Acclimation to one environmental factor may constrain acclimation to another. Synechococcus elongatus (sp. PCC7942), growing under continuous light in high inorganic carbon (Ci; approximately 4 mm) and low-Ci (approximately 0.02 mm) media, achieve similar photosynthetic and growth rates under continuous low or high light. During acclimation from low to high light, however, high-Ci cells exploit the light increase by accelerating their growth rate, while low-Ci cells maintain the prelight shift growth rate for many hours, despite increased photosynthesis under the higher light. Under increased light, high-Ci cells reorganize their photosynthetic apparatus by shrinking the PSII pool and increasing Rubisco pool size, thus decreasing the photosynthetic source-to-sink ratio. Low-Ci cells also decrease their reductant source-to-sink ratio to a similar level as the high-Ci cells, but do so only by increasing their Rubisco pool. Low-Ci cells thus invest more photosynthetic reductant into maintaining their larger photosystem pool and increasing their Rubisco pool at the expense of population growth than do high-Ci cells. In nature, light varies widely over minutes to hours and is ultimately limited by daylength. Photosynthetic acclimation in S. elongatus occurs in both high and low Ci, but low-Ci cells require more time to achieve acclimation. Cells that can tolerate low Ci do so at the expense of slower photosynthetic acclimation. Such differences in rates of acclimation relative to rates of change in environmental parameters are important for predicting community productivity under variable environments.

Lobaria pulmonaria (L.) Hoffm. is an epiphytic lichen common to temperate deciduous forests where it copes with large changes in temperature and light levels through repeated annual cycles. Samples of L. pulmonaria were taken from a deciduous forest in southeastern Canada at 35-day intervals from February 1999 to February 2000 and also from a rare population in an evergreen forest in March and August 1999. At field-ambient temperatures and light levels, the realised photosystem II (PSII) electron transport was low both in the summer and winter, with transient peaks in the spring and autumn. In contrast, the seasonal pattern of potential electron transport measured at a fixed 20 degrees C peaked in winter, showing the importance of temperature in driving photosynthesis to low levels in the winter despite an acclimation of electron-transport potential to exploit the high ambient light. Realised gross CO2 uptake was correlated with PSII electron transport at mechanistically plausible rates at all sampling sites in the summer but not in the winter, indicating electron diversion away from CO2 fixation in the winter. Chlorophyll content was highest in the dark summer months. The amount of ribulose-1,5-bisphosphate carboxylase-oxygenase (RuBisCO) large subunit (LSU) was highest in spring. Changes in the level of this hyperabundant protein and in the activity of PSII maintained a relatively constant rate of maximum CO2 uptake per RuBisCO LSU from April through November, despite great changes in the seasonal light and temperature. L. pulmonaria acclimates between light and temperature stress in the winter months to light-limitation in the dark summer months. Transition intervals in the spring and autumn, with warm, bright and wet conditions, are likely the most amenable times for growth.

The oceans globally constitute an important sink for carbon dioxide (CO2) due to phytoplankton photosynthesis. However, the marine environment imposes serious restraints to carbon fixation. First, the equilibrium between CO2 and bicarbonate (HCO3  −) is pH dependent, and, in normal, slightly alkaline seawater, [CO2] is typically low (approximately 10 μ  m). Second, the rate of CO2 diffusion in seawater is slow, so, for any cells unable to take up bicarbonate efficiently, photosynthesis could become carbon limited due to depletion of CO2 from their immediate vicinity. This may be especially problematic for those dinoflagellates using a form II Rubisco because this form is less oxygen tolerant than the usually found form I enzyme. We have identified a carbonic anhydrase (CA) from the free-living marine dinoflagellate Lingulodinium polyedrum that appears to play a role in carbon acquisition. This CA shares 60% sequence identity with δ-class CAs, isoforms so far found only in marine algae. Immunoelectron microscopy indicates that this enzyme is associated exclusively with the plasma membrane. Furthermore, this enzyme appears to be exposed to the external medium as determined by whole-cell CA assays and vectorial labeling of cell surface proteins with 125I. The fixation of 14CO2 is strongly pH dependent, suggesting preferential uptake of CO2 rather than HCO3  −, and photosynthetic rates decrease in the presence of 1 mm acetazolamide, a non-membrane-permeable CA inhibitor. This constitutes the first CA identified in the dinoflagellates, and, taken together, our results suggest that this enzyme may help to increase CO2 availability at the cell surface.

Lobaria pulmonaria (L.) Hoffm. survives large changes in ambient light and temperature between winter and summer in temperate deciduous forests. Potential photosystem II (PSII) electron transport, measured at 20°C using chlorophyll fluorescence analysis, was consistent in thallus samples taken in March and August from a temperate deciduous forest, while the potential for nonphotochemical quenching (NPQ) was higher in March than in August. NPQ was, however, similar in March and August in a population from a coniferous site with a permanently closed canopy. Thalli measured at a typical March field temperature showed a depression of PSII electron transport, qp and gross CO2 uptake and a rise in realized NPQ. Xanthophyll cycle pigments were more abundant in the March than in the August samples in the deciduous forest populations but did not change significantly in the permanently closed-canopy population. In August, relatively low NPQ correlated with xanthophyll pool size in the deciduous forest samples. The more intense NPQ derived from low temperature and low CO2-electron demand in the March samples, however, was not correlated with xanthophyll pool size. Thus, most of the NPQ observed in the high light exposure March samples was not explained by variation in xanthophyll pool size.Key words: carotenoids, excitation quenching, lichen, nonphotochemical quenching, PSII electron transport.