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

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.

Enhancement of CO2 biofixation and lipid production by Chlorella vulgaris using coloured polypropylene film

Chlorella vulgaris was cultivated with light at different wavelengths (λ_max) and irradiation intensities (I) by applying a coloured tape (CT) as a simple, inexpensive light filter. C. vulgaris was cultivated in a standard medium using blue (CT_B), green (CT_G), red (CT_R), yellow (CT_Y) and white (CT_W) CT to filter the light, as well the unfiltered light (U). The influence of λ_max and I on specific growth rate (μ), nutrient removal efficiency (% RE of total nitrogen, TN, and phosphorus, TP), CO₂ fixation rate (RC) and lipid productivity (P_lipid) were evaluated. The highest biomass concentration X_max of 2.26 g L^-1 was measured for CT_W with corresponding μ, TN and TP RE, RC and P_lipid values of 0.95 d^-1, 92% and 100%, 0.67 g L^-1 d^-1 and 83.6 mg L^-1 d^-1, respectively. The normalised μ and P_lipid for U were significantly lower than in CT_W of 33-50% and 75%, respectively. The corresponding non-normalised parameter values for CT_B were significantly lower at 0.45 d^-1, 0.18 g L^-1, 15% and 37%, 0.03 g L^-1 d^-1 and 1.2 mg L^-1 d^-1. Results suggest a significant impact of I and λ_max, with up to a 50% increase in growth and nutrient RE from optimising these parameters.

Metabolic modeling of Chlamydomonas reinhardtii: energy requirements for photoautotrophic growth and maintenance

In this study, a metabolic network describing the primary metabolism of Chlamydomonas reinhardtii was constructed. By performing chemostat experiments at different growth rates, energy parameters for maintenance and biomass formation were determined. The chemostats were run at low irradiances resulting in a high biomass yield on light of 1.25 g  mol(-1). The ATP requirement for biomass formation from biopolymers (K(x)) was determined to be 109 mmol g(-1) (18.9 mol mol(-1)) and the maintenance requirement (m(ATP)) was determined to be 2.85 mmol g(-1) h(-1). With these energy requirements included in the metabolic network, the network accurately describes the primary metabolism of C. reinhardtii and can be used for modeling of C. reinhardtii growth and metabolism. Simulations confirmed that cultivating microalgae at low growth rates is unfavorable because of the high maintenance requirements which result in low biomass yields. At high light supply rates, biomass yields will decrease due to light saturation effects. Thus, to optimize biomass yield on light energy in photobioreactors, an optimum between low and high light supply rates should be found. These simulations show that metabolic flux analysis can be used as a tool to gain insight into the metabolism of algae and ultimately can be used for the maximization of algal biomass and product yield. ELECTRONIC SUPPLEMENTARY MATERIAL: The online version of this article (doi:10.1007/s10811-011-9674-3) contains supplementary material, which is available to authorized users.

Effect of O₂:CO₂ ratio on the primary metabolism of Chlamydomonas reinhardtii

High oxygen:carbon dioxide ratios may have a negative effect on growth and productivity of microalgae. To investigate the effect of O₂ and CO₂ concentrations and the ratio between these on the metabolism of Chlamydomonas reinhardtii we performed turbidostat experiments at different O₂:CO₂ ratios. These experiments showed that elevated O₂ concentrations and the corresponding increase in the ratio of O₂:CO₂ common in photobioreactors led to a reduction of growth and biomass yield on light with 20-30%. This is most probably related to the oxygenase activity of Rubisco and the resulting process of photorespiration. Using metabolic flux modeling with measured rates for each experiment we were able to quantify the ratio of the oxygenase reaction to the carboxylase reaction of Rubisco and could demonstrate that photorespiration indeed can cause the reduction in biomass yield on light. The calculated ratio of the oxygenase reaction to the carboxylase reaction was 16.6% and 20.5% for air with 2% CO₂ and 1% CO₂, respectively. Thus photorespiration has a significant impact on the biomass yield on light already at conditions common in photobioreactors (air with 2% CO₂).

A photo microcalorimetric system for studies of plant tissue

A gas perfusion microcalorimeter, primarily designed for studies of plant tissue, has been equipped with light guides in order to allow studies under the influence of light (UV-vis), and with variations in gas phase composition. The calorimetric system consists of three twin microcalorimeters of the heat conduction type positioned in a thermostated water bath. The biological sample is placed in a 20-ml photo calorimetric vessel. Light is introduced by means of a quartz rod connected to one of the arms of a bifurcated light cable. The other arm of the cable is connected to a second microcalorimeter, serving as the photo calorimetric reference. The gas flow leaving the main calorimeter will pass the reaction vessel of the third microcalorimeter, where CO2 is trapped by NaOH solution. The measurement with this calorimeter will lead to values for the rate of uptake/release of CO2 by the biological sample. Test experiments have been conducted with pieces of spinach leaf, and values have been derived for the uptake of light energy at different concentrations of CO2. From the calorespirometric results a value was derived for the Gibbs energy of formation of photosynthate from CO2 and H2O.