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Crassulacean acid metabolism (CAM), a key adaptation of photosynthetic carbon fixation to limited water availability, is characterized by nocturnal CO2 fixation and daytime CO2 re-assimilation, which generally results in improved water-use efficiency. However, CAM plants display a remarkable degree of photosynthetic plasticity within a continuum of diel gas exchange patterns. Genotypic, ontogenetic and environmental factors combine to govern the extent to which CAM is expressed. The ecological diversity of CAM is mirrored by plasticity in a range of biochemical and physiological attributes. In C3/CAM-intermediate plants, limited water availability can induce or enhance the expression of CAM. CAM induction is controlled by a combination of transcriptional, post-transcriptional and post-translational regulatory events. Early events in CAM induction point to a requirement for calcium and calcium-dependent protein kinase activities. Gene discovery efforts, improved transformation technologies and genetic models for CAM plants, coupled with detailed physiological investigations, will lead to new insights into the molecular genetic basis of induction processes and the circadian oscillator that governs carbon flux during CAM. Future integration of genomic, biochemical and physiological approaches in selected CAM models promise to provide a detailed view of the complex regulatory dynamics involved in CAM induction and modulation by water deficit. Such information is expected to have broad significance as the ecological and agricultural importance of CAM species increases in the face of global warming trends and the associated expansion of desertification in semi-arid regions around the world.

To test the possibility of inorganic carbon limitation of the marine unicellular alga Emiliania huxleyi (Lohmann) Hay and Mohler, its carbon acquisition was measured as a function of the different chemical species of inorganic carbon present in the medium. Because these different species are interdependent and covary in any experiment in which the speciation is changed, a set of experiments was performed to produce a multidimensional carbon uptake scheme for photosynthesis and calcification. This scheme shows that CO2 that is used for photosynthesis comes from two sources. The CO2 in seawater supports a modest rate of photosynthesis. The HCO3- is the major substrate for photosynthesis by intracellular production of CO2 (HCO3- + H+ → CO2 + H2O → CH2O + O2). This use of HCO3- is possible because of the simultaneous calcification using a second HCO3- , which provides the required proton (HCO3- + Ca2+ → CaCO3 + H+). The HCO3- is the only substrate for calcification. By distinguishing the two sources of CO2 used in photosynthesis, it was shown that E. huxleyi has a K½ for external CO2 of ‘‘only’’ 1.9 ± 0.5 µM (and a Vmax of 2.4 ± 0.1 pmol·cell-1·d-1). Thus, in seawater that is in equilibrium with the atmosphere ([CO2] = 14 µM, [HCO3-] = 1920 µM, at fCO2 = 360 µatm, pH = 8, T = 15° C), photosynthesis is 90% saturated with external CO2. Under the same conditions, the rate of photosynthesis is doubled by the calcification route of CO2 supply (from 2.1 to 4.5 pmol·cell-1·d-1). However, photosynthesis is not fully saturated, as calcification has a K½ for HCO3- of 3256 ± 1402 µM and a Vmax of 6.4 ± 1.8 pmol·cell-1·d-1. The H+ that is produced during calcification is used with an efficiency of 0.97 ± 0.08, leading to the conclusion that it is used intracellularly. A maximum efficiency of 0.88 can be expected, as NO3- uptake generates a H+ sink (OH- source) for the cell. The success of E. huxleyi as a coccolithophorid may be related to the efficient coupling between H+ generation in calcification and CO2 fixation in photosynthesis.

Germanic acid (/sup 68/Ge) was used as a tracer to study the uptake and incorporation of silicic acid by natural phytoplankton populations in the Gulf of California. The technique was useful in measuring silicic acid incorporation rates as low as 0.5 nM h/sup -1/. The incorporation followed saturation kinetics. K/sub s/ values at two stations were 1.59 and 2.53 ..mu..M. The incorporation was light dependent, maximum rates being achieved at 0.28 x 10/sup 21/ photons cm/sup -2/ d/sup -1/ (0.044 ly min/sup -1/; 9 percent of surface irradiance) and half-maximum rate at 0.02 x 10/sup 21/ photons cm/sup -2/ d/sup -1/ (0.003 ly min/sup -1/; 0.6 percent of surface irradiance); rate of incorporation in the dark was 44 percent of that at light saturation. Enrichment with silicic acid caused stimulation of photosynthetic carbon fixation at two stations.

Dunaliella tertiolecta (Butcher) was grown in chemostat culture over a wide range of ammonium limited growth rates. The addition of ammonium caused a rapid temporary suppression of photosynthetic carbon fixation. The magnitude of ammonium induced photosynthetic suppression increased with the severity of ammonium limitation. Cells growing at rates greater than ca. 80%μmax exhibited no photosynthetic suppression in response to additions of the limiting nutrient. The duration of photosynthetic suppression was related to the concentration of added ammonium. Immediately following the suppression, photosynthesis was enhanced with respect to the controls. The eventual degree of enhancement increased with the concentration of added ammonium. Steady-state cellular chlorophyll concentrations, photosynthetic rates, and assimilation numbers are reported.

Organelles morphologically similar to microbodies have been found in several tissues of atmospheric species of Tillandsia from different habitats. The presence of catalase was demonstrated by the DAB reaction thus confirming the microbody nature of these organelles. They are a feature of the Tillandsia species with normal photosynthetic carbon fixation and with CAM. Their size is consistently small. The nucleoid observed in the microbodies shows a characteristic morphology which has not been reported before within other plant microbodies. This nucleoid is composed of minute tubular structures, for which the authors here propose a three-dimensional arrangement.

Seven species of marine microalgae including chlorophytes, diatoms, and a chrysomonad, of tropical, Antarctic, and temperate origin, were used to measure photosynthetic carbon fixation as affected by concentration of dissolved O2 at two levels of CO2. An apparent O2 inhibition of carbon fixation was observed in some species, but others were affected little or not at all by concentration of dissolved O2. In several cases, O2 appeared necessary for maximal carbon fixation.