Algae are a diverse group of photosynthetic eukaryotes in the kingdom Protoctista. Rhodomonas salina and Skeletonema costatum are two species of alga that live in seawater. Fig. 1.1 shows a scanning electron micrograph of R. salina, which is a unicellular organism. R. salina is red in colour. [Figure 1.1] Fig. 1.2 shows a scanning electron micrograph of S. costatum, which is also a unicellular organism. The individual cells can group together in a chain. S. costatum is yellow-brown in colour. [Figure 1.2]