Interpretation Of Fast Repetition Rate (frr)fluorescence: Signatures Of Phytoplankton community Structure Versus Physiological State

摘要

Introduction of active chlorophyll a fluorescence protocols, in particular fast repetition rate (FRR) fluorometry, to oceanography and limnology 15 yr ago has enabled rapid assessment of photosyn-thetic physiology in situ. The FRR protocol generates simultaneous measurements of Photosystem II (PSII) effective absorption cross sections (termed σ_(PSII)) and photochemical efficiency (termed F_v/F_m). Both F_v/F_m and σ_(PSII) measurements have been utilised to examine the effects of physiological stress on the photosynthetic apparatus of phytoplankton in an ever growing number of fluorescence-based studies. However, it is now becoming clearer that in situ values of F_v/F_m and σ_(PSII) also contain taxonomic information. Here, we present a synthesis of previously unpublished and published data, which show that F_v/F_m and σ_(PSII) vary principally with broad-scale changes in community structure. These patterns observed in situ conform to trends observed in laboratory-grown cultures of a range of phytoplankton taxa. The magnitudes of variability in F_v/F_m and σ_(PSII) driven by changes in phytoplankton community structure often exceed that induced by nutrient limitation (as determined from controlled nutrient addition experiments). An exception to this general trend occurs in high-nutrient, low-chlorophyll a (HNLC) regions, where strong phenotypic changes in F_v/F_m and σ_(PSII) have been repeatedly demonstrated on relief of iron limitation. Overall, FRR fluorescence measurements of both F_v/F_m and σ_(PSII) in natural populations represent a combination of the taxonomic 'signature' (values of F_v/F_m and σ_(PSII) determined by the taxa present) within the phytoplankton community that is further modified according to the (photo-) physiological status. As such, fluorescence-based investigations of mixed populations must account for potential variations in phytoplankton community structure before interpretations of physiological status are made.