Diatoms such as this specimen of Coscinodiscus are photosynthetic. You are viewing an individual single-celled specimen. The yellowish-brown spots in the specimen are chloroplasts. In this photo, the opal shell is not highlighted.
Although round or centric diatoms such as this one are common, diatoms exhibit a wide range of shapes. View a variety of specimens to get a feel for the look of diatoms.
Photograph by Wim van Egmond.
For more information about diatoms, check out:
Big Diatoms: Photographing Coscinodiscus