Different fixatives and chloridric acid concentrations in microphytobenthic primary production estimates using radiolabeled carbon: Their use and misuse

In literature, different fixatives have been used to stop microphytobenthic photosynthetic activity estimated using the 14C technique, and different concentrations of HCl have been used to remove the excess labeled bicarbonate. To standardize these two steps of the 14C method, we designed a series of experiments using sublittoral muddy sediments colonized by microphytobenthos. The first aim was to identify which fixative among formaldehyde, glutaraldehyde, and HCl was most effective in arresting photosynthetic activity. Formaldehyde increases cell membrane permeability leading to a loss of assimilated carbon because an underestimate of disintegrations per minute (DPM) of up to 64% was obtained when compared with HCl values. Glutaraldehyde led to an overestimate of DPM due to its autofluorescence. We propose that the best way to stop photosynthetic activity is by using HCl. The second aim of the article was to define which HCl concentration from 0.1N to 5N was sufficient to remove the excess labeled bicarbonate. The residual inorganic 14C led to an overestimate of DPM values when mild acids were added. None of the acid treatments completely removed the inorganic labeled carbon because of the sediment matrix effect. However, a further pH decrease, caused by too strong acid normalities, could digest the organic carbon pool. Besides dark estimates, blank production estimates should be assessed before each primary production experiment to consider the sediment matrix effect.