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In vivo estimation of the photosystem II photochemical efficiency of individual microphytobenthic cells using high‐resolution imaging of chlorophyll a fluorescence

Rates of primary production by intertidal microphytobenthos within biofilms have been shown to be very high. An essential step toward assessing the contribution of individual species to this level of production is the in vivo measurement of photosynthetic efficiency from individual cells. A strong relationship between photosystem II photochemical efficiency and the fluorescence parameter Fq'/Fm′ (where Fq′ = Fm′ − F′) has been established within higher plants and unicellular algae. Calculation of Fq′/Fm′ requires measurement under constant light (at the F′ level of fluorescence) and during a pulse of saturating light (at the Fm′ level of fluorescence). High‐resolution imaging of chlorophyll fluorescence at the F′ and Fm′ levels has allowed the construction of Fq′/Fm′ images from individual cells of several species of diatom and Euglena sp. within intact biofilms. No species differences in the values of Fq′/Fm′ were observed at low levels of incident light. However, Euglena sp. showed significantly higher Fq′/Fm′ values at moderate to high incident light levels than all of the diatom species. Endogenous rhythms of vertical migration during tidal exposure and peaks in photosystem II photochemical efficiency at low tide could also be followed using this technique. Clear differences were observed in the migration of individual taxa to the surface of the biofilm. Images of Fq′/Fm′ were also used to assess the scale of heterogeneity for this parameter. Overall, these data demonstrate that high‐resolution imaging of chlorophyll fluorescence is a valuable technique that allows for determination of the photosystem II photochemical efficiency from different microphytobenthic taxa within biofilms.