Properties of the light saturation curve of photosynthesis and ribulose-1,5-bisphosphate carboxylase (RuBPC) activity are shown to change qualitatively in a natural population of marine phytoplankton during a spring bloom. Evidence is presented to show that these changes constitute photoadapative responses to increasing irradiance. As irradiance increased during the bloom, both the level of light-saturated photosynthesis (
Pm) and the initial slope of the light saturation curve (α = photosynthetic efficiency) increased whether those parameters were normalized to chlorophyll
a concentration (
Pmb, α
b) or to cell numbers (
Pmc, α
c). The magnitudes of these changes were such that
Ik (=
Pm/α, the photoadaptation parameter) did not change, but
Im, the light intensity at which photosynthesis becomes saturated, increased. RuBPC activity, both chlorophyll
a (RuBPC
b) and cell number normalized (RuBPC
c), also increased during the bloom. We suggest that these adaptations were achieved by simultaneously increasing the number of photosynthetic units, proportionately decreasing the photosynthetic unit size, and increasing both the concentrations of the enzymes of the dark reactions and possibly also of photosynthetic electron transport components.We also observed diminished levels of photoinhibition in the high light adapted cells late in the bloom and have suggested that this was a consequence of the same suite of physiological changes.
In situ carbon fixation per cell increased during the bloom whereas no change occurred in this parameter when normalized to chlorophyll
a concentration. Although these photoadaptive responses thus permitted carbon to be fixed
in situ more rapidly per cell, at a constant efficiency with respect to investment of energy in the photosynthetic apparatus, they did not result in a change in growth rate. Based on consideratios of the role of time scale in physiological adaptation, however, it is suggested that the observed alterations in photosynthesis with increasing irradiance might permit a cell to more rapidly fill an energy quota for division, possibly an advantage in a mixing environment in which energy is patchily distributed, both spatially and temporalyy.Phosphoenolpyruvate carboxylase activity when normalized to chlorophyll
a (PEPC
b) did not change during the bloom while chlorophyll
a normalized dark carbon fixation decreased sharply and was quantitatively small compared to PEPC
b. On this basis and considering that RuBPC
b increased during the bloom, it is suggested that, although PEPC may be involved in dark carbon fixation, its most important quantitative role is probably an indirect one in light dependent photosynthesis.We have also considered the relevance of laboratory results on photoadaptation to interpretations of field studies and have suggested that batch culture studies must be treated with caution but that turbidistat and semi-continuous methods provide reasonable simulations of natural conditions.
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