SummaryExcess illumination damages the photosynthetic apparatus with severe implications with regard to plant productivity. Unlike model organisms, the growth of Chlorella ohadii, isolated from desert soil crust, remains unchanged and photosynthetic O 2 evolution increases, even when exposed to irradiation twice that of maximal sunlight.Spectroscopic, biochemical and molecular approaches were applied to uncover the mechanisms involved.D1 protein in photosystem II (PSII) is barely degraded, even when exposed to antibiotics that prevent its replenishment. Measurements of various PSII parameters indicate that this complex functions differently from that in model organisms and suggest that C. ohadii activates a nonradiative electron recombination route which minimizes singlet oxygen formation and the resulting photoinhibition. The light-harvesting antenna is very small and carotene composition is hardly affected by excess illumination. Instead of succumbing to photodamage, C. ohadii activates additional means to dissipate excess light energy. It undergoes major structural, compositional and physiological changes, leading to a large rise in photosynthetic rate, lipids and carbohydrate content and inorganic carbon cycling.The ability of C. ohadii to avoid photodamage relies on a modified function of PSII and the dissipation of excess reductants downstream of the photosynthetic reaction centers. The biotechnological potential as a gene source for crop plant improvement is self-evident.

Previous studies of the mitochondrial carbonic anhydrase (mtCA) of Chlamydomonas reinhardtii showed that expression of the two genes encoding this enzyme activity required photosynthetically active radiation and a low CO 2 concentration. These studies suggested that the mtCA was involved in the inorganic carbon-concentrating mechanism. We have now shown that the expression of the mtCA at low CO 2 concentrations decreases when the external NH 4 ϩ concentration decreases, to the point of being undetectable when NH 4 ϩ supply restricts the rate of photoautotrophic growth. The expression of mtCA can also be induced at supra-atmospheric partial pressure of CO 2 by increasing the NH 4 ϩ concentration in the growth medium. Conditions that favor mtCA expression usually also stimulate anaplerosis. We therefore propose that the mtCA is involved in supplying HCO 3 Ϫ for anaplerotic assimilation catalyzed by phosphoenolpyruvate carboxylase, which provides C skeletons for N assimilation under some circumstances.In algae and plants, the tricarboxylic acid (TCA) cycle plays a fundamental biosynthetic role (Beardall and Raven, 1990). The removal of intermediates from this cycle to feed other biosynthetic pathways (of which amino acid synthesis is often quantitatively the most important) requires that the cycle is replenished of its intermediates via anaplerotic reactions (Beardall and Raven, 1990; Norici and Giordano, 2002). The anaplerotic reactions make use of inorganic carbon to build the C4 compounds in demand in the TCA cycle, via ␤-carboxylation (Beardall and Raven, 1990; Norici and Giordano, 2002). The provision of inorganic carbon for these reactions can be crucial to sustain amino acid and protein synthesis (among others; Norici and Giordano, 2002). Anaplerotic ␤-carboxylation therefore represents a pivotal intersection among the metabolisms of C and N. Consequently, mechanisms must exist to ensure that there is sufficient inorganic carbon to maintain anaplerosis at an appropriate rate, especially in conditions in which the dissolved inorganic carbon (DIC) in the cytosol may be limited, competition with other DIC-requiring pathways (mostly photosynthesis) is significant, and N assimilation is fast. Respiration and photorespiration are conveniently located (spatially and functionally) sources of CO 2 to supply reactions that replenish the TCA cycle. A mechanism that recovers respiratory CO 2 would therefore be a very effective way to ensure an appropriate flux of C to the TCA cycle via anaplerosis. However, respiration and photorespiration produce DIC in the form of CO 2 , whereas many ␤-carboxylases such as phosphoenolpyruvate carboxylase (PEPc) and pyruvate carboxylase (PC) require HCO 3 Ϫ (Chollet et al., 1996; Norici and Giordano, 2002). Thus, if the uncatalyzed rate of CO 2 conversion to HCO 3 Ϫ is not sufficiently high, enzymatic hydration of CO 2 to HCO 3 Ϫ may be necessary (Raven and Newman, 1994; Huang and Chapman, 2002). Reactions of this sort are catalyzed by carbonic anhydrases (CAs), whose activity is wi...

Contents SummarySulfur emission from marine phytoplankton has been recognized as an important factor for global climate and as an entry into the biogeochemical S cycle. Despite this significance, little is known about the cellular S metabolism in algae that forms the basis of this emission. Some biochemical and genetic evidence for regulation of S uptake and assimilation is available for the freshwater model alga Chlamydomonas . However, the marine environment is substantially different from most freshwaters, containing up to 50 times higher free sulfate concentrations and challenging the adaptive mechanisms of primary and secondary S metabolism in marine algae. This review intends to integrate ecological and physiological data to provide a comprehensive view of the role of S in the oceans.

Information on interaction of C and N at the cellular level is lacking for ecologically relevant phytoplankton species. We examined the effects of NO 3 -availability on C and N fluxes in the widely distributed marine coccolithophore Emiliania huxleyi. Cells were cultured at replete (∼280 μM) and ambient (∼10 μM) NO 3 -, the latter representing a typical surface water nitrate concentration of the North Atlantic Ocean during spring. While growth rates and C to N ratios were not altered by the NO 3 -availability, organic C and N as well as inorganic C quotas were reduced under ambient NO 3 -. Growth at ambient NO 3 -caused a higher proportion of fixed C to be allocated to lipids relative to carbohydrates and especially to proteins. Ambient NO 3 --grown cells showed lower ). The CO 2 uptake and the maximum light use efficiency of photosynthesis (α) were unaffected by the concentration of NO 3 -. The affinities of NR for NO 3 -, of NiR for NO 2 -, of GS for Glu, and of the inorganic carbon uptake system for HCO 3 -were higher under ambient NO 3 -(ambient/replete: K m NR = 0.074/0.099 mM; K m NiR = 1.69/3.14 mM; K m GS = 1.62/ 3.81 mM; K m HCO3 -= 195/524 μM). Our data suggest that a concerted regulation of the intracellular CO 2 and NO 3 -concentrations is required to maintain balanced C and N metabolic fluxes resulting in a constant C to N ratio.

Fourier transform infrared (FTIR) spectra were measured from cells of Microcystis aeruginosa and Protoceratium reticulatum, whose growth rates were manipulated by the availability of nutrients or light. As expected, the macromolecular composition changed in response to the treatments. These changes were species-specific and depended on the type of perturbation applied to the growth regime. Microcystis aeruginosa showed an increase in the carbohydrate-to-protein ratio with decreased growth rates, under nutrient limitation, whereas light limitation induced a decrease of the carbohydrate-to-protein ratio with decreasing proliferation rates. The macromolecular pools of P. reticulatum showed a higher degree of compositional homeostasis. Only when the lowest light irradiance and nutrient availability were supplied, an increase of the carbohydrate-to-protein FTIR absorbance ratio was observed. A species-specific partial least squares (PLS) model was developed using the whole FTIR spectra. This model afforded a very high correlation between the predicted and the measured growth rates, regardless of the growth conditions. On the contrary, the prediction based on absorption band ratios generally used in FTIR studies would strongly depend on growth conditions. This new computational method could constitute a substantial improvement in the early warning systems of algal blooms and, in general, for the study of algal growth, e.g. in biotechnology. Furthermore, these results confirm the suitability of FTIR spectroscopy as a tool to map complex biological processes like growth under different environmental conditions.

ABSTRACT. Inorganic carbon acquisition, fixation and allocation, and silicic acid and orthophosphate uptake were also studied. The C : P ratio was below the Redfield ratio, especially at LL. In HL cells, N quota was halved, C quota was similar, silica quota was lower, growth rate and long-term net primary productivity were almost doubled, relative to LL cells. The HL : LL cell quota ratios were 6 for lipid, 0.5 for protein and 0.4 for carbohydrate. Phosphoenolpyruvate carboxylase (PEPc) and glutamine synthetase (GS) activities were unaffected by the growth irradiance; phosphoenolpyruvate carboxykinase (PEPck) was 2.5-fold more active in LL cells. This suggests that in S. marinoi, C4 photosynthesis is unlikely, PEPc is anaplerotic and PEPck may be involved in the conversion of lipid C to carbohydrates, especially in LL cells. Because about 50% of the cost for the production of an HL cell is caused by lipid biosynthesis, we propose that the preferential allocation of C to lipid at HL takes advantage of the relatively high volumebased energy content of lipids, in an organism that reduces its size at each vegetative cell division.

Sulfur is one of the critical elements in living matter, as it participates in several structural, metabolic and catalytic activities. Photosynthesis is an important process that entails the use of sulfur during both the light and carbon reactions. Nearly half of global photosynthetic carbon fixation is carried out by phytoplankton in the aquatic environment. Aquatic environments are very different from one another with respect to sulfur content: while in the oceans sulfate concentration is constantly high, freshwaters are characterized by daily and seasonal variations and by a wide range of sulfur concentration. The strategies that algal cells adopt for energy and resource allocation often reflect these differences. In the oceans, the amount and chemical form of sulfur has changed substantially during the course of the Earth's history; it is possible that sulfur availability played a role in the evolution of marine phytoplankton communities and it may continue to have appreciable effects on global biogeochemistry and ecology. Phytoplankton is also the main biogenic source of sulfur; sulfur can be released into the atmosphere by algal cells as dimethylsulfide, with possibly important repercussions on global climate. These and related matters are discussed in this review.

Short-term acclimation response of individual cells of Thalassiosira weissflogii was monitored by Synchrotron FTIR imaging over the span of 75 minutes. The cells, collected from batch cultures, were maintained in a constant flow of medium, at an irradiance of 120 μmol m-2 s-1 and at 20 °C. Multiple internal reflections due to the micro fluidic channel were modeled, and showed that fringes are additive sinusoids to the pure absorption of the other components of the system. Preprocessing of the hyperspectral cube (x, y, Abs(λ)) included removing spectral fringe using an EMSC approach. Principal component analysis of the time series of hyperspectral cubes showed macromolecular pool variations (carbohydrates, lipids and DNA/RNA) of less than 2% after fringe correction.