperatures drop during fall (autumn), lots of organisms adjust their biology to increase stress tolerance in advance of winter. When this happens in natural environmental conditions (e.g., a pond; Figure 7.7A), we call it acclimatization. Researchers often want to study these changes in laboratory settings as well. When organisms adjust their biology under certain conditions in controlled environments (e.g., in an aquarium; Figure 7.7B), we call it acclimation. To understand the difference between acclimation and acclimatization, let’s talk about crickets. The spring field cricket (Gryllus veletis) lives in southern Canada and cannot survive freezing (ice within its body) during the summer, but during fall they become freeze-tolerant through acclimatization. We can also mimic fall conditions in the lab, for example by placing the crickets in an incubator that slowly decreases the temperature and amount of day light over a period of several weeks. Crickets that become freeze-tolerant in this incubator do so via acclimation. The acclimation process does not exactly replicate natural conditions (e.g., changes in humidity, presence of natural food), but still causes similar biological changes that enhance stress tolerance.