Algae differ from organisms such as bacteria, fungi and protozoa in having a chemical economy based upon photosynthesis, in which the accumulation rather than the breakdown of organic matter predominates. In bulk of material involved this algal type of metabolism perhaps exceeds any other. The total yield of carbohydrate synthesised in the oceans, in which algae are the only photosynthetic organisms, has been estimated to be from 1.6 to 15.5 x lO10 tons of carbon fixed per year and is evidently at least as much as that from land plants. All animals in the sea must derive their food from this plant synthesis, these algae thus support the fisheries upon which man relies for food. In soil and in freshwaters algal metabolism is generally on a lesser scale, but is nevertheless of considerable importance. Algae contribute to the fertility of soil by nitrogen fixation and as a source of trace elements.
The metabolic processes which characterise the algae may be similar to land plants in the initial act of photosynthesis and in many cases similar enzyme systems may be present, however from a study of the structure of the main components there are also metabolic activities peculiar to the algae.
Carbohydrate extracts of algae have many present day uses in the drugs, cosmetics, pharmaceuticals, textile, food and fishing industries. Seaweeds have been used as a source of food since ancient times and nowadays are valuable as a source of minerals, trace elements and vitamins for animal feeding stuffs. The possibility of the growth of unicellular algae in culture as a source of human food on prolonged space flights in the solar system is being investigated.
The algae may be classified on morphological grounds. The photosynthetic pigments often afford the simplest and most certain means of recognising representatives of different classes.