SUMMARY Cyanobacteria are ecologically important photosynthetic prokaryotes that also serve as popular model organisms for studies of photosynthesis and gene regulation. Both molecular and ecological studies of cyanobacteria benefit from real-time information on photosynthesis and acclimation. Monitoring in vivo chlorophyll fluorescence can provide noninvasive measures of photosynthetic physiology in a wide range of cyanobacteria and cyanolichens and requires only small samples. Cyanobacterial fluorescence patterns are distinct from those of plants, because of key structural and functional properties of cyanobacteria. These include significant fluorescence emission from the light-harvesting phycobiliproteins; large and rapid changes in fluorescence yield (state transitions) which depend on metabolic and environmental conditions; and flexible, overlapping respiratory and photosynthetic electron transport chains. The fluorescence parameters FV/FM, FV′/FM′,qp,qN, NPQ, and φPS II were originally developed to extract information from the fluorescence signals of higher plants. In this review, we consider how the special properties of cyanobacteria can be accommodated and used to extract biologically useful information from cyanobacterial in vivo chlorophyll fluorescence signals. We describe how the pattern of fluorescence yield versus light intensity can be used to predict the acclimated light level for a cyanobacterial population, giving information valuable for both laboratory and field studies of acclimation processes. The size of the change in fluorescence yield during dark-to-light transitions can provide information on respiration and the iron status of the cyanobacteria. Finally, fluorescence parameters can be used to estimate the electron transport rate at the acclimated growth light intensity.

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During winter and early spring, evergreen boreal conifers are severely stressed because light energy cannot be used when photosynthesis is pre-empted by low ambient temperatures. To study photosynthetic performance dynamics in a severe boreal climate, seasonal changes in photosynthetic pigments, chloroplast proteins and photochemical efficiency were studied in a Scots pine forest near Zotino, Central Siberia. In winter, downregulation of photosynthesis involved loss of chlorophylls, a twofold increase in xanthophyll cycle pigments and sustained high levels of the light stress-induced zeaxanthin pigment. The highest levels of xanthophylls and zeaxanthin did not occur during the coldest winter period, but rather in April when light was increasing, indicating an increased capacity for thermal dissipation of excitation energy at that time. Concomitantly, in early spring the D1 protein of the photosystem II (PSII) reaction centre and the light-harvesting complex of PSII dropped to their lowest annual levels. In April and May, recovery of PSII activity, chloroplast protein synthesis and rearrangements of pigments were observed as air temperatures increased above 0 1C. Nevertheless, severe intermittent low-temperature episodes during this period not only halted but actually reversed the physiological recovery. During these spring low-temperature episodes, protective processes involved a complementary function of the PsbS and early lightinduced protein thylakoid proteins. Full recovery of photosynthesis did not occur until the end of May. Our results show that even after winter cold hardening, photosynthetic activity in evergreens responds opportunistically to environmental change throughout the cold season. Therefore, climate change effects potentially improve the sink capacity of boreal forests for atmospheric carbon. However, earlier photosynthesis in spring in response to warmer temperatures is strongly constrained by environmental variation, counteracting the positive effects of an early recovery process. PPFD 5 photosynthetic photon flux density (mmol photons m À2 s À1 ) PSI or PSII 5 photosystem I or photosystem II, respectively PsbS 5 protein involved in nonphotochemical quenching RC 5 reaction centre of the photosystems V 5 violaxanthin Z 5 zeaxanthin

Marine phytoplankton show complex community structures and biogeographic distributions, the net results of physiological and ecological trade-offs of species responses to fluctuating, heterogeneous environments. We analysed photosynthesis, responses to variable light and macromolecular allocations across a size panel of marine centric diatoms. The diatoms have strong capacities to withstand and exploit fluctuating light, when compared with picophytoplankton. Within marine diatoms, small species show larger effective cross-sections for photochemistry, and fast metabolic repair of photosystem II after photoinactivation. In contrast, large diatoms show lower susceptibility to photoinactivation, and therefore incur lower costs to endure short-term exposures to high light, especially under conditions that limit metabolic rates. This size scaling of key photophysiological parameters thus helps explain the relative competitive advantages of larger versus smaller species under different environmental regimes, with implications for the function of the biogenic carbon pump. These results provide a mechanistic framework to explain and predict shifts in marine phytoplankton community size structure with changes in surface irradiance and mixed layer depth.

A continuous seawater sulfate sulfur isotope curve for the Cenozoic with a resolution of ϳ1 million years was generated using marine barite. The sulfur isotopic composition decreased from 19 to 17 per mil between 65 and 55 million years ago, increased abruptly from 17 to 22 per mil between 55 and 45 million years ago, remained nearly constant from 35 to ϳ2 million years ago, and has decreased by 0.8 per mil during the past 2 million years. A comparison between seawater sulfate and marine carbonate carbon isotope records reveals no clear systematic coupling between the sulfur and carbon cycles over one to several millions of years, indicating that changes in the burial rate of pyrite sulfur and organic carbon did not singularly control the atmospheric oxygen content over short time intervals in the Cenozoic. This finding has implications for the modeling of controls on atmospheric oxygen concentration.Global changes in climate and atmospheric chemistry are intimately related to the sedimentary S cycle (1, 2). Seawater sulfate represents one of the main reservoirs of dissolved S (3). Evidence for large-scale transfers of S between different sedimentary reservoirs is provided by the evaporite-based isotope record of oceanic sulfate (4 -6). Here, we present a high-resolution, continuous seawater S isotope age curve ( Fig. 1) that could help to elucidate the factors affecting the cycles of S and C and atmospheric oxygen content over geologic time.The concentration of dissolved sulfate in the ocean and its ␦ 34 S value are controlled by (i) a balance between the flux of S to the oceans from continental weathering and its isotopic value [␦ 34 S of this source is variable but is typically lower than seawater, 0 to 10 per mil (1)], and (ii) the deposition rate of S-bearing sediments, including evaporite sulfates with ␦ 34 S values approximately equal to that of seawater, and pyrite sulfide with a large isotopic fractionation signature [for example, modern ␦ 34 S values of pyrites average about -20 per mil (7)]. Volcanism and hydrothermal activity are also sources of reduced sulfur to the ocean [␦ 34 S values of these sources vary from about 0 to 3.5 per mil (1,8,9)]. Therefore, considerable S isotopic variations can be generated. For example, during periods of enhanced pyrite formation and burial, the light S isotope is preferentially removed from the ocean and the ␦ 34 S value of seawater sulfate increases. These and other fluctuations are recorded in contemporaneous evaporites and marine barite. Suitability of barite for recording seawater sulfate isotopic composition. Records of the S isotopic composition of seawater sulfate have been obtained mostly from isotope data on marine evaporite sulfates [for reviews see (5,6,10)], but the geological record of marine evaporites is episodic, with gaps of millions of years (5). Evaporites are also susceptible to diagenesis, their age control is often problematic (3), and a purely marine origin of some is questionable (11). In another attempt, Burdett et al. (12) used the S...