experimental feeding of copepod Calanus finmarchicus (Gunner) on phytoplankton cultures 1 1 I 



Walker and Whisenand, 1951; Rice, 1953). The error is not likelv to be large 

 for Rice has shown that in cultures a week old with a low phosphorus concentration 

 only about 2 % is exchangeable. Most of our cultures were used when more than a week 

 old, and when the phosphorus in solution had fallen to a low value. 



It was, however, thought advisable to measure the digestion of some of the same 

 organisms labelled with other radioactive isotopes. One of the most important 

 elements in the plant cell is carbon, and by growing cultures using ^^C it was possible 

 to measure the uptake and digestion in a way similar to that used with phosphorus. 



There are certain disadvantages in the use of "C which are not met when using ^^p. 

 Carbon is present in sea water as carbonate, bicarbonate and CO^, and it is never 

 limiting so that the plant cells cannot be expected to remove all the carbon present. 

 The cells have therefore to be removed and re-suspended in non-radioactive water 

 before use. Again carbon is liberated as CO2 in respiration while the culture is growing, 

 and also during the experiments, and although the value of this second figure may be 

 small it will reduce the amount apparently used. 



METHODS 



The method adopted was in general similar to that used in the earlier experiments with radioactive 

 phosphorus. The cultures were grown in sterile 250 ml conical flasks, to which were added 100 ml 

 Erdschreiber culture medium and 50 ml of algal culture. The radioactive carbon was added in the 

 form of bicarbonate, and 50 i^c was found to be a convenient quantity for this volume of culture 

 medium. The flasks were kept airtight by means of rubber stoppers to avoid possible loss of '^C to 

 the air as COj. The cultures were allowed to grow either in diff'use daylight or close to a fluorescent 

 tube. They grew rapidly and well, and took up an unexpectedly high proportion (over 50",,) of the 

 added ^^C in a fev,' days. For use in an experiment the culture was centrifuged at about 2500 r.p.m. 

 for five minutes, the supernatant liquid decanted, membrane-filtered sea water added, and the 

 organisms dispersed. This process was repeated twice more, and the cells finally suspended in mem- 

 brane-filtered sea water in a dilution suitable for the experiment. For filtering the sea water a Gradocol 

 membrane was used with an average pore diameter of about Q-9\j.. 



Since the (3-radiation of ^^C is relatively weak and readily absorbed, it is necessary either to correct 

 for absorption or to make all the samples tested strictly comparable. 



The second procedure was adopted. The methods used are discussed by Calvin, Heidelbergpr, 

 Reid, Tolbert and Yankwich (1949). For sea water samples, uniform flat-bottomed dishes (plan- 

 chettes) of 25 mm diameter were used. With a 0-2 ml delivery pipette the sample was put in the middle 

 of the planchette and one drop of N/10 NaOH added. A circle of lens tissue of a diameter slightly 

 less than the bottom of the planchette was then laid on the sample to spread the drop evenly so that, 

 on drying, a layer of uniform thickness would be obtained. Drying was carried out on a hot plate. 

 An attempt was made to increase the accuracy by using 0-5 ml instead of 0-2 ml, but it was unsuccess- 

 ful because the self-absorption was relatively greater than with 0-2 ml, and the samples dried less 

 uniformly. 



The object of adding the NaOH was to avoid the exchange of CO.> between the air and the bicar- 

 bonate of the sea water. If this procedure is not adopted, there may be a serious loss, and variable 

 results will be obtained (Calvin, et ai, p. 123). The lens tissue should not be silicone treated, since 

 this type tends to curl up on drying. There was no tendency to curl with the photographic Ions tissue 

 used, so that the addition of collodion was unnecessary. 



To ensure that no change in absorption of the radiation was caused by variations in salinii\. the 

 same sample of sea water was used throughout the experiments. 



The activity of each sample was measured by exposing it in a holder at a constant distance from 

 the mica end-window of a G.-M. counter, and the counts were recorded on a scaling unit. The results 

 are expressed as counts per minute, but since we know the concentration of cells in the culture and 

 the activity of both whole culture and filtrate, we can also express them as cell equivalents. 



The bodies and faecal pellets of the Calanus have an appreciable self-absorption, and even after 

 they had been torn up by needles the losses were considerable. Good duplicates were obtained by 



