Understanding the role of chlorophyll fluorescence in nutrient stress

Abstract

Phytoplankton exert a dominant influence on the biogeochemical cycling of the oceans, but iron limitation in this dynamic environment can exert a control on photosynthesis. Phytoplankton evolved coping mechanisms to overcome and alleviate the effects of iron limitation. One mechanism is the alteration of the thylakoid membrane and the expression of chlorophyll-binding proteins, which can alter the variable chlorophyll fluorescence signal. Firstly, a study of the chlorophyll-binding iron-stress induced protein, IsiA, in Synechocystis PCC 6803 revealed a 60% increase under iron limitation, in agreement with the theoretical increase. On progressive iron-stress IsiA continued to accumulate without a concomitant increase in _PSI, while Fv/Fm, a measure of photochemical efficiency, continued to decrease. Secondly an oceanographic study to the high latitude North Atlantic in which chlorophyll fluorescence kinetics were used to measure the response to iron addition of in situ phytoplankton populations. The difference in the Fv/Fm between nutrient amended and control treatments (_(Fv/Fm)) was used as a measure of the relative degree of iron stress. The combined observations of both longterm (> 24 h) and short-term (24 h) indicated variability in the seasonal cycle of iron stress, with phytoplankton iron stress developing during the transition from prebloom to peak bloom conditions. Thirdly, similar physiological characteristics were also observed in an oceanographic study in the Ross Sea. The results further confirmed the highly variable response across temporal and spatial scales, but also within different phytoplankton groups. Consistent across all three studies is the reduction in Fv/Fm as the result of an elevated Fo signal, representing potentially unbound chlorophyll-binding proteins. These unbound chorophyll-binding proteins can dominate the total cellular chlorophyll, at least in culture, and reflect a large resource investment. These proteins may provide a rapid source of chlorophyll upon iron resupply. Irrespective of the underlying causes of unbound chlorophyll-binding proteins, the potential large scale expression of such complexes provides a powerful diagnostic tool with which to investigate iron stress in situ.

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ORCID for C.M. Moore: orcid.org/0000-0002-9541-6046

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