Monitoring migration and measuring biomass in benthic biofilms: the effects of dark/far red adaptation and vertical migration on fluorescence measurements

Pulse modulated fluorescence has increasingly been used as an ecological tool to examine changes in the vertical distribution of microphytobenthic cells within the upper layers of estuarine sediments (most often using the minimum fluorescence yield F-O) as well as to indicate the health of the community (using the maximum PS II quantum efficiency F-v/F-m). However, the practicalities of in situ measurements, often dictates that short dark adaptation periods must be used (similar to15 min). The use of far-red light as an alternative to dark adaptation was investigated in natural migratory microphytobenthic biofilms and artificial non-migratory biofilms. Prolonged periods of darkness (similar to24 h) were not adequate to achieve 'true' measurements of F-O and F-v/F-m, which require complete oxidation of Q(A) and full reversal of non-photochemical quenching (NPQ). In some instances, stable values were only achieved using far-red light. Prolonged exposure to dark/far-red light led to a downwards migration of cells in natural assemblages, as seen by a reduction in both F-O and the maximum fluorescence yield (F-m). In non-migratory biofilms, Fm increased in the dark and far-red treatments, indicating a reversal of NPQ, whereas F-O decreased in far-red light but increased in the dark. It is suggested that far-red light and darkness differentially affected the balance between NPQ reversal and Q(A) oxidation that lead to the measured F-O yield. The use of far-red light as an alternative to dark adaptation is discussed and the implications of short (e.g., 15 min) dark adaptation times used in situ are discussed with reference to the vertical migration of cells within sediment biofilms.