SIZE FRACTIONATION OF PHOTOSYNTHESIZING PHYTOPLANKTON 



by 



Robert W. Holmes 



To estimate the size ranges of photosynthesiz- 

 ing phytoplankton in tropical waters, three 

 simple experiments were performed on SCOPE. 

 In each experiment a surface water sample of 

 0.5 or 1.0 liter was inoculated with approxi- 

 mately 20 uc of C 1 and placed just below the 

 sea surface for incubation. After 2-3 hours' 

 incubation an aliquot was taken from the sam- 

 ple and passed through a series of filters in 

 the following order: a disk of No. 20 bolting 

 silk (mesh size 106u x 106u by measurement), a 

 disk of nylon bolting material (mesh size 30u 

 x 3Cu by measurement), an AA Millipore filter 

 (pore size specified by the manufacturer as 

 0.8u t 0.05m), and lastly an HA Millipore fil- 

 ter (pore size likewise specified as 0.1*5u 1 

 0.02u). In addition in 2 experiments another 

 aliquot was filtered directly through an HA 

 Millipore filter. The pieces of netting and 

 filters were dried and counted in the usual 

 manner (see p. ?)• The results of these ex- 

 periments are given in Table 6. 



From these data it can be readily seen that 

 the activity of organisms retained by the 

 bolting silk and nylon bolting material rep- 

 resented a small fraction of the total activ- 

 ity. In two out of three experiments only 

 about one-half of the total activity was re- 

 tained by the AA Millipore filter. 



It is difficult to believe that all of the 

 activity passed by the AA Millipore filter was 

 contained in bacterial cells. Dark-bottle 

 C fixation in experiments of 6 hours' dura- 

 tion in these same waters usually averaged 

 10°/ o and never exceeded l8°/ of the light- 

 bottle uptake. It seems more plausible to 

 suggest two alternative explanations. Extreme- 

 ly small photosynthesizing organisms (less than 

 about lu) may have been present in these waters 

 and passed through the AA filter and/or the 

 bulk of material passed by the AA Millipore 

 filter may have been cell fragments produced by 

 the rupture and disintegration of some of the 

 cells as they impinged upon the membrane-filter 

 surface during filtration. Unfortunately the 



water samples collected for the purpose of 

 flagellate enumeration and identification have 

 not yet been examined carefully but it appears 

 from a cursory examination that the smallest 

 naked flagellate visible in these samples are 

 between 1-1.5 l-i in "diameter." Organisms 

 smaller than those observed in the fixed mater- 

 ial may exist in the sea but may not have been 

 preserved adequately enough to permit enumera- 

 tion or identification. However, it seems un- 

 likely that a significant portion of the total 

 photosynthesizing biomass could have been such 

 organisms . 



It seems more plausible that the material pass- 

 ing through the AA millipore filter was large- 

 ly in the form of protoplasmic fragments 

 released from fragile cells which ruptured on 

 the filter surface. That small naked flagel- 

 lates do disintegrate as a result of filtration 

 has been observed by the author and by Dr. 

 W. Rodhe (personal communication) by comparing 

 the flagellate abundances on cleared Millipore 

 filters with those in unfiltered samples. Con- 

 firmation of this fragmentation hypothesis 

 has also been observed by the author and Dr. 

 R. Lasker (unpublished results) who used 

 radioactive bacteria-free cultures of 

 C hi amy domo na s sp. Of nine aliquot s, three 

 were filtered through AA Millipore filters, 

 three through HA, and three through PH. No 

 essential. difference in the activities of the 

 HA and PH filter membranes was observed where- 

 as the activity in the AA Millipore filter 

 averaged 12-19°/o less than that observed on 

 the HA or PH filters. The Chlamydomonas em- 

 ployed in this study was quite healthy and the 

 cells averaged about 8u in "diameter." A some- 

 what greater difference was observed in another 

 experiment when the filtrate of a nonbacteria- 

 free culture from the AA filter, was passed 



successively through an HA and PH filter 



here the AA filter passed about 32°/ of the 

 activity retained by all three filters. This 

 apparent difference in retention is probably 

 the result of fragmentation caused by the fil- 

 tration through the AA Millipore filter and 

 the passage of some bacteria less than 0.8 \i 

 in size. 



69 



