Algae frequently get a bad press. Pond slime is a problem in garden pools, algal blooms can produce toxins that incapacitate or kill animals and humans and even the term seaweed is pejorative - a weed being a plant growing in what humans consider to be the wrong place. Positive aspects of algae are generally less newsworthy - they are the basis of marine food webs, supporting fisheries and charismatic marine megafauna from albatrosses to whales, as well as consuming carbon dioxide and producing oxygen. Here we consider what algae are, their diversity in terms of evolutionary origin, size, shape and life cycles, and their role in the natural environment and in human affairs.

Division of Plant Science University of Dundee at the James Hutton Institute Invergowie Dundee DD2 5DA UK

Laboratorio di Fisiologia delle Alghe e delle Piante Dipartimento di Scienze della Vita e dell'Ambiente Università Politecnica delle Marche via Brecce Bianche 60131 Ancona Italy; Institute of Microbiology ASCR Algatech Trebon 37981 Czech Republic

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