Re: What is the difference between the plankton and diatoms?

Aimee,

Let's start at the top and work our way down to more and more refined 
classification.  First the plankton are a group of organisms subject to the 
movement of the water in which they live. While many are capable of 
small-scale movement, their large scale movements and distribution are 
determined by currents, thus they are the "drifters" in the sea (and lakes, 
too). At this level, "plankton" includes ZOOPLANKTON, or animal drifters - 
non-photosynthetic, heterotrophic organisms (such as copepods, and many 
larval forms of sessile animals, and jellyfish), BACTERIOPLANKTON, bacteria 
(including both photosynthetic cyanobacteria and non-photosynthetic, or 
heterotrophic, bacteria), and the PHYTOPLANKTON, which refers primarily to 
microalgae and cyanobacteria (also called blue-green algae) but this is a 
slightly misleading label. "Phytos" means plant in latin, but most 
phytoplankton are not true plants. The presence of true plants in the 
plankton is limited. The plankton also includes members of the fungi and 
viruses.

Separating zooplankton and phytoplankton seems to be the fairly obvious 
functional difference between "animals" and "plants", but read on.

The subset of plankton known as phytoplankton includes diatoms, 
dinoflagellates, coccolithophorids, and cyanobacteria. At least these are 
the major components of the phytoplankton of the sea. The cyanobacteria are 
true prokaryotic bacteria, sometimes included in bacterioplankton, 
sometimes in phytoplankton because they photosynthesize. The 
dinoflagellates, coccolithophorids and diatoms are considered Protists, not 
true plants despite similarities in how they perform photosynthesis. The 
Protista, in the 5 Kingdom classification scheme, are not true plants 
(Kingdom Planta) due to their simple life histories and lack of specialized 
body tissues. True plants include some single celled organisms such as 
Chlorella, but for the most part include those organisms we generally think 
of as plants from seaweeds to trees. Their representation in the plankton 
is limited to planktonic sexual stages and a small number of special cases 
such as the floating seaweed, Sargassum. 

However, things can get complicated here. Some species of dinoflagellates, 
for example, are not photosynthetic. They do not produce their food from 
inorganic substrates but rather eat (usually bacteria and other 
phytoplankton) and some do both depending on what is available. Here the 
distinction between 'phyto'plankton and 'zoo'plankton can get blurred.

So dinoflagellates complicate the distinction between phyto- and 
zoo-plankton. Remember, though, that the distinction is a functional one. 
Phytoplankton photosynthesize. So some dinoflagellates are zoo- and some 
are phytoplankton. The 'switch-hitters' are both.

This confusion is not the case for diatoms. Diatoms (Bacillariophyceae) are 
obligate autotrophs. They require sunlight for growth. There are some that 
can absorb organic compounds to supplement their nutrition, but they cannot 
live only that way. Diatoms are a diverse group of photosynthetic 
unicellular (although some form chains) organisms characterized by their 
siliceous (glass) "shells". These shells, called frustules, are the main 
determinant of classification. The diatoms are divided into two 
subgroups, based on the symmetry of their frustules. If the 
frustule is radially symmetrical, they are part of the "Centric" group 
(Centrales) while those that are bilaterally symmetrical belong in the 
"Pennate" group (Pennales). Frustules come in a variety of shapes and can 
be highly ornamented. Since they persist after death, they are also the 
subject of paleoclimate research as frustules from sediments can be 
identified. Knowing who lived in the waters above the sediment can give 
clues to past environmental conditions based on the tolerance range of the 
species that are found. They are classified in the classical Linnean method 
of morphological characters (size, shape, etc.). But more and more research 
is being done using molecular biology to examine the genetic relationships. 
Sometimes two speices that look quite alike are not closely related 
genetically. This has caused a minor revolution within the science of 
classification of organisms (taxonomy)

I am including Dr. R. Sweets' Diatom HomePage address where you can find 
many links, images and other information about this interesting group of 
organisms.
http://www.indiana.edu/~diatom/diatom.html

Good Luck!