The Chlorophyta are very diverse aquatic plants with over 8000 species ranging from fresh to marine conditions. However, about 90% are fresh water. They contain chlorophyll a and b. They store starch as a food reserve inside organelles called plastids. The classification is confusing and not agreed upon by researchers. Most green algae have firm cell walls. Some contain flagella while others have calcified shells.
Heterotrophic representatives would include protozoa and small animals like rotifers and gastrotriches. Some arthropods can also live their entire life in the water column including water fleas, ostrocods and copepods. Heterotrophic animals can ingest their food (i.e., paramecium) or nutrients can be adsorbed through their root or body system (i.e., fungi). There is also the benthic or bottom living environment. These organisms spend most of their time in the slimy world of mud. Also Included in this section are organisms that live in the water column as plankton, but can live in the benthic (bottom) environment.
Blue-green algae or cyanobateria are common in the holoplankton . Some are harmful in that they may add to the pollution of lakes and rivers by their rapid growth, but most are benign. Cyanobacteria are one of the most primitive organisms found in the fossil record, making massive mounds 600 million years ago called stromatolites.
The Protista are single-celled organisms that have a true nucleus (eukaryotic). Protista may be either autotrophic or heterotrophic. Movement by protists is dependent upon certain physical characteristics. Some protozoa have pseudopodia which can extend the cell membrane and push forward or surround a food particle, such as an amoeba does. A protist that possesses a single tail-like structure is called a flagellate. The flagellum will beat back and forth and propel the organism through the water. Some Protozoa are covered with tiny hair-like structures called cilia which move back and forth quickly propelling the organisms through the water. A paramecium is an example of a ciliate. Some Protozoa have axopodia, or pencil-like structures, that help them to be planktonic or floaters in the water. A heliozoan is an example of protozoa with axopodia.
There are many debates about whether protozoa are all one-celled organisms or whether they are all one-celled organisms that are heterotrophs. Scientists, who study these groups, debate on how to classify some of these organisms, like euglena and dinoflagellate. With more study these groups will be better understood.