This is possible because they contain chlorophyll . Diatoms are thus a major food resource for marine and freshwater microorganisms and animal larvae, and are a major source of atmospheric oxygen. Diatoms are a major component of plankton , free-floating microorganisms of marine or freshwater environments. Whales may carry dense growths of diatoms on their skin. This particular diatom occurs as individual cells, while others, like the Melosira shown below at right, may grow together in long colonial chains.
Diatoms only construct new walls during cell division.
After the cell divides, the epitheca and hypotheca separate, and new valves are laid down between them. Because the frustule cannot grow once it has been laid down, the mean size of a dividing population of diatoms gets smaller and smaller with time. Fortunately, diatoms can also reproduce sexually, producing offspring that secrete a wholly new cell wall from scratch. For more photographs of diatoms, visit the Algal Microscopy and Image Digitization server at Bowling Green State University. The Diatoms .