microphyte

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun Any of various minute plants or other photosynthetic organisms, such as cyanobacteria and diatoms.

from The Century Dictionary.

• noun A microscopic plant, especially one that is parasitic in its habits.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Bot.) A very minute plant, one of certain unicellular algæ, such as the germs of various infectious diseases are believed to be.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun biology Any microscopic plant or alga