Photoadaptation in marine phytoplankton: variations in ribulose 1,5-bisphos-phate activity

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Abstract
The rates of photosynthesis and the activity of ribulose 1,5-bisphosphate carboxylase (RuBPCase) were concurrently measured for phytoplankton in laboratory cultures and isolated from natural populations. Both photosynthetic capacity (Pmax) and RuBPCase activity were greater for phytoplankton adapted to high than low irradiances. Using single-species radiosotope techniques, the irradiance-dependent rates of photosynthesis and the RuBPCase activity of Ditylum brightwellii and Ceratium lineatum, 2 common coastal phytoplankton, were measured at a Chesapeake Bay, USA, plume front. The photosynthetic characteristics and RuBPCase were clearly dependent on the photic regime. The species-specific Pmax and RuBPCase activity was similar for phytoplankton isolated throughout the water column on the well-mixed, oceanic side of the front and within the surface mixed-layer on the stratified, estuarine side of the front. In contrast, these photosynthetic parameters were significantly lower for the same isolated from species isolated from below the pycnocline upstream of the frontal interface. Changes in RuBPCase correlated (p = 0.01) with those in Pmax; for light-limited and -saturated phytoplankton in culture and from natural populations, the relationship was linear and significant. The coordinate pairs of Pmax and RuBPCase from all the laboratory and field, i.e. all stations and depths, experiments were fit to the regression (n = 84) RuBPCase = -1.686(.+-.7.675) + 0.677.cntdot.Pmax(.+-.0.043); r2 = 0.872. Variations in Pmax are commonly used to describe the photoadaptations of phytoplankton from different photic regimes. The results of this study suggest that the temporal and spatial variations in RuBPCase may be a useful photoadaptive parameter for phytoplankton in nature.