248 Marine Microbiology 



when results of other nivestigations are compared, the porosities 

 of filters employed must be considered. In view of the significant 

 loss of activity through HA filters, measurements of marine 

 primary production at the University of Washington and Scripps 

 Institution of Oceanography are now made with PH filters. 



Although only eight filtration series have dark bottle repli- 

 cates (Table 1), there is no indication that dark uptake by bac- 

 teria or other heterotrophic organisms can account for the greater 

 amount of activity retained by the finer porosity filters. We be- 

 lieve there are three possible explanations which account for the 

 results obtained; these should not be considered mutually ex- 

 clusive. The first is that finer porosity filters prevent the passage 

 of radioactive fragments and/or exudates resulting from me- 

 chanical fragmentation of cells during filtration. The second ex- 

 planation supposes the presence in sea water of exceedingly mi- 

 nute autotrophic organisms which pass through all but the finest 

 filters. The last and least likely possibility would assume the 

 existence of bacterial symbionts which may live in close asso- 

 ciation with the phytoplankton. Such symbionts could become 

 labelled in the light by incorporating into their protoplasm radio- 

 active substances excreted by the photosynthesizing phytoplank- 

 ton. Such bacteria might pass through the coarser filters and be 

 retained by the finer filters. 



SUMMARY AND CONCLUSIONS 



Recovery of C^^-labelled natural phytoplankton communi- 

 ties on fine plankton nets and on membrane filters of graded 

 porosities was measured successively, using waters collected near 

 San Juan Island in Puget Sound. The finest plankton net (35 x 35 

 IX mesh) usually failed to retain half of the assimilated C^'* that 

 was recovered on the 0.45 /i filter, and in some cases, recovery 

 with the plankton nets was below 10 per cent. More than half of 

 the photosynthesis, therefore, was usually eflFected by algae that 

 pass through a 35 /-<. net. The range of membrane filter porosities 

 used in this study extended well below the known dimensions 

 of the smallest marine algae, yet recovery of assimilated C^^ 

 was commonly greater with a pore size of 0.22 /x than 0.4 i^l or 

 larger. Greater retention of assimilated organic matter on the 



