Light distribution, transfer and utilization in the marine red alga Porphyra perforata from photoacoustic energy-storage measurements

David (Dave) Charles Fork (1929-2020): a gentle human being, a great experimenter, and a passionate researcher

We provide here an overview of the remarkable life and outstanding research of David (Dave) Charles Fork (March 4, 1929-December 13, 2021) in oxygenic photosynthesis. In the words of the late Jack Edgar Myers, he was a top 'photosynthetiker'. His research dealt with novel findings on light absorption, excitation energy distribution, and redistribution among the two photosystems, electron transfer, and their relation to dynamic membrane change as affected by environmental changes, especially temperature. David was an attentive listener and a creative designer of experiments and instruments, and he was also great fun to work with. He is remembered here by his family, coworkers, and friends from around the world including Australia, France, Germany, Japan, Sweden, Israel, and USA.

Shmuel Malkin (1934-2017) : Listening to photosynthesis and making music

We present here the life and work of Shmuel Malkin (1934-2017), an accomplished scientist and a gifted musician who touched the lives of many around the world. His early scientific work addressed the dynamics of light harvesting and electron transport in photosynthesis. Later, he used photoacoustic and photothermal methodologies to explore all aspects of photosynthesis. As a musician, Shmuel played the piano often for family and friends but after his formal retirement, he produced a body of original musical compositions, many of which were performed publicly. Throughout his life, Shmuel was a caring and deeply thoughtful man, respected and loved by colleagues, family, and friends. This tribute presents a summary of Shmuel's work as well as remembrances written by his wife, Nava Malkin, their son, Eyal Malkinson, and many of his colleagues: Michael Havaux from France; Sandra and Marcel Jansen from Ireland; David Cahen, Marvin Edelmann, Joop and Onnie de Graaf, Jonathan Gressel, Uri Pick, Yona Siderer, and Elisha Tel-Or from Israel; Ulrich Schreiber from Germany; James Barber and Alison Telfer from the UK; Govindjee, Stephen Herbert and Thomas Sharkey from the USA. Minnie Ho and Iris Malkin of the USA wrote contributions about Shmuel's music.

Bill Arnold and calorimetric measurements of the quantum requirement of photosynthesis-once again ahead of his time

The approach of photocalorimetry to decide on the true quantum requirement of photosynthesis - one of the main issues of the research in the first half of the century and a source of a bitter debate - is described. Bill Arnold's original approach to get into the true answer is reflected from the point of view of present day calorimetric techniques.

Photosynthetic energy storage in aquatic leaves measured by photothermal deflection

In a study of photosynthetic energy storage efficiency (ES), the adaxial surface of the leaves of Vallisneria americana exhibited the highest ES values (22%) of the four aquatic plants examined. V. americana leaves have a dispersed structure and it was possible to measure the energy storage properties of the epidermal cells independently of the rest of the leaf. The abaxial epidermis had a higher value of ES at zero light fluence than the adaxial epidermis but ES in the abaxial epidermis declined much more rapidly with light fluence. Thus the abaxial epidermis is more suited to lower light fluences than the adaxial epidermis. ES declined as the pH rose from 4.0 to 8.0 at a constant dissolved inorganic carbon concentration. This paralleled the change from carbon dioxide to bicarbonate and suggests that these leaves utilise CO2 more efficiently than bicarbonate. ES increased by about 50% at pH 8.0 as leaf sections further from the leaf tip were examined which demonstrates that the older epidermal cells are less well able to use bicarbonate. Exposure to 30 min of a saturating light fluence caused the epidermal chloroplasts to move from the periclinal walls to the anticlinal walls. This decreased the photothermal signal by increasing the thermal diffusion distance and lowering the light fluence due to greater chloroplast shading. The latter effect increased ES. It appears that chloroplast movement could assist the epidermis to survive harmful light fluences.

Changes in the cyanobacterial photosynthetic apparatus during acclimation to macronutrient deprivation

When the cyanobacterium Synechococcus sp. Strain PCC 7942 is deprived of an essential macronutrient such as nitrogen, sulfur or phosphorus, cellular phycobiliprotein and chlorophyll contents decline. The level of β-carotene declines proportionately to chlorophyll, but the level of zeaxanthin increases relative to chlorophyll. In nitrogen- or sulfur-deprived cells there is a net degradation of phycobiliproteins. Otherwise, the declines in cellular pigmentation are due largely to the diluting effect of continued cell division after new pigment synthesis ceases and not to net pigment degradation. There was also a rapid decrease in O2 evolution when Synechococcus sp. Strain PCC 7942 was deprived of macronutrients. The rate of O2 evolution declined by more than 90% in nitrogen- or sulfur-deprived cells, and by approximately 40% in phosphorus-deprived cells. In addition, in all three cases the fluorescence emissions from Photosystem II and its antennae were reduced relative to that of Photosystem I and the remaining phycobilisomes. Furthermore, state transitions were not observed in cells deprived of sulfur or nitrogen and were greatly reduced in cells deprived of phosphorus. Photoacoustic measurements of the energy storage capacity of photosynthesis also showed that Photosystem II activity declined in nutrient-deprived cells. In contrast, energy storage by Photosystem I was unaffected, suggesting that Photosystem I-driven cyclic electron flow persisted in nutrient-deprived cells. These results indicate that in the modified photosynthetic apparatus of nutrient-deprived cells, a much larger fraction of the photosynthetic activity is driven by Photosystem I than in nutrient-replete cells.

State adaptations in the cyanobacterium Synechcoccus 6301 (PCC): Dependence on light intensity or spectral composition?

A profile of high light to intense self-shading conditions was constructed using a white light source and cultures of the cyanobacterium Synechococcus 6301; this profile approximates to a natural self-shading gradient of decreasing light intensity and PS II/PS I excitation ratio. Samples of S.6301 were placed along this profile and allowed to state adapt. To separate the effects of light intensity and wavelength on state adaptation, samples were also placed in a shade profile produced by a white light source and neutral density filters. After adaptation, samples were fixed in their resulting state by the addition of glutaraldehyde, and fluorescence measurements were made at 35° C or -160 °C. It is concluded: 1. Under conditions of deep shade (<5 μmol m(-2)s(-1) PAR) and weak shade (>200 μmol m(-2)s(-1) PAR), cells adapt to a low PS II fluorescence state (state 2); in moderate shade (20-60 μmol m(-2)s(-1)PAR) cells adapt to a high PS II fluorescence state (state 1). We suggest these findings provide evidence for the operation of different factors on the control of state adaptations in cyanobacteria; one set operates at low light and another at high light intensities. 2. Under conditions of self-shading, there is little evidence to support the contention that state adaptations in cyanobacteria are produced by wavelength-dependent changes in the PS II/PS I excitation ratio, instead, it appaers they are produced by changes in the intensity of incident irradiation. 3. The observed fluorescence changes do not appear to involve major changes in the phycobilisome sensitisation of PS II and PS I. Instead, it appears that these changes are effected by alterations in Φ(F) of PS II (i.e. changes in PS II excitation density caused by alterations in the rate constants controlling spillover to PS I, photochemistry, fluorescence emission or thermal deactivation.

A photoacoustic study of water infiltrated leaves

Photoacoustic measurements of photosynthetic energy storage were conducted on water infiltrated pea and sugar maple leaves. The samples were vacuum infiltrated with pure water or with a suitable buffer. The use of such methodology permitted an accurate determination of the energy storage parameter at low modulation frequencies, where in non-infiltrated leaves oxygen evolution dominates the photoacoustic signal and does not allow energy storage measurements. Differences between infiltration media were not essential, however the use of pure water as infiltration medium sometimes caused instability of the measured energy storage, particularly at longer experimental time. Values of energy storage in individual samples ranged mostly between 0.2 to 0.35. Measured as a function of the modulation frequency, energy storage was found to be constant from about 10 to 200 Hz for pea leaves. In sugar maple leaves, the energy storage slightly increased between 100 and 500 Hz. Obtaining an accurate value for energy storage also allowed an accurate estimation of the O2 evolution contribution to the photoacoustic signal of an unfiltrated leaf. In a maple leaf its frequency dependence showed only the effect of diffusion in the entire frequency range (10-500 Hz). Energy storage transients were observed after long periods (ca. 1/4-2 hrs) of dark adaptation upon the transition to light. In this case the initial energy storage was roughly about 1/2 that of the steady state value indicating strong PS I activity, while PS II was transiently incompetent. Energy-storage increased during illumination in a way to correspond to photosynthetic induction events as previously measured by fluorescence and O2 evolution. Transients in energy storage were also found following high light to low light transitions (i.e., switch off of the saturating background light), that paralleled similar transients in oxygen evolution, showing initial transient inactivation followed by progressive reactivation of PS II.

Photosynthetic energy storage of Photosystems I and II in the spectral range of photosynthetically active radiation in intact sugar maple leaves

The relative activity of Photosystems (PS) I and II in the spectral range between 400 and 720 nm was studied by measuring photosynthetic energy storage (ES) of an intact sugar maple leaf using photoacoustic spectroscopy. ES, determined with a modulated (80 Hz) monochromatic light beam in the presence of saturating intensity of background non-modulated white light, indicated the total energy stored by both photosystems (EST). Using background far-red light, ES of PS I (ESPS I) was quantified. ESPS II was derived from EST-ESPS I. EST dependence on intensity and wavelength of modulated light was studied at 470, 560, 640 and 680 nm. EST was maximum in red light and minimum in blue light. It decreased with an increase in modulated light intensity. The ratio ESPS II/ESPS I, measured at 640 nm, remained nearly constant with an increase in modulated light intensity. The relative quantum yield of EST spectrum showed two peaks around 610 and 660 nm, and declined sharply after 680 nm, revealing a clear red drop. ESPS I spectrum presented peaks around 610 and 670 nm, and a minimum between 440 and 470 nm. ESPS I was observed beyond 700 nm up to 720 nm, indicating the energy stored by cyclic electron transport. ESPS II spectrum showed broad peaks, around 460, 490, 600 and 660 nm, and a shoulder between 530 and 560 nm. ESPS II was always higher than ESPS I between 400 and 690 nm and reached zero around 700 nm.

The use of photothermal radiometry in assessing leaf photosynthesis: I. General properties and correlation of energy storage to P700 redox state

Energy storage measurements by modulated photothermal radiometry (PTR) were carried out on intact leaves to assess the value of the PTR method for photosynthesis research. In particular, correlations to the redox state of P700 under various conditions were examined. PTR monitors modulated light conversion to heat by sensing the resulting modulated infra-red radiation emitted from the leaf. It is, therefore, a complementary method to photoacoustics for estimating energy storage and its time variation, particularly under controlled leaf atmosphere.With modulated light-1 (λ>690 nm) the energy storage approached zero and P700 was maximally oxidized. When background light of shorter wavelength (λ<690 nm-light-2) was added, energy storage momentarily increased (a manifestation of Emerson enhancement) while P700 was reduced. The values of both parameters varied as a function of the background light intensity, keeping a mutual linear relationship. Following the initial change, there was a slow reversal transient of P700 oxidation with a parallel decrease in energy storage. Temporal correlation to P700 redox state after dark adaptation was observed also for the energy storage measured in modulated light 2 when combined with background actinic light of medium intensity (about 50 W m(2)). Under these circumstances P700 was almost totally oxidized initially and then gradually reduced while energy storage was initially low and then increased parallel to P700 reduction.A comparison between the maximum energy storage in modulated light 1, enhanced by background light 2, to the energy storage with short wavelength light (where light tends to be more evenly distributed) indicates a comparable contribution to energy storage from each active photosystem. The above experiments indicate that energy storage contribution from PS I is directly related to the extent of openness of its reaction-centers.While some aspects of the data call for more experimentation, these experiments already establish PTR as a valuable method to monitor photosynthetic energy storage activity in vivo, particularly when used simultaneously with other non-invasive methods.

A gas-permeable photoacoustic cell

A photoacoustic cell assembly is described that is permeable to CO2 and other gases but not water vapor. As a replacement for the usually employed solid cover, this cell uses a cover containing a small fritted glass disk that holds a small piece of 6.4 μm Teflon film against the sample.With the above arrangement it was possible to increase the rate of O2 evolution measured photoacoustically about 3 times in Zea mays leaves and about 1.7 times in Phaseolus vulgaris leaves upon adding CO2 to the gas stream. The extent of energy storage was also enhanced with supplemental CO2 in Zea and Ulva but less so in Phaseolus. The maximum improvements of photosynthetic activities were obtained when the gas stream contained 2.5-5% CO2. These high concentrations were presumably necessary as the result of a high resistance to diffusion through the gas-permeable cover.