Susceptibility of some marine phytoplankton species to cell breakage during filtration and post-filtration rinsing

Several fragile phytoplankton species among a diverse group of 13 species were found to be very susceptible to cell breakage when exposed to the air under vacuum between the filtration and rinsing steps used to terminate 14C fixation experiments and remove residual [ 14C]HCO 3 −. Up to 60% of fixed carbon after 15-min incubation was found in the rinse. Losses were even greater when cultures were pulsed with NH 4 + at the start of the incubation, probably because rapid NH 4 + uptake leads to the accumulation of large pool of soluble and low molecular weight compounds. Most likely, the cells, when exposed to the air, are subject to extreme osmotic shock and rupture. Unaccountable losses of 14C occurred with polycarbonate filiters relative to glass-fiber filters. In addition, vacuum pressure differentials >25–100 mmHg across polycarbonate filters also caused cell breakage that led to the accumulation of 14C in the filtrate. Avoiding air exposure of the filter between the filtration and rinsing steps or eliminating the rinsing step entirely and acid-soaking or fuming the filters led to virtually complete recovery of fixed carbon. Our results based on 15-min incubations may not be directly comparable with longer-term incubations, but they do serve highlight our concerns about filtration procedures in general.