NATURE ADRIFT 



I / 1, 000 millimetre). They look like discs of opaque white cellophane. These 

 of course, pass water very slowly indeed even when the speed is increased by 

 applying a vacuum pump below or air pressure above the filter. As the 

 numbers of these smallest organisms are so great, this is not usually a severe 

 drawback, and small samples of water are sufficient. The difficulty is to get 

 them oft' the filter for later examination ! This is virtually impossible so that 

 one looks at them on the filter by making it transparent with a medium of 

 the same refractive index 1-47, or one dissolves away the filter. Neither is 

 completely satisfactory and the method is mainly used for chemical or 

 optical assessment of the amount of pigment, usually the chlorophyll, that 

 can be chemically extracted from the organisms left on the filter. A recent 

 method that complements rather than replaces the chlorophyll estimation 

 makes use of radioactive isotopes ; the chlorophyll method indicates what is 

 there, but the isotope method reveals what is being produced. Of these the 

 most useful is carbon-T4; and the amount of O^ present can be asssessed 

 with a Geiger counter. If we add a known but very small amount of C^"* to 

 a sample of sea water with a known amount of ordinary carbon, C^^, 

 there will be a measurable amount of radioactivity, but not enough to kill 

 the organisms. The carbon is not, of course, in the solid black state, but as 

 part of a soluble compound like a carbonate or as dissolved carbon dioxide. 

 All living plants absorb carbon during their metabolism (see also Chapter 9) 

 and will absorb C^'* at just about the same ratio as the ordinary carbon O^. 

 The ratio of C^'* to C^" absorbed by the organisms will therefore be about 

 the same as it was in the sea-water sample. After a carefully timed period of 

 exposure to a constant light and at a constant temperature, we can then 

 filter the sample through a membrane filter and measure the radioactivity 

 of the filter. If no living plants were present all the C^^ would go through 

 the filter; the amount of activity shown by the filter thus represents the 

 anount of plant metabolic activity during the period of the experiments. 

 Under constant conditions we can relate this to the productivity of the 

 water. 



If we want to examine our smallest organisms microscopically, we can 

 adopt a different technique, the second, which is by ccntnfuging the water. 

 Centrifuging is a method of spinning a sample at great speed, up to about 

 20,000 revolutions per minute, in a series of carefully balanced glass tubes— 

 they need to be carefully balanced to avoid the shatter that would result from 

 unbalance at high speed. Plankton, particularly dead plankton, is slightly 

 heavier than water and, after a few minutes centrifuging, the organisms are 

 concentrated at the tips of the tubes. After gradual slowing and stopping of 

 the spin they can be extracted with a fine pipette. Such pressure tends to 

 distort fragile and delicate organisms and make identification difficult, so the 



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