Interpretation of fast repetition rate (FRR) fluorescence: signatures of phytoplankton community structure versus physiological state

Introduction of active chlorophyll a fluorescenceprotocols, in particular fast repetition rate(FRR) fluorometry, to oceanography and limnology15 yr ago has enabled rapid assessment of photosyntheticphysiology in situ. The FRR protocol generatessimultaneous measurements of Photosystem II (PSII)effective absorption cross sections (termed ?PSII) andphotochemical efficiency (termed Fv/Fm). Both Fv/Fmand ?PSII measurements have been utilised to examinethe effects of physiological stress on the photosyntheticapparatus of phytoplankton in an ever growing numberof fluorescence-based studies. However, it is nowbecoming clearer that in situ values of Fv/Fm and ?PSIIalso contain taxonomic information. Here, we presenta synthesis of previously unpublished and publisheddata, which show that Fv/Fm and ?PSII vary principallywith broad-scale changes in community structure.These patterns observed in situ conform to trendsobserved in laboratory-grown cultures of a range ofphytoplankton taxa. The magnitudes of variability inFv/Fm and ?PSII driven by changes in phytoplanktoncommunity structure often exceed that induced bynutrient limitation (as determined from controllednutrient addition experiments). An exception to thisgeneral trend occurs in high-nutrient, low-chlorophylla (HNLC) regions, where strong phenotypic changesin Fv/Fm and ?PSII have been repeatedly demonstratedon relief of iron limitation. Overall, FRR fluorescencemeasurements of both Fv/Fm and ?PSII in natural populationsrepresent a combination of the taxonomic ‘signature’(values of Fv/Fm and ?PSII determined by thetaxa present) within the phytoplankton communitythat is further modified according to the (photo-) physiologicalstatus. As such, fluorescence-based investigationsof mixed populations must account for potentialvariations in phytoplankton community structure beforeinterpretations of physiological status are made.