Diatoms are photosynthetic algae, and like the cyanobacteria and brown seaweeds they are classified as Chromista rather than plants. The cell wall is made of silicon which forms a highly ornamented and symmetrical exoskeleton. Morphologically they are so diverse that the shape is species specific and used as the basis of the taxonomic classification.
Found in marine and freshwater habitats they are one of the most common forms of phytoplankton. Some diatoms exist as free floating cells and the planktonic species often have morphological adaptations that allow them to remain suspended in the water. Others are attached to surfaces by stalks or mucilage pads.
There are approximately 2800 species of freshwater diatoms recorded in the British Isles but in the Outer Hebrides little is known about which species are present or their distribution.
All images are by Chris Johnson, unless annotated otherwise.