Diatoms and sponge spicules. Coloured scanning electron micrograph (SEM) of a circular arrangement of various diatoms and sponge spicules. Diatoms are planktonic unicellular algae. They have a mineralised cell wall (frustule) divided into two halves. The frustule contains silica and provides protection and support. Sponges are primitive aquatic organisms that filter food from the water. They are a loose aggregation of cells supported by spicules and needles composed of silica