Phytoplankton

Most of the plants in the ocean are planktonic, single celled algae known as phytoplankton. Though some are large enough to be collected in fine meshed nets, many types can only be collected by filtering large amounts of water. The food web begins with phytoplankton, upon which all other organisms in the ocean depend either directly, or indirectly. Phytoplankton are found throughout the world's oceans, even under polar ice, wherever light and nutrient requirements are satisfied.

Because phytoplankton are photosynthetic organisms, they have the same essential requirements as land-based plants, i.e. solar energy, CO2, nutrients and water. Phytoplankton however are without roots, leaves, stems, etc. so have other adaptive strategies for meeting their requirements. Their small size is an adaptive strategy, serving two purposes;

1. Small size increases the surface area to volume ratio (SA/V) for increased uptake efficiency of dilute nutrients.
2. A high SA/V also slows sinking rate, thus increasing residence time in the upper, sunlit waters of the ocean.

Reproductive rate for these organisms is expressed as an increase in cell numbers. For unicellular organisms, this increase is exponential.

N0 +DN = N0 e ^ kt

Where N0 is the beginning population of cells, DN is the number produced during time t, and k is a growth constant in units of generations per day. A useful way of expressing growth rate is in terms of doubling time (d), defined as the time required for the population to double. The doubling time for phytoplankton population can be derived thus,

N0 + DN = 2N0 = N0e ^kd

2 = e ^kd

ln 2 = kd

ln 2 / k = d = 0.69 / k

The growth constant k then is related to the doubling time d by

k = 0.69 / d

There are three major phytoplankton groups; diatoms, coccolithophores, and dinoflagellates. Diatoms have a pillbox like structure made of silica surrounding the cell. Since silica is a relatively insoluble in seawater, diatoms form a major component of the sediments in areas of high productivity, most notably in a belt around the Antarctic and in the North Pacific. If the sediment is at least 30% diatom frustrules it is known as diatomaceous ooze. The silica frustrules are very ornamented with pores. Centric diatoms are round, pennates are elongated. Some species form chains. Many are spined, further increasing surface area, and retarding predation. Coccolithophores are covered by calcareous plates or discs called coccoliths. When the cell dies, these coccoliths sink thus contributing to the sediments. They are usually quite small. Dinoflagellates are usually armored by cellulose plates that fit together like pieces of a jigsaw puzzle. Cellulose is organic, and thus does not contribute to the sediments. Dinoflagellates have flagella, whip-like structures which are used for motility. Some dinoflagellates produce bioluminescence or "living light" while others are responsible for red tides.

Portions adapted from; Hallegraeff, G. M. 1988. Plankton: a Microscopic World. QH 90.8 P5 H35 1988.

Last Updated: 17 August 1999

jwormuth@ocean.tamu.edu

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