Zbigniew S. Kolber

Bacterial photosynthesis in surface waters of the open ocean

The oxidation of the global ocean by cyanobacterial oxygenic photosynthesis, about 2,100 Myr ago, is presumed to have limited anoxygenic bacterial photosynthesis to oceanic regions that are both anoxic and illuminated. The discovery of oxygen-requiring photosynthetic bacteria about 20 years ago changed this notion, indicating that anoxygenic bacterial photosynthesis could persist under oxidizing conditions. However, the distribution of aerobic photosynthetic bacteria in the world oceans, their photosynthetic competence and their relationship to oxygenic photoautotrophs on global scales are unknown. Here we report the first biophysical evidence demonstrating that aerobic bacterial photosynthesis is widespread in tropical surface waters of the eastern Pacific Ocean and in temperate coastal waters of the northwestern Atlantic. Our results indicate that these organisms account for 2–5% of the photosynthetic electron transport in the upper ocean.

Representing key phytoplankton functional groups in ocean carbon cycle models: Coccolithophorids

Carbonates are the largest reservoirs of carbon on Earth. From mid-Mesozoic time, the biologically catalyzed precipitation of calcium carbonates by pelagic phytoplankton has been primarily due to the production of calcite by coccolithophorids. In this paper we address the physical and chemical processes that select for coccolithophorid blooms detected in Sea-viewing Wide Field-of-view Sensor (SeaWiFS) ocean color imagery. Our primary goal is to develop both diagnostic and prognostic models that represent the spatial and temporal dynamics of coccolithophorid blooms in order to improve our knowledge of the role of these organisms in mediating fluxes of carbon between the ocean, the atmosphere, and the lithosphere. On the basis of monthly composite images of classified coccolithophorid blooms and global climatological maps of physical variables and nutrient fields, we developed a probability density function that accounts for the physical chemical variables that predict the spatiotemporal distribution of coccolithophorids in the world oceans. Our analysis revealed that areas with sea surface temperatures (SST) between 3° and 15°C, a critical irradiance between 25 and 150 µmol quanta m-2 s-1, and decreasing nitrate concentrations (N/t < 0) are selective for upper ocean large-scale coccolithophorid blooms. While these conditions favor both Northern and Southern Hemisphere blooms of the most abundant coccolithophorid in the modern oceans, Emiliania huxleyi, the Northern and Southern Hemisphere populations of this organism are genetically distinct. Applying amplified fragment length polymorphism as a marker of genetic diversity, we identified two major taxonomic clades of E. huxleyi; one is associated with the Northern Hemisphere blooms, while the other is found in the Southern Hemisphere. We suggest a rule of “universal distribution and local selection”: that is, coccolithophorids can be considered cosmopolitan taxa, but their genetic plasticity provides physiological accommodation to local environmental selection pressure. Sea surface temperature, critical irradiance, and N/t were predicted for the years 2060–2070 using the NCAR Community Climate System Model to generate future monthly probability distributions of coccolithophorids based upon the relationships observed between the environmental variables and coccolithophorid blooms in modern oceans. Our projected probability distribution analysis suggests that in the North Atlantic, the largest habitat for coccolithophorids on Earth, the areal extent of blooms will decrease by up to 50% by the middle of this century. We discuss how the magnitude of carbon fluxes may be affected by the evolutionary success of coccolithophorids in future climate scenarios.

Nitrogen- and irradiance-dependent variations of the maximum quantum yield of carbon fixation in eutrophic, mesotrophic and oligotrophic marine systems

Abstract Natural variability of the maximum quantum yield of carbon fixation ( φ C max ), as determined from the initial slope of the photosynthesis-irradiance curve and from light absorption measurements, was studied at three sites in the northeast tropical Atlantic representing typical eutrophic, mesotrophic and oligotrophic regimes. At the eutrophic and mesotrophic sites, where the mixed layer extended deeper than the euphotic layer, all photosynthetic parameters were nearly constant with depth, and φ C max averaged between 0.05 and 0.03 molC (mol quanta absorbed) −1 , respectively. At the oligotrophic site, a deep chlorophyll maximum (DCM) existed and φ C max varied from ca 0.005 in the upper nutrient-depleted mixed layer to 0.063 below the DCM in stratified waters. firstly, φ C max was found roughly to covary with nitrate concentration between sites and with depth at the oligotrophic site, and secondly, it was found to decrease with increasing relative concentrations of non-photosynthetic pigments. The extent of φ C max variations directly related to nitrate concentration was inferred from variations in the fraction of functional PS2 reaction centers ( f ), measured using fast repetition rate fluorometry. Covariations between f and nitrate concentration indicate that the latter factor may be responsible for a 2-fold variation in φ C max . Moreover, partitioning light absorption between photosynthetic and non-photosynthetic pigments suggests that the variable contribution of the non-photosynthetic absorption may explain a 3-fold variation in φ C max , as indicated by variations in the effective absorption cross-section of photosystem 2 ( σ PS2 ). Results confirm the role of nitrate in φ C max variation, and emphasize those of light and vertical mixing.

Non-photochemical fluorescence quenching and the diadinoxanthin cycle in a marine diatom

The diadinoxanthin cycle (DD-cycle) in chromophyte algae involves the interconversion of two carotenoids, diadinoxanthin (DD) and diatoxanthin (DT). We investigated the kinetics of light-induced DD-cycling in the marine diatom Phaeodactylum tricornutum and its role in dissipating excess excitation energy in PS II. Within 15 min following an increase in irradiance, DT increased and was accompanied by a stoichiometric decrease in DD. This reaction was completely blocked by dithiothreitol (DTT). A second, time-dependent, increase in DT was detected ∼ 20 min after the light shift without a concomitant decrease in DD. DT accumulation from both processes was correlated with increases in non-photochemical quenching of chlorophyll fluorescence. Stern-Volmer analyses suggests that changes in non-photochemical quenching resulted from changes in thermal dissipation in the PS II antenna and in the reaction center. The increase in non-photochemical quenching was correlated with a small decrease in the effective absorption cross section of PS II. Model calculations suggest however that the changes in cross section are not sufficiently large to significantly reduce multiple excitation of the reaction center within the turnover time of steady-state photosynthetic electron transport at light saturation. In DTT poisoned cells, the change in non-photochemical quenching appears to result from energy dissipation in the reaction center and was associated with decreased photochemical efficiency. D1 protein degradation was slightly higher in samples poisoned with DTT than in control samples. These results suggest that while DD-cycling may dynamically alter the photosynthesis-irradiance response curve, it offers limited protection against photodamage of PS II reaction centers at irradiance levels sufficient to saturate steady-state photosynthesis.

Compensatory changes in Photosystem II electron turnover rates protect photosynthesis from photoinhibition

AbstractExposure of algae or higher plants to bright light can result in a photoinhibitory reduction in the number of functional PS II reaction centers (n) and a consequential decrease in the maximum quantum yield of photosynthesis. However, we found that light-saturated photosynthetic rates (Pmax) in natural phytoplankton assemblages sampled from the south Pacific ocean were not reduced despite photoinhibitory decreases in n of up to 52%. This striking insensitivity of Pmax to photoinhibition resulted from reciprocal increases in electron turnover (

Cyclic electron flow around Photosystem II in vivo.

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Natural Versus Anthropogenic Factors Affecting Low-Level Cloud Albedo over the North Atlantic.

)through the remaining functional PS II centers. Similar insensitivity of Pmax was also observed in low light adapted cultures of Thalassiosira weissflogii (a marine diatom), but not in high light adapted cells where Pmax decreased in proportion to n. This differential sensitivity to decreases in n occurred because