Anton F. Post

Kinetics of light-intensity adaptation in a marine planktonic diatom

The marine planktonic diatom Thalassiosira weisflogii was grown in turbidostat culture under both continuous and 12 hL: 12 hD illumination regimes in order to study the kinetics of adaptation to growth-irradiance levels. In both illumination regimes adaptation to a higher growth-irradiance level was accompanied by an increase in cell division rates and a decrease in chlorophyll a cell-1. The rates of adaptation for both processes, derived from first order kinetic analysis, equaled each other in each experiment. The results suggest that during the transition from low-to-high growth-irradiance levels chlorophyll a is diluted by cell division and is not actively degraded. Introduction of a light/dark cycle lowered the rate of adaptation. In transitions from high-to-low growth-irradiance levels there was a sharp drop in growth rates and a slow increase in chlorophyll a cell-1 under both continuous and intermittent illumination. In the 12 hL:12hD cycle there was a circadian rhythm in chlorophyll a cell-1, where cellular chlorophyll contents increased during the light cycle and decreased during the dark cycle. This circadian rhythm was distinctly different from light intensity adaptation. For kinetic analysis of light intensity adaptation in a 12 hL: 12 hD cycle, the circadian periodicity was separated from the light intensity response by subjecting the data to a Kaiser window optimization digital filter. Kinetic parameters for light-intensity adaptation were resolved from the filtered data. The kinetics of lightintensity adaptation of marine phytoplankton are discussed in relation to their spatial variations and time scales of mixing.

Geosmin production in the cyanobacterium Oscillatoria brevis

The cyanobacterium Oscillatoria brevis (Kütz.) Gom., strain NIVA CYA 7, was used to investigate how geosmin production is related to the synthesis of chlorophyll a, phycobiliproteins and β-carotene under nitrogen (NH4+) and light limiting conditions. Chemostat samples were used to inoculate batch cultures that were treated with inhibitors of isoprenoid synthesis, norflurazon and dimethazone, and gabaculine that inhibits tetrapyrrole synthesis. Dimethazone decreased and norflurazon increased geosmin production under light limited conditions, as was expected due to their sites of action in the isoprenoid pathway. This effect was not so pronounced in nitrogen limited cultures due to the additional effect of increasing nitrogen deficiency during the experimental period. Norflurazon was the only inhibitor that uncoupled geosmin production completely from β-carotene formation which indicates a strikt coupling between geosmin and β-carotene biosynthesis. From the observed increase of geosmin production relative to pigment synthesis after norflurazon treatment it was suggested that isoprenoid precursors are directed to geosmin synthesis when the demand for pigment precursors is very low. Within the framework of this study the data strongly support the hypothesis of geosmin formation via the isoprenoid pathway in Oscillatoria brevis as was found for actinomycetes.

Transient states of geosmin, pigments, carbohydrates and proteins in continuous cultures of Oscillatoria brevis induced by changes in nitrogen supply

Transitions in the growth limiting factor from light (I) to nitrogen (N) and vice versa caused changes in geosmin production, protein and carbohydrate content, and the synthesis of pigments such as chlorophyll a (Chl a), phycobiliproteins (PBPs), and β-carotene of the cyanobacterium Oscillatoria brevis. Following I→N transition the first 150h, the decrease in protein content was compensated for by an increase of carbohydrates, and thereby, a constant biomass level was maintained in this period. Thereafter, biimass dropped to 15% of its initial level. A decrease in geosmin and pigment content was observed during transition from I→N-limited growth. However, geosmin increased relative to phytol (Chl a) and β-carotene which may indicate that a lowered demand for phytol and β-carotene during N-limited growth allows isoprenoid precursors to be directed to geosmin rather than to pigment synthesis. Synthesis of Chl a and β-carotene at the expense of geosmin was suggested for the observed start of increase in geosmin production only at the time that Chl a and β-carotene had reached their I-limited steady state. Transition from nitrogen to light limited growth caused an acceleration of metabolism shown by a rapid decrease in carbohydrate content accompanied by an increase in protein content. The growth rate of the organisms temporarily exceeded the dilution rate of the culture and the biomass level increased 6-fold. Due to the only modest changes in geosmin production (2-fold) compared to changes in biomass level (6-fold) during I-or N-limited growth, environmental factors seem to have limited effect on geosmin production.

Influence of light period length on photosynthesis and synchronous growth of the green alga Scenedesmus protuberans

Scenedesmus protuberans was grown in turbidostat cultures with various light/dark cycles. Cultures with a light period length of 4–16 h showed a spontaneous partial synchronization of the population. The maximal growth rate was related to the maximal rate of cell division when growing with a L/D cycle. The same μmax was found with continuous illumination. Photosynthetic activities showed daily variations, that were largely influenced by partial synchronization. For cultures with a light period length of 4–8 h photosynthetic activities showed high variations but no daily patterns. These variations were due to changes in population composition. Cultures with a light period length of 12–16 h showed photosynthetic activities with little variation, but with distinct daily patterns related to the phase of the life cycle of the synchronized part of the population. “Young” cells exhibited high photosynthetic activities, while for “old” cells photosynthesis was low, thus reflecting the metabolic needs of the cells...

The Influence of Periodicity in Light Conditions, as Determined by the Trophic State of the Water, on the Growth of the Green Alga Scenedesmus Protuberans and the Cyanobacterium Oscillatoria Agardhii

The green alga Scenedesmus protuberans and the cyanobacterium Oscillatoria agardhii were grown in dilute suspension in continuous cultures using the turbidostat technique and a nutrient-rich medium, in order to measure growth rate as a function of photoperiod length. A saturating irradiance was applied. The total length of the light-dark cycles was uniformly 24 hours.

Energy distribution between Photosystems I and II in the photosynthetic prokaryote Prochlorothrix hollandica involves a chlorophyll ab antenna which associates with Photosystem I

Abstract Prochlorothrix hollandica is a photosynthetic prokaryote, whose main thylakoid intrinsic chlorophyll a b antenna copurifies with PS I and is both structurally and functionally distinct from chloroplast LHC II. The 35 kDa apoprotein of the antenna forms the main target for light/redox controlled reversible phosphorylation (Post, A.F., Gal, A., Ohad, I., Milbauer, K.M. and Bullerjahn, G.S. (1992) Biochim. Biophys. Acta 1100, 75–82). The occurrence of state 1 → 2 transitions in cells illuminated with light 1 (710 nm) and light 2 (652 nm) was shown from differences in fluorescence properties using the chlorophyll fluorescence induction technique. The same technique showed that the redox state of the PQ pool responded to light conditions, being more oxidized in dark-incubated and light-1-illuminated cells. Following the transfer of light-1-treated cells to light 2 conditions, state 2 was reached in approx. 10 min. Addition of the phosphatase inhibitor NaF locked cells in state 2. These observations lend support to the hypothesis that the molecular mechanism driving the state 1 → 2 transitions involves the reversible phosphorylation of the main chlorophyll a b antenna. 77 K fluorescence spectra of whole cells and of PS I complexes obtained from detergent-treated thylakoids showed strong energy coupling between the antenna and PS I preferentially. Fluorescence quenching measurements showed an increase in PS I activity during a state 1 → 2 transition. These observations suggest that during a state 1 → 2 transition an increasing fraction of the excitation energy arriving from the antenna is diverted to PS I. The antenna copurified with PS I complexes under all conditions examined. We have summarized the principal differences between eukaryotic and prokaryotic chlorophyll a b antennae. A model for the regulation of photosynthetic activity in P. hollandica is proposed and it involves light controlled reversible phosphorylation of the chlorophyll a b antenna. Our model claims that in state 1 (non-phosphorylating conditions) the bulk chlorophyll a b antenna is shared by both photosystems. Under phosphorylating conditions (state 2) the antenna associates more tightly with PS I, effectively reducing energy transfer to PS II centers. It is noted that reversible phosphorylation of a chlorophyll a b antenna may have evolved as a regulatory mechanism prior to the evolution of chloroplast LHC II.

Transient state characteristics of adaptation to changes in light conditions for the cyanobacterium Oscillatoria agardhii

Transitions in growth irradiance level from 92 to 7 μEm-2 s-1 and vice versa caused changes in the pigment contents and photosynthesis of Oscillatoria agardhii. The changes in chlorophyll a and C-phycocyanin contents during the transition from high to low irradiance (H→L) were reflected in photosynthetic parameters. In the L→H transition light utilization efficiencies of the cells changed faster than pigment contents. This appeared to be related to the lowering of light utilization efficiencies of photosynthesis. As a possible explanation it was hypothesized that excess photosynthate production led to feed back inhibition of photosynthesis. Time-scales of changes in the maximal rate of O2 evolution were discussed as changes in the number of reaction centers of photosystem II in relation to photosynthetic electron transport. Parameters that were subject to change during irradiance transitions obeyed first order kinetics, but hysteresis occurred when comparing H→L with L→H transients. Interpretation of first order kinetic analysis was discussed in terms of adaptive response vs changes in growth rate.

Transient state characteristics of changes in light conditions for the cyanobacterium Oscillatoria agardhii

The cyanobacterium Oscillatoria agardhii was subjected to changes in irradiance and to changes in light period. During transient states parameters as growth rate, pigment contents, photosynthetic activities and pool sizes of carbohydrate and proteins were followed. The changes in pigments and photosynthesis were similar for irradiance transitions and transitions in light period length. Carbohydrates served for the supply of carbon and energy during adaptation to low light conditions until a basal level of 125 μg · mg dry wt-1 was reached. After transfer to high light conditions excess carbon fixation led to the storage of carbohydrate reserve polymers up to 600 μg · mg dry wt-1. During adaptation to longer light periods cells showed an overcapacity for carbohydrate accumulation even in the presence of a high carbohydrate content at the start of the light period. A model for the feed back repression of photosynthesis related to carbohydrate accumulation was presented. In all cases protein synthesis was directly maximized under the given conditions. Growth rate defined as specific rate of change in carbon showed the fastest response after a shift in light conditions. It was concluded that adaptation of O. agardhii to changes in light conditions was directed to the optimization of growth. The observation that carbohydrate is used to supply carbon and/or energy during adaptation leads to the conclusion that changes on survival in low light depend on carbohydrate level, the efficiency of its conversion in cell material and the maintenance requirements. Such a survival strategy enables cyanobacteria to cope succesfully with light limiting conditions.

Light 2 directed changes in the effective absorption cross-section of Photosystem II in Synechocystis 27170 are related to modified action on the donor side of the reaction center

Synechocystis 27170 was grown in green, orange and red light in order to adapt cells to light preferentially absorbed by phycoerythrin, phycocyanin and chlorophyll a, respectively. Chromatically adapted cells were used to study effective absorption cross-sections of RCII, σeff, in state 1 and state 2. σeff for state 1 and state 2 in orange and green light grown cells was approximately twice that for red light grown cells. Light 1 and 2 both induced increased σeff in an intensity dependent way. The effect saturated at 40 μ mol quanta m−2 s−1 of light 2. Increased σeff invoked by light 1 was related to PSI activity only, since light 1 did not close significant numbers of RCII. Light 2 also effected an increased σeff, but in an inverse relationship with the fraction of open RCII traps. These data support the model of modulated energy transfer from PSII chlorophyll a to PSI as the mechanism for state 1 > 2 transitions in cyanobacteria. Although matching each other in light 1, PSII fluorescence yields deviated substantially from O2-flash yields in light 2 over a wide intensity range. Since the half-time of Q−A was equal in state 1 and 2, it was concluded that light 2 has a modifying effect on the RCII donor side, probably causing longer half-time of P+680, which delays the reopening of closed RCII. These results were interpreted as a feature of the RCII related to an increased half-time of an exciton in the PSII chlorophyll a antenna. This phenomenon is thought to be functional in facilitating energy transfer from PSII chlorophyll a to PSI.