This paper reports experimental measurements of the radiation characteristics of green algae used for carbon dioxide fixation via photosynthesis. The generated biomass can be used to produce not only biofuels but also feed for animal and food supplements for human consumptions. Particular attention was paid to three widely used species namely Botryococcus braunii, Chlorella sp., and Chlorococcum littorale. Their extinction and absorption coefficients were obtained from normal–normal and normal–hemispherical transmittance measurements over the spectral range from 400 to 800 nm. Moreover, a polar nephelometer was used to measure the scattering phase function of the microorganisms at 632.8 nm. It was observed that for all strains, scattering dominates over absorption. The magnitudes of the extinction and scattering cross-section are functions of the size, shape, and chlorophyll content of each strain in a non-trivial manner. Absorption peaks at 435, 475, and 676 nm corresponding to chlorophyll a and chlorophyll b. The results can be used for scaling and optimization of CO2 fixation in ponds or photobioreactors as well as in the development of controlled ecological life support systems.

This study reports a factor 5.5 increase in hydrogen production of Anabaena variabilis ATCC 29413 using Allen-Arnon medium compared with BG-11 and BG-11o media. The results were obtained with a flat panel photobioreactor made of acrylic and operated in two stages at 30oC. Stage 1 aims at converting carbon dioxide into biomass by photosynthesis while Stage 2 aims at producing hydrogen. During Stage 1, the photobioreactor is irradiated with 65 umol/m2/s of light and sparged with a mixture of air and carbon dioxide. During Stage 2, irradiance is increased to 150 umol/m2/s and the photobioreactor is sparged with pure argon. The parameters continuously monitored are (1) the cyanobacteria concentration, (2) the pH, (3) the dissolved oxygen concentration, (4) the nitrate and (5) the ammonia concentrations in the medium, and (6) the hydrogen concentration in the effluent gas. The three media BG-11, BG-11o, and Allen-Arnon are tested under otherwise similar conditions. The light to biomass energy conversion efficiency varied between 5.5 and 10.5% and was similar for all media. The cyanobacteria concentrations during Stage 2 were 1.10 and 1.17 kg dry cell/m3 with BG-11 and Allen-Arnon media, respectively, while it could not exceed 0.76 kg dry cell/m3 with medium BG-11o. The average specific hydrogen production rates were about 1 and 0.9 L/kg dry cell/h in media BG-11 and BG-11o, respectively. In contrast, it was about 5.6 L/kg dry cell/h in Allen-Arnon medium. The maximum light to hydrogen energy conversion efficiencies achieved were 0.26%, 0.16%, and 1.32% for BG-11, BG-11o, and Allen-Arnon media, respectively. The larger specific hydrogen production rates, efficiencies, and cyanobacteria concentrations achieved using Allen-Arnon medium are attributed to the presence of vanadium, and higher concentrations of molybdenum, magnesium, calcium, sodium, and potassium in the medium.

This paper presents a parametric study simulating light transfer in a photobioreactor containing gas bubbles and filamentous cyanobacteria Anabaena variabilis suspended in water. To the best of our knowledge, this paper presents for the first time a model for such system (i) using a consistent set of radiation characteristics of the medium derived from experimental data and from Mie theory, (ii) accounting for anisotropic scattering by both the bubbles and the filamentous microorganisms, (iii) considering the spectral dependency of radiation characteristics in the spectral range from 400 to 700 nm using a box model, and (iv) evaluating light transfer in a photobioreactor containing genetically engineered microorganisms with reduced pigment content. The steady-state one-dimensional radiation transfer equation is solved using the modified method of characteristics and a quadrature with 24 directions per hemisphere adapted to forward scattering media. The parameters investigated include the bacteria concentration, the bubble radius and the void fraction, as well as the approximate scattering phase function. It was established that the strongly forward scattering by the bubbles must be accounted for and the truncated phase function is recommended. In the absence of bubbles, ignoring in-scattering by the bacteria leads to errors as high as 20%. On the other hand, accounting for in-scattering with isotropic phase function gives acceptable results. Finally, genetically reducing the pigment content of the microorganisms by an order of magnitude increases the significance of forward scattering of light by the microorganisms. This in turn, increases the penetration depth and can be accounted for by either the Henyey-Greenstein or the truncated phase function approximations.

This experimental study reports, for the first time, the radiation characteristics of the unicellular green algae Chlamydomonas reinhardtii strain CC125 and its truncated chlorophyll antenna transformants tla1, tlaX, and tla1-CW+. Photobiological hydrogen production is a sustainable alternative to thermochemical and electrolytic technologies with the possible advantage of carbon dioxide mitigation. However, scale-up of photobioreactors from bench top to industrial scale is made difficult by excessive absorption and waste of light energy as heat and fluorescence. This results in limited light penetration into the photobioreactor and low solar to hydrogen energy conversion efficiency. To overcome these challenges, the algae Chlamydomonas reinhardtii have been genetically engineered with reduced pigment concentrations in their photosystems. This can improve the performance of photobioreactors by increasing the saturation irradiance of algae and quantum efficiency of photobiological hydrogen production. The extinction and absorption coefficients of all strains studied are obtained from normal-normal and normal-hemispherical transmittance measurements over the spectral range from 300 to 1,300 nm. Moreover, a polar nephelometer is used to measure the scattering phase function of the microorganisms at 632.8 nm. It is established that the wild strain C.reinhardtii CC125 has major absorption peaks at 435 and 676 nm, corresponding to the in vivo absorption peaks of chlorophyll a, and at 475 and 650 nm corresponding to those of chlorophyll b. The genetically engineered strains have less chlorophyll pigments than the wild strain and thus have smaller absorption cross-sections. In particular, the mutant tlaX features a significant reduction in chlorophyll b concentration. For all mutants, however the reduction in the absorption cross-section is accompanied by an increase in scattering cross-section. Although scattering becomes the dominant phenomenon contributing to the overall extinction of light, it is mainly in the forward direction. Thus, to fully assess the effect of genetic engineering on light transport in the photobioreactor requires careful radiation transfer analysis using the radiation characteristics reported in this study.

Aims: The objective of this study is to develop kinetic models based on batch experiments describing the growth, CO2 consumption, and H2 production of Anabaena variabilis ATCC 29413-U as functions of irradiance and CO2 concentration.

Methods and Results: A parametric experimental study is performed for irradiances from 1120 to 16100 lux and for initial CO2 mole fractions from 0.03 to 0.20 in argon at pH 7.0 with nitrate in the medium. Kinetic models are successfully developed based on the Monod model and on a novel scaling analysis employing the CO2 consumption half-time as the time scale.

Conclusions: Monod models predict the growth, CO2 consumption, and O2 production within 30%. Moreover, the CO2 consumption half-time is an appropriate time scale for analyzing all experimental data. In addition, the optimum initial CO2 mole fraction is 0.05 for maximum growth and CO2 consumption rates. Finally, the saturation irradiance is determined to be 5,170 lux for CO2 consumption and growth whereas, the maximum H2 production rate occurs around 10,000 lux.

Significance and Impact: The study presents kinetic models predicting the growth, CO2 consumption,and H2 production of A.variabilis. The experimental and scaling analysis methods can be generalized to other microorganisms.

In this paper we present preliminary results obtained from fluorescence lifetime measurements on human skin using time-correlated single photon counting (TCSPC) techniques. Human skin was exposed to light from a pulsed LED of 700 ps pulse width at a wavelength of 375 nm and fluorescence decays were recorded at fourdifferent emission wavelengths (442, 460, 478 and 496 nm) using a photomultiplier tube (PMT) coupled to a monochromator. Measurements were carried out on the left and right palms of subjects recruited for the study after obtaining consent using a UCLA IRB approved consent form. The subjects recruited consisted of 18 males and 17 females with different skin complexions and ages ranging from 10 to 70 years. In addition, a set of experiments were also performed on various locations including the palm, the arm and the cheek of a Caucasian subject. The fluorescence decays thus obtained were fit to a three-exponential decay model in all cases and were approximately 0.4, 2.7 and 9.4 ns, respectively. The variations in these lifetimes with location, gender, skin complexion and age are studied. It is speculated that the shorter lifetimes correspond to free and bound NADH while the longer lifetime is due to AGE crosslinks.