Life within life

The organisms and environments so far described in this chapter are largely free living. Many organisms, however, are not free living but live within, or in close association with, another organism. Such an association is called a symbiosis (which means 'living together'). The term 'symbiosis' does not imply any harm or benefit to either of the partners in the association. Where both partners gain some benefit from the association, it is called 'mutualism', and 'parasitism' where one partner lives at the expense of the other. The larger organism in a symbiotic association is called the 'host' and the smaller the 'parasite' or 'symbiont'. Being parasitised is not an unusual situation. Most animals contain parasites, usually of several types. Most groups of organisms have representatives which are parasites or symbionts. Some groups of microorganisms consist exclusively of parasites (for example, viruses), as do some phyla of animals (for example, the spiny-headed worms or Acanthocephala). There may well be more parasitic than free-living animals on Earth.

The environment of many parasitic animals consists of the inside of another animal. This means they are faced with some unusual challenges compared with a free-living species. The intestine is the most common site for parasitism, so let us look at the features of the inside of the intestine of a human as a place to live. It is dark and the intestine undergoes constant muscular movement to keep the food stirred, to aid digestion and to assist its passage through the gut. This, and the flow of food, means that a parasite is in constant danger of losing its footing and being swept out of the body. Many parasites have various types of attachment organs, consisting of a variety of suckers, clamps and hooks, which enable them to hang on to the wall of the intestine and to maintain their position. The inside of the intestine is low in oxygen and parasites, at least in some parts of the gut, have to cope with anoxic conditions. The stomach is very acidic, due to the secretion of hydrochloric acid which helps break up the food. Intestinal parasites have to survive passage through the acid conditions of the stomach and some even live there. The host is continually secreting enzymes into the intestine, to digest the food, which the parasite has to neutralise in order to prevent them from dissolving its own tissues. On the plus side, the parasite is protected from the external environment in relatively constant conditions and is provided with a ready supply of food by its host.

A mutualistic symbiosis operates to the benefit of both partners. We have already met some of these. The tube worms living around deep-sea hydrothermal vents have a mutualistic association with chemo-trophic bacteria. The tube worms harbour the bacteria in their tissues and supply them with the oxygen and sulphides they need via the blood. The bacteria are able to live in a favourable position close to the vent since they are within the body of the tube worm which is anchored to the seafloor. This prevents them from being swept away. In return, the tube worms receive the products of the bacteria's metabolism as food, without which they would be unable to live in the hydrothermal vent habitat. You could say that the tube worm has acquired the ability to utilise a novel food source (the sulphides from the vents) by forming a mutualistic relationship with the bacterium. Angela Douglas of the University of York suggests that we might even regard mutualistic symbiosis as an alternative mechanism of evolution. Rather than acquiring a new ability through natural selection operating on random mutations, the tube worm has acquired the ability to utilise sulphides as a food source by forming an association with a chemotrophic bacterium. There are other examples. Many invertebrate animals contain algae. They have acquired the ability to harvest the energy in sunlight by forming a mutualistic association with a photosynthetic organism. Animals do not have the ability to digest cellulose, the material which makes up the cell wall of plants, on their own. A cow can feed on grass because there are bacteria and protozoa within some of the chambers of its stomach which produce cellulase, the enzyme necessary to digest cellulose. Many plant-eating animals have acquired the ability to digest cellulose via a mutualistic association with microorganisms. It is now widely accepted that animals and plants themselves developed via mutualistic associations between microorganisms. Mitochondria (the energy-producing structures of plant and animal cells) developed from bacterial symbionts and chloroplasts (the sites of photosynthesis in plant cells) developed from a mutualistic association with a photo-synthetic microorganism.

the features of extreme environments In this chapter, we have seen that there are a wide variety of extreme environments on Earth. They are considered to be extreme for a variety of reasons, including high or low temperatures, lack of water, high or low pH, high pressures, high exposure to radiation (particularly ultraviolet), high salt concentrations, exposure to toxins and low nutrient availability. These stresses rarely act on their own and organisms are exposed to a combination of stresses (Table 2.1). We can divide them into two broad groups: terrestrial and aquatic (including salt lakes, soda lakes and hot springs). Water availability is a major factor in terrestrial habitats, but not, of course, in aquatic habitats. The great bulk of water in most aquatic habitats also has a buffering effect and so changes in

Table 2.1 Features of extreme environments

Habitat

Temperature

Water Pressure Oxygen

PH

Toxins Nutrients

Salts Ultraviolet radiation

Hot deserts

J

^ 1 1 1

1

1 1

J

Cold deserts

n

^ 1 1 1

1

1 1

J

Temporary deserts

n

^ 1 1 1

1

1 1

1

Salt lakes

i

1111

J

J J

}

Soda lakes

i

1 1 1 J

J

J J

Polar regions

^ 1 1 1

1

1 M

J

Mountains

n

^ 1 ^ 1

1

1 1

J

Temperate winter

^ 1 1 1

1

1 1

1

Deep sea

i

1 J 11

1

I 1

I

Hydrothermal vents

j

1 J 1 }

J

J J

I

Cold seeps

i

1 J 11

J

J 1

Deep subsurface

j

1 J I" J

} J

} J

I

Hot springs

j

1111

J

J J

1

Parasitic

i

1 1 }

}

1 J

J is high, i' is low and 1 is normal temperature, and other conditions, are likely to be slow. We might say that, overall, extreme terrestrial habitats are more extreme than extreme aquatic habitats.

Some extreme environments are constantly extreme. This favours organisms with capacity adaptations which can grow and reproduce under the extreme conditions. Examples include: the deep sea (high pressure), polar oceans (freezing temperatures), and hydrothermal vents and hot springs (high temperatures). Environments where extreme conditions are temporary and there are periods of less extreme conditions when growth and reproduction can occur may tend to favour resistance adaptation, at least in some organisms. Examples include: deserts, polar regions and mountains. Again, this reveals a difference between aquatic and terrestrial environments.

The following chapters will look at how some of the organisms which live in extreme environments survive the stresses to which they are exposed.

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