Retention of Dissolved Compounds by Membrane Filters as an Error in the ¹⁴c Method of Primary Production Measurement¹

SUMMARY Membrane filters retain 14 C bicarbonate, 14 C glycollate, and other 14 C labeled substances from filtrates of algal cultures and lake water. By refiltering different volumes of the filtrate from algal cultures and from lake water after incubation with 14 C bicarbonate, it was shown that the labeled material retained was not proportional to volume but showed a saturation effect with increasing volume filtered. When the radioactivity retained by a filter is divided by the volume filtered, decreasing values are obtained with increased volume filtered. This radioactivity may represent a significant addition to the radioactivity in particulate material on the filters, resulting in a similar type of curve when different volumes of lake water or cultures are filtered. Values of radioactivity per milliliter were constant using Chlorella in Chu 10 medium. However, the curve could, be obtained by increasing pH and bicarbonate concentrations in the medium and on resuspending the algae in Lake Ontario (winter) filtrate. The values of cpm/ml retained from filtrates were low in Cryptomonas cultures and the curve was not obtained unless population density was reduced, thus increasing the relative contribution to the radioactivity on the filters. The curve was not always obtained in lake water. It was significant in 10 out of 14 experiments in Lake Ontario and in 2 out of 5 experiments in Grenadier Pond. Changes in lake water rather than in experimental techniques were probably responsible. On 2 occasions when values of cpm/ml were constant in Lake Ontario, addition of sodium bicarbonate without a pH change resulted, in a significant curve. Our experiments do not disprove the possibility of cell damage during Millipore filtration, however, it has been shown that 14 C labeled substances retained from solutions can account for the entire range of decreasing values as a function of volume filtered.