That shrinking is sometimes offset by the creation of auxospores, which can lie dormant to survive periods of unfavorable conditions.
The bodies of these organisms, when embedded in sediments, can be used to monitor the change in salinity concentration of a body of water over time. These organisms deposit new cell wall material inside their existing walls, and, as a result, gradually decrease in size after successive mitotic divisions. That shrinking is sometimes offset by the creation of auxospores, which can lie dormant to survive periods of unfavorable conditions. The bipartite walls of this organism each have an end-piece called a valve, and are joined at the cingulum. Those found in freshwater are usually the pennate type, contrasted with the centric type, and these organisms contain yellow-brown chloroplasts. For 10 points, name this group of algae whose silica-containing cell wall is called a frustule, the most abundant phytoplankton.
The bodies of these organisms, when embedded in sediments, can be used to monitor the change in salinity concentration of a body of water over time.
{diatoms} [prompt on {algae} or {phytoplankton} before mention]