Daily and seasonal dynamics of remotely sensed photosynthetic efficiency in tree canopies.

The photosynthesis of various species or even a single plant varies dramatically in time and space, creating great spatial heterogeneity within a plant canopy. Continuous and spatially explicit monitoring is, therefore, required to assess the dynamic response of plant photosynthesis to the changing environment. This is a very challenging task when using the existing portable field instrumentation. This paper reports on the application of a technique, laser-induced fluorescence transient (LIFT), developed for ground remote measurement of photosynthetic efficiency at a distance of up to 50 m. The LIFT technique was used to monitor the seasonal dynamics of selected leaf groups within inaccessible canopies of deciduous and evergreen tree species. Electron transport rates computed from LIFT measurements varied over the growth period between the different species studied. The LIFT canopy data and light-use efficiency measured under field conditions correlated reasonably well with the single-leaf pulse amplitude-modulated measurements of broadleaf species, but differed significantly in the case of conifer tree species. The LIFT method has proven to be applicable for a remote sensing assessment of photosynthetic parameters on a diurnal and seasonal scale; further investigation is, however, needed to evaluate the influence of complex heterogeneous canopy structures on LIFT-measured chlorophyll fluorescence parameters.

Remote chlorophyll fluorescence measurements with the laser-induced fluorescence transient approach.

The interaction of plants with their environment is very dynamic. Studying the underlying processes is important for understanding and modeling plant response to changing environmental conditions. Photosynthesis varies largely between different plants and at different locations within a canopy of a single plant. Thus, continuous and spatially distributed monitoring is necessary to assess the dynamic response of photosynthesis to the environment. Limited scale of observation with portable instrumentation makes it difficult to examine large numbers of plants under different environmental conditions. We report here on the application of a recently developed technique, laser-induced fluorescence transient (LIFT), for continuous remote measurement of photosynthetic efficiency of selected leaves at a distance of up to 50 m. The ability to make continuous, automatic, and remote measurements of photosynthetic efficiency of leaves with the LIFT provides a new approach for studying the interaction of plants with the environment and may become an important tool in phenotyping photosynthetic properties in field applications.

VARIATION IN THE ABUNDANCE OF SYNECHOCOCCUS SP. CC9311 NARB MRNA RELATIVE TO CHANGES IN LIGHT, NITROGEN GROWTH CONDITIONS AND NITRATE ASSIMILATION(1).

Synechococcus- and Prochlorococcus-specific narB genes that encode for an assimilatory nitrate reductase are found in coastal to open-ocean waters. However, it remains uncertain if these picocyanobacteria assimilate nitrate in situ. This unknown can potentially be addressed by examining narB mRNA from the environment, but this requires a better understanding of the influence of environmental factors on narB gene transcription. In laboratory experiments with Synechococcus sp. CC9311 cultures exposed to diel light fluctuations and grown on nitrate or ammonium, there was periodic change in narB transcript abundance. This periodicity was broken in cultures subjected to a doubling of irradiance (40-80 µmol photons · m(-2) · s(-1) ) during the mid-light period. Therefore, the irradiance level, not circadian rhythm, was the dominant factor controlling narB transcription. In nitrate-grown cultures, diel change in narB transcript abundance and nitrate assimilation rate did not correlate; suggesting narB mRNA levels better indicate nitrate assimilation activity than assimilation rate. Growth history also affected narB transcription, as changes in narB mRNA levels in nitrogen-deprived CC9311 cultures following nitrate amendment were distinct from cultures grown solely on nitrate. Environmental sampling for narB transcripts should consider time, irradiance, and the growth status of cells to ecologically interpret narB transcript abundances.

NITROGEN FIXATION, HYDROGEN CYCLING, AND ELECTRON TRANSPORT KINETICS IN TRICHODESMIUM ERYTHRAEUM (CYANOBACTERIA) STRAIN IMS101(1).

This study describes the relationships between dinitrogen (N2 ) fixation, dihydrogen (H2 ) production, and electron transport associated with photosynthesis and respiration in the marine cyanobacterium Trichodesmium erythraeum Ehrenb. strain IMS101. The ratio of H2 produced:N2 fixed (H2 :N2 ) was controlled by the light intensity and by the light spectral composition and was affected by the growth irradiance level. For Trichodesmium cells grown at 50 µmol photons · m(-2) · s(-1) , the rate of N2 fixation, as measured by acetylene reduction, saturated at light intensities of 200 µmol photons · m(-2) · s(-1) . In contrast, net H2 production continued to increase with light levels up to 1,000 µmol photons · m(-2) · s(-1) . The H2 :N2 ratios increased monotonically with irradiance, and the variable fluorescence measured using a fast repetition rate fluorometer (FRRF) revealed that this increase was accompanied by a progressive reduction of the plastoquinone (PQ) pool. Additions of 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB), an inhibitor of electron transport from PQ pool to PSI, diminished both N2 fixation and net H2 production, while the H2 :N2 ratio increased with increasing level of PQ pool reduction. In the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), nitrogenase activity declined but could be prolonged by increasing the light intensity and by removing the oxygen supply. These results on the coupling of N2 fixation and H2 cycling in Trichodesmium indicate how light intensity and light spectral quality of the open ocean can influence the H2 :N2 ratio and modulate net H2 production.

Hydrogen cycling by the unicellular marine diazotroph Crocosphaera watsonii strain WH8501.

The hydrogen (H2) cycle associated with the dinitrogen (N2) fixation process was studied in laboratory cultures of the marine cyanobacterium Crocosphaera watsonii. The rates of H2 production and acetylene (C2H2) reduction were continuously measured over the diel cycle with simultaneous measurements of fast repetition rate fluorometry and dissolved oxygen. The maximum rate of H2 production was coincident with the maximum rates of C2H2 reduction. Theoretical stoichiometry for N2 fixation predicts an equimolar ratio of H2 produced to N2 fixed. However, the maximum rate of net H2 production observed was 0.09 nmol H2 µg chlorophyll a (chl a)⁻¹ h⁻¹) compared to the N2 fixation rate of 5.5 nmol N2 µg chl a⁻¹ h⁻¹, with an H2 production/N2 fixation ratio of 0.02. The 50-fold discrepancy between expected and observed rates of H2 production was hypothesized to be a result of H2 reassimilation by uptake hydrogenase. This was confirmed by the addition of carbon monoxide (CO), a potent inhibitor of hydrogenase, which increased net H2 production rates ∼40-fold to a maximum rate of 3.5 nmol H2 µg chl a⁻¹ h⁻¹. We conclude that the reassimilation of H2 by C. watsonii is highly efficient (> 98%) and hypothesize that the tight coupling between H2 production and consumption is a consequence of fixing N2 at nighttime using a finite pool of respiratory carbon and electrons acquired from daytime solar energy capture. The H2 cycle provides unique insight into N2 fixation and associated metabolic processes in C. watsonii.

Diel changes in bacteriochlorophyll a concentration suggest rapid bacterioplankton cycling in the Baltic Sea.

Aerobic anoxygenic phototrophs were recently found to constitute a significant portion of the marine microbial community. These bacteria use bacteriochlorophyll-containing reaction centers to perform photoheterotrophic metabolism. A new instrument for routine measurements of both chlorophyll a and bacteriochlorophyll a was used for monitoring anoxygenic phototrophs in the Baltic Sea in late summer 2003. Bacteriochlorophyll a concentration ranged from 8 to 50 ngl(-1), with an average bacteriochlorophyll/chlorophyll ratio of 4.2 x 10(-3). Moreover, diel trends in bacteriochlorophyll a signals were observed, with a distinct decline occurring during daylight hours. Based on laboratory measurements this phenomenon was ascribed to the complete inhibition of bacteriochlorophyll synthesis by light, which, in combination with a concurrent turnover of the cells, resulted in a pigment decline. Following this explanation, we postulate that bacteriochlorophyll a can serve as a natural 'pulse-and-chase' marker, allowing estimation of the mortality rates of anoxygenic phototrophs from the rates of pigment decline. Based on this assumption, we suggest that the Baltic photoheterotrophic community was characterized by high turnover rates, in a range of 0.7-2 d(-1).

Sequential assembly of photosynthetic units in Rhodobacter sphaeroides as revealed by fast repetition rate analysis of variable bacteriochlorophyll a fluorescence.

The development of functional photosynthetic units in Rhodobacter sphaeroides was followed by near infra-red fast repetition rate (IRFRR) fluorescence measurements that were correlated to absorption spectroscopy, electron microscopy and pigment analyses. To induce the formation of intracytoplasmic membranes (ICM) (greening), cells grown aerobically both in batch culture and in a carbon-limited chemostat were transferred to semiaerobic conditions. In both aerobic cultures, a low level of photosynthetic complexes was observed, which were composed of the reaction center and the LH1 core antenna. Interestingly, in the batch cultures the reaction centers were essentially inactive in forward electron transfer and exhibited low photochemical yields F(V)/F(M), whereas the chemostat culture displayed functional reaction centers with a rather rapid (1-2 ms) electron transfer turnover, as well as a high F(V)/F(M) of approximately 0.8. In both cases, the transfer to semiaerobiosis resulted in rapid induction of bacteriochlorophyll a synthesis that was reflected by both an increase in the number of LH1-reaction center and peripheral LH2 antenna complexes. These studies establish that photosynthetic units are assembled in a sequential manner, where the appearance of the LH1-reaction center cores is followed by the activation of functional electron transfer, and finally by the accumulation of the LH2 complexes.

On the origin of cis-vaccenic acid photodegradation products in the marine environment.

The origin of 11-hydroxyoctadec-trans-12-enoic and 12-hydroxyoctadec-trans-10-enoic acids (photodegradation products of cis-vaccenic acid) in the marine environment was investigated. cis-Vaccenic acid is commonly used as a bacterial biomarker; however, in heterotrophic bacteria the observed rates of cis-vaccenic acid photodegradation are negligible. Here, two hypotheses explaining the source of the photoproducts were tested. According to the first hypothesis, the photoproducts originate from aerobic anoxygenic bacteria, i.e., photoheterotrophic organisms using bacteriochlorophyll-containing reaction centers. Alternatively, the photoproducts come from a heterotrophic bacterial community closely associated with senescent phytoplanktonic cells. cis-Vaccenic acid photodegradation was detected in both experimental setups. However, a detailed comparison of the cis-vaccenic acid photodegradation patterns with those observed in particulate matter samples of the DYFAMED station (Mediterranean Sea) suggests that photodegradation of heterotrophic bacteria attached to senescent phytoplanktonic cells constitutes the more likely source of cis-vaccenic acid oxidation products detected in situ.

Isolation and characterization of Erythrobacter sp. strains from the upper ocean.

Seven strains of marine aerobic anoxygenic phototrophs belonging to the genus Erythrobacter were isolated. The strains were characterized regarding their physiological and biochemical properties, 16S rDNA and pufM gene sequences, morphological features, substrate preference, as well as pigment and lipid composition. All strains had functional type-2 reaction centers containing bacteriochlorophyll, served by small, light-harvesting complex 1, and were photosynthetically competent. In addition, large pools of carotenoids were found, but only some of the accessory pigments transfer energy to the reaction centers. All of the isolates were facultative photoheterotrophs. They required an organic carbon substrate for growth; however, they are able to supplement a significant fraction of their metabolic requirements with photosynthetically derived energy.