diatom.org PHYTOPLANKTON Creative Commons License

The phytoplankton is an ecological group of photosynthetic microorganisms adapted to living suspended in the open water of lakes and oceans.

Phytoplankton from Crystal Lake, Vermont, June 2012.

The "phyto" (from the Greek for plant) in the name reflects the ability of individuals to photosynthesize, a characteristic shared with groups more commonly considered "plants", such as flowering plants, conifers, ferns, and mosses. Photosynthesis using green, yellow, orange, red and bluish-green pigments captures light energy and converts it to the chemical energy which drives the metabolism of the phytoplankton. Growth and reproduction set the phytoplankton up as food (as prey) for small grazing animals (its predators), which in turn are food for trout, pike, bass and other higher carnivores. The functional role of the phytoplankton in aquatic ecosystems is the same as that of the macroscopic grasses in terrestrial ecosystems -- the prairies of North America, the pampas of South America, the steppes of Asia, the savannas of Africa and Australia -- which have supported great herds of grazing animals.

Coincident with its conversion of light energy into the chemical energy of organic molecules, the phytoplankton produces more oxygen than it consumes and is thus is a net source of oxygen for a lake and the atmosphere and the organisms which depend on it.The phytoplankton also consumes more carbon dioxide than it releases, the difference reflecting its growth in biomass.

Being small -- cells typically less than 200 µm and colonies rarely more than a few millimeters -- phytoplankters form the base of a lake's food chains and webs, powering the entire living lake system. Also, being small, and even though many of them are quite capable of locomotion on a local scale, phytoplankers have their location on a larger scale determined primary by water turbulence and the movement of water masses. From a human perspective they thus have been seen as buffeted by forces larger and more powerful than ones they can generate themselves, leading to their characterization as "wanderers" (from the Greek planktos), not fixed or attached. Fish swim, plankters are largely adrift.

The tree has been highly pruned to show the general relations among major subgroups (color-coded ellipses)
which contribute to the phytoplankton.
Click on the groups for more information about them and the tree

The branching tree-like diagram above is a time graph. The base of the tree represents the origin of life (c. 3.5 billion years ago) and stemming from it are three main branches representing the 3 major domains of life (Bacteria, Archaea and Eukaryotes, indicated by the dashed blue lines). As one moves away from the origin time increases toward the present, which is represented by present day groups (color-coded ellipses) depicted at the tips of the branches. The ellipses represent groups that have organism that occur in the phytoplankton of Vermont lakes, and taken as whole they form the artificial group called algae.

The tree is most commonly used to express the time since two groups shared a common ancestor, the "propinquity of secent of two groups" -- or more commonly, how closely related two groups are. For example, among Eukaryotes, the diatoms and chrysophytes share a common ancestor (point b). This common ancestor is viewed as having given rise in one branch to the chrysophytes and in an independent branch to the diatoms. Also, the diatoms and chrysopyhtes (as a group) share a common ancestor with the dinoflagellates (point c), but that common ancestor lived and diverged before the common ancestor of the chrysophytes and diatoms. Folllowing this same means of comparison as first approximation for relatedness, diatoms are less closely related to land plants and charophytes than to dinoflagellates, and least closely related to cyanobacteria among all the algal groups shown.

Eight major clades (colored ellipses) have representatives common in the Vermont lake plankton (the names of organisms in these groups are typically color-coded, as shown below, through these web pages).


The relationships among the algal groups are more complex than repesented above; to follow those complexities look at "Diatoms, endosymbiosis and horizontal gene transfer" (Marginalia no. 10).

The phytoplankton corresponds to no single group actually existing in nature. Instead it is a composite of parts of several natural groups -- cyanobacteria, chlorophytes, euglenophytes, rhodophytes, chrysophytes, dinoflagellates, diatoms and charophytes -- groups historically recognized as "algae." About 1870 the phytoplankton was given initial recognition and a name to contrast with the then-known primary habitat of microorganisms in a lake - its bottom, where they attached to and richly covered rocks, submerged plants, sediments and other substrates. What gives the group phytoplankton coherence is thus not any particular common ancestry, or any particular anatomical structure, or even necessarily a "wandering" existence, but rather its being "adapted to living suspended" in open water, that is, being able to photosynthesize enough in the sunlit regions of open water to be able to surive and reproduce sufficiently to seed the next generation of phytoplankters. "Adapted" is used here in its strong Darwinian evolutionary sense of the phytoplankters having features and characteristics shaped by natural selection.

At first members of the phytoplankton may be perceived by us as simply different colored shapes. But in accord with our marvelling at the adaptive variation in the skeletal structures of vertebrates and the pollination schemes of flowering plants, so too do phytoplankters offer us striking examples of microorganisms shaped to their circumstances by the process of differential survival and reproduction -- by natural selection. We pursue these evolutionary threads -- regarding phytoplankton structure, biochemistry, cellular organization, behavior, ecology and life history -- in the pages on this website.

Charles Darwin, Origin of Species (1859), pages 485-486 [underscores not in original].

When we no longer look at an organic being as a savage looks at a ship, as at something wholly beyond his comprehension; when we regard every production of nature as one which has had a history; when we contemplate every complex structure and instinct as the summing up of many contrivances, each useful to the possessor, nearly in the same way as when we look at any great mechanical invention as the summing up of the labour, the experience, the reasoning, and even the blunders of numerous workmen; when we thus view each organic being, how far more interesting, I speak from experience, will the study of natural history become!

Painting of HMS Beagle in seaways of Tierra del Feugo, South America,
by Conrad Martens, ship's artist (1833). Darwin was the principal
naturalist on this expedition (1832-1836). Image in Public Domain.

Page updated 22 January 2015
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