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 co ntribution of individual species to this level of production is the in vivo measurement of photosynthetic efficiency from Individual cells. A strong r elationship between photosystem II photochemical efficiency and the fluores cence parameter F-q'/F-m' (where F-q' = F-m' - F') has been established wit hin higher plants and unicellular algae. Calculation of F-q'/F-m' requires measurement under constant light lat the F' level of fluorescence) and duri ng a pulse of saturating light (at the F-m' level of fluorescence). High-re solution imaging of chlorophyll fluorescence at the F' and F-m' levels has allowed the construction of F-q'/F-m' images from individual cells of sever al species of diatom and Euglena sp. within intact biofilms. No species dif ferences in the values of F-q'/F-m' were observed at low levels of incident light. However, Euglena sp, showed significantly higher F-q'/F-m' values a t moderate to high incident light levels than all of the diatom species. En dogenous rhythms of vertical migration during tidal exposure and peaks in p hotosystem II photochemical efficiency at low tide could also be followed u sing this technique. Clear differences were observed in the migration of in dividual taxa to the surface of the biofilm. Images of F-q'/F-m' were also used to assess the: scale of heterogeneity for this parameter. Overall, the se data demonstrate that high-resolution imaging of chlorophyll fluorescenc e is a valuable technique that allows for determination of the photosystem II photochemical efficiency from different microphytobenthic taxa within bi ofilms.