by Janet Grischy

Created on: December 06, 2008   Last Updated: January 25, 2010

Algae are not quite plants. They use photosynthesis to make food from sunlight, water, and carbon dioxide, the way plants do, and they use the same organelle to do it, the chloroplast. However, algae are simpler than plants.

They are mostly single celled, though they may also grow in filamentous colonies like the spirogyra or as tall seaweeds like kelp. Even algae that are not unicellular still do not have differentiated cells the way plants do; they are mostly colonies of similar cells. On the other hand, algae do have a nucleus and a chloroplast enclosed inside separate membranes as plants do.

The photosynthesizing chloroplast is possibly the remains of an invader. It has the circular DNA of cyanobacteria, different from the strands of DNA in the nucleus of algae. Most scientists think cyanobacteria once lived inside the algae, in a mutually beneficial arrangement called endosymbiosis. Eventually, the functions of the bacteria were reduced and the two evolved into one organism.

One of the most common groups of algae is the diatoms. There are 200 or so genera of diatoms, and about 100,000 species. They are found worldwide, wherever there is sufficient moisture. As part of the assortment that is plankton, they live in and on the oceans, pelagic, floating on or near the sunlit surface. They create about 45% of the primary production, the food that is produced through photosynthesis, in the sea. Almost all diatoms are one-celled, although they can exist as colonies shaped like filaments, fans, or stars. Most, though not all, are microscopic.

They wear a hard coating made of glassy silicate. This shell, the often beautifully complex frustule, is split in two asymmetrically. When they reproduce they divide. Each new organism gets half of the shell, and grows a new, slightly smaller second shell to match it. This makes succeeding generations smaller and smaller.

Eventually, the smallest generation in some kinds of diatom reproduces sexually, becoming large again. This arrangement gives the diatom the advantages of both forms of reproduction. When it reproduces by fission, asexually, it increases its numbers rapidly, often doubling in a day. When it reproduces sexually, it introduces genetic variation that helps it evolve to meet changing conditions, although this kind of reproduction is more expensive in terms of energy and time.

Diatoms keep their chloroplasts inside several sets of membranes. Within the membranes are DNA, some proteins, and stacks of membrane

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