Pinnularia

Habitat

Pinnularia is a predominantly fresh-water alga, usually found in ponds and moist soil. They can also be found in springs, estuaries, sediments, and oceans. Members of this genus are most commonly found in 40 cm (1 ¹⁄₄ ft) of water, at 5 °c (41 °F).

Internal structure

The cytoplasm is arranged approximately in layers conforming to the shape of the cell's walls. A large central vacuole is present with the nucleus suspended in its centre by a transverse cytoplasmic bridge. Two chloroplasts are present along the sides of the cells, and contain chlorophyll a, c, beta-carotene and fucoxanthin pigments. One or two pyrenoids are usually present in each chloroplast, although like many heterokont algae Pinularia tend to store their energy as fat. The cytoplasm also contains chrysolaminarin and some volutin.

Reproduction

Pinnularia like most diatoms, can reproduce by simple cell division. Nuclear division occurs by mitosis and cell divides into two parts. Each daughter receives one of the parent cell's thecae, which becomes that cell's Epitheca. The cell then synthesizes a new hypotheca. Thus one daughter is the same size as the parent, and one is slightly smaller. With subsequent generations the average cell size of a Pinnularia population gradually becomes smaller. When a minimum average size is reached auxospore formation occurs and sexual reproduction restores the population's average cell size.