The base of marine life is a large complex group of
organisms known as plankton. Plankton are not defined by size nor
taxonomy (i.e., biological classification), but rather because
they are passively carried by water motion. Although some plankton can
swim, their ability to is generally less than the strength of water movement.
Most planktonic organisms are small (less than a 1 mm or 0.039 inch),
although larger jellyfish are also considered plankton. Plankton include
a wide variety of organisms such as algae, bacteria, single-celled animals.
Many marine organisms -- including barnacles, lobsters, crabs, starfish,
etc. -- begin their life with a planktonic stage. Plankton can be broadly
divided into phytoplankton (plants or photosynthetic organisms),
zooplankton (animals), and bacteria. Phytoplankton carry
out photosynthesis and are the base of the food chain in the ocean. Zooplankton
are heterotrophic consumers. Bacteria decompose organic matter
into its non-living constituents.
Below we focus on the key phytoplankton
groups that contribute to ocean color.
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Diatoms have rigid cell walls consisting
of two closely fitting halves; they are like miniature greenhouses,
protecting internal plant matter within glass walls. These algae occur
as single cells, chains and colonies. Cell sizes range from 5 to 1000
micrometers. Centric diatoms have circular, triangular, or
pillbox shapes. Pennate diatoms are elongate with bilateral
symmetry. Diatoms may have spines or other projections and some
are toxic.
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Dinoflagellates are generally smaller than diatoms
(can be as small as 10 micrometers). They usually occur as single
cells. Typical dinoflagellate forms have a body surface with two grooves,
each having a "whip-like" flagellum they use to swim. Some
dinoflagellates are plant-like (get energy from the Sun), others are
animal-like (consume other organisms for energy), and some are both.
There are bioluminescent dinoflagellates
and toxic species, too.
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Coccolithophores surround themselves with plates
made of calcite. Known as coccoliths, these plates have a variety
of shapes. Single coccolithophores are commonly smaller than 20 micrometers
across and are often enclosed by over 30 plates. Coccolithophores
can form enormous blooms, turning seas an opaque turquoise
color.
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Phytoflagellates are diverse, crossing ten classes
of algae. These motile organisms are so small they must be identified
using powerful microscopes or biochemical techniques. Phytoflagellates
are very abundant and often dominate the phytoplankton when larger
forms -- i.e., diatoms and dinoflagellates -- are low in concentration.
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Photosynthetic bacteria are some of the smallest
-- and largest-- phytoplankton. They exist as small single cells (less
than 1 micrometer in diameter) but some forms clump together to form
"bundles" that can be seen by eye. Photosynthetic bacteria
are major contributors to oceanic primary productivity. One type,
Prochlorococcus, may be the most abundant species on earth.
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Some background material from "Visit to an Ocean Planet" CD-ROM, Copyright 1998, California Institute of Technology and its licenses.
