The bacteria and fungi that live in the sea cover the full range of morphology, life history and physiology that is found in these organisms. All four of the major classes of fungi are represented in the sea (see Table 1.1), and marine bacteria include photosynthetic and chemosynthetic species, as well as aerobic and anaerobic heterotrophs. Thus, there is little, apart from their ecology, to distinguish marine microorganisms from those of terrestrial and freshwater habitats. This contrasts with the situation among flowering plants (see Chapter 1) and suggests that life in the sea is not fundamentally different from life on land for bacteria and fungi, and that the transition from sea to land — and back again — may have occurred many times during the evolution of these groups. Only about 1% of the known species of fungi are found in the sea, but this is not thought to mean that the marine environment is physically unfavourable for fungal growth. It is usually attributed to the restricted amount and variety of organic substrates available in the sea, and to the low selective pressure exerted by the relatively constant environment. The same arguments probably apply to the bacteria but the greater difficulty of defining a species in this group makes it impossible to obtain a reliable estimate of the numbers of bacterial species in either marine or terrestrial habitats.