The increases in atmospheric pCO 2 over the last century are accompanied by higher concentrations of CO 2 (aq) in the surface oceans. This acidification of the surface ocean is expected to influence aquatic primary productivity and may also affect cyanobacterial nitrogen (N)-fixers (diazotrophs). No data is currently available showing the response of diazotrophs to enhanced oceanic CO 2 (aq). We examined the influence of pCO 2 [preindustrial $ 250 ppmv (low), ambient $ 400, future $ 900 ppmv (high)] on the photosynthesis, N fixation, and growth of Trichodesmium IMS101. Trichodesmium spp. is a bloom-forming cyanobacterium contributing substantial inputs of 'new N' to the oligotrophic subtropical and tropical oceans. High pCO 2 enhanced N fixation, C : N ratios, filament length, and biomass of Trichodesmium in comparison with both ambient and low pCO 2 cultures. Photosynthesis and respiration did not change significantly between the treatments. We suggest that enhanced N fixation and growth in the high pCO 2 cultures occurs due to reallocation of energy and resources from carbon concentrating mechanisms (CCM) required under low and ambient pCO 2 . Thus, in oceanic regions, where light and nutrients such as P and Fe are not limiting, we expect the projected concentrations of CO 2 to increase N fixation and growth of Trichodesmium. Other diazotrophs may be similarly affected, thereby enhancing inputs of new N and increasing primary productivity in the oceans.

Summary• Acclimation of hyperaccumulators to heavy metal-induced stress is crucial for phytoremediation and was investigated using the hyperaccumulator Thlaspi caerulescens and the nonaccumulators T. fendleri and T. ochroleucum .• Spatially and spectrally resolved kinetics of in vivo absorbance and fluorescence were measured with a novel fluorescence kinetic microscope.• At the beginning of growth on cadmium (Cd), all species suffered from toxicity, but T. caerulescens subsequently recovered completely. During stress, a few mesophyll cells in T. caerulescens became more inhibited and accumulated more Cd than the majority; this heterogeneity disappeared during acclimation. Chlorophyll fluorescence parameters related to photochemistry were more strongly affected by Cd stress than nonphotochemical parameters, and only photochemistry showed acclimation.• Cd acclimation in T. caerulescens shows that part of its Cd tolerance is inducible and involves transient physiological heterogeneity as an emergency defence mechanism. Differential effects of Cd stress on photochemical vs nonphotochemical parameters indicate that Cd inhibits the photosynthetic light reactions more than the Calvin-Benson cycle. Differential spectral distribution of Cd effects on photochemical vs nonphotochemical quenching shows that Cd inhibits at least two different targets in/around photosystem II (PSII). Spectrally homogeneous maximal PSII efficiency ( F v / F m ) suggests that in healthy T. caerulescens all chlorophylls fluorescing at room temperature are PSII-associated.Key words: acclimation, cadmium (Cd), heterogeneity, imaging and spectral measurements of chlorophyll fluorescence kinetics, metal sequestration, photosynthetic performance. as q CN = ( F m -F m ′ )/ F m = 'complete nonphotochemical quenching of Chl fluorescence' (i.e. with normalisation to F m ); OD, optical density; PSII, photosystem II; RC, photosynthetic reaction centre; Φ PSII = Φ e = ( F m ′ − F t ′ )/ F m ′ = effective quantum yield of photochemical energy conversion in actinic light (Genty et al. , 1989). Here, the values of this parameter were calculated also for responses to saturating flashes during the relaxation period after the end of actinic light in order to follow the return of the system to its dark-acclimated state as measured by F v / F m . AbbreviationsNew Phytologist (2007) 175 : [655][656][657][658][659][660][661][662][663][664][665][666][667][668][669][670][671][672][673][674]

The targets of heavy metal (here Cu2+ and Zn2+) attack on the photosynthetic apparatus of algae belonging to different phyla were investigated. Experiments with the green alga Scenedesmus quadricauda confirmed previous findings that according to the irradiance level two different phenomena occur, which were further characterized by specific changes in several photosynthetic parameters. The reaction occurring under low irradiance (shade reaction) is characterized by heavy metal substitution of Mg2+ in chl molecules bound predominantly in the light harvesting complex II of Chlorophyta (LHC II). Under high irradiance (sun reaction) the LHC II chls are inaccessible to substitution and the damage occurs in the PSII reaction center instead. Algae with antenna proteins other than the LHC II did not show the two types of heavy metal attack at different irradiances. In red algae (Antithamnion plumula), low Cu2+ concentrations induced the sun reaction even at very low irradiance. In brown algae (Ectocarpus siliculosus) the shade reaction occurred even in saturating irradiance. These results also indicate that despite some similarity in their features, the primary step of the sun reaction and photoinhibition is different.

We investigated interactions between photosynthesis and nitrogen fixation in the non-heterocystous marine cyanobacterium Trichodesmium IMS101 at the single-cell level by two-dimensional (imaging) microscopic measurements of chlorophyll fluorescence kinetics. Nitrogen fixation was closely associated with the appearance of cells with high basic fluorescence yield (F 0 ), termed bright cells. In cultures aerated with normal air, both nitrogen fixation and bright cells appeared in the middle of the light phase. In cultures aerated with 5% oxygen, both processes occurred at a low level throughout most of the day. Under 50% oxygen, nitrogen fixation commenced at the beginning of the light phase but declined soon afterwards. Rapid reversible switches between fluorescence levels were observed, which indicated that the elevated F 0 of the bright cells originates from reversible uncoupling of the photosystem II (PSII) antenna from the PSII reaction center. Two physiologically distinct types of bright cells were observed. Type I had about double F 0 compared to the normal F 0 in the dark phase and a PSII activity, measured as variable fluorescence (F v 5 F m 2 F 0 ), similar to normal non-diazotrophic cells. Correlation of type I cells with nitrogen fixation, oxygen concentration, and light suggests that this physiological state is connected to an up-regulation of the Mehler reaction, resulting in oxygen consumption despite functional PSII. Type II cells had more than three times the normal F 0 and hardly any PSII activity measurable by variable fluorescence. They did not occur under low-oxygen concentrations, but appeared under high-oxygen levels outside the diazotrophic period, suggesting that this state represents a reaction to oxidative stress not necessarily connected to nitrogen fixation. In addition to the two high-fluorescence states, cells were observed to reversibly enter a low-fluorescence state. This occurred mainly after a cell went through its bright phase and may represent a fluorescence-quenching recovery phase.Biological fixation of atmospheric nitrogen is performed by certain cyanobacteria when bioavailable forms of nitrogen (nitrate and ammonia) are limited. Nitrogen fixation is catalyzed by an essentially anaerobic enzyme, nitrogenase, which is irreversibly inhibited in vitro when exposed to molecular oxygen (for review, see Postgate, 1998). Diazotrophic cyanobacteria are the only nitrogen-fixing organisms that produce molecular oxygen as a by-product of photosynthesis and must prevent nitrogenase from being damaged by oxygenic photosynthesis (for review, see Gallon, 1992Gallon, , 2001Bergman et al., 1997;Berman-Frank et al., 2003). Most diazotrophic cyanobacteria achieve this by separating photosynthesis and nitrogen fixation either spatially, by differentiating highly specialized cells called heterocysts, or temporally, by fixing nitrogen at night (usually found in unicellular diazotrophic cyanobacteria). In contrast, filamentous non-heterocystous marine cyanobacteria of the genus Trichodesmium execut...

No abstract