Experimental measurements of the radiation characteristics of Anabaena variabilis ATCC 29413-U and Rhodobacter sphaeroides ATCC 49419
Introduction
Photobioreactors are enclosures used for cultivating microorganisms that utilize light as their energy source for their growth and subsequent product formation [1]. Traditionally, they have been used for a wide range of applications including production of pharmaceuticals, food additives for humans, feed for animals, and cosmetic chemicals [2]. They also find applications in environmental engineering such as wastewater treatment, heavy metal removal, and mitigation [2]. More recently, they have been considered for hydrogen production [3], [4], [5], [6], [7]. The productivity and efficiency of photobioreactors are mainly limited by light transfer [8], [9], [10], [11]. Light transfer within an absorbing and scattering medium, such as a photobioreactor, is governed by the radiative transport equation (RTE), which is an integro-differential equation expressed in dimensionless optical coordinates [12],where is the spectral intensity expressed in W/m2/sr. Here, is the unit vector in the line-of-sight direction and is the solid angle around the direction . The non-dimensional spectral optical thickness and single scattering albedo are defined, respectively, asandwhere , , and are the absorption, scattering, and extinction coefficients expressed in , respectively. The scattering phase function represents the probability of the radiation propagating in direction to be scattered in direction , and is normalized such thatNote that the variables , , , and are often denoted by , , , and , respectively, in the ocean optics literature [13]. Eq. (1) indicates that the extinction, absorption, and scattering coefficients together with the scattering phase function and the single scattering albedo are major parameters needed to solve the radiation transfer. Therefore, accurate modeling of light transfer in photobioreactors as well as detecting algal blooms by satellite remote sensing [14], for example require the knowledge of these radiation characteristics.
Moreover, the types of microorganisms commonly used in photobioreactors for hydrogen production include cyanobacteria (or blue-green algae) [6], green algae [15], purple sulfur [16], and purple non-sulfur bacteria [17]. Two of the widely studied photobiological hydrogen producers are the cyanobacteria Anabaena variabilis ATCC 29413-U [18] and the purple non-sulfur bacteria Rhodobacter sphaeroides ATCC 49419 [17], [19]. A. variabilis is a blue-green microorganism composed of cells of approximately in diameter forming filaments more than long. They absorb radiation mainly in the visible part of the electromagnetic spectrum. On the other hand, R. sphaeroides is a purple, elliptical shaped unicellular microorganism consisting of cells having equivalent diameters of approximately . It absorbs radiation both in the visible and in the near infrared parts of the electromagnetic spectrum. Fig. 1 shows micrographs of A. variabilis and R. sphaeroides under investigation.
This study contributes to the development of photobiological production of hydrogen as a sustainable and renewable energy source. More specifically, it aims at determining the radiation characteristics of both the filamentous cyanobacteria A. variabilis and the unicellular purple bacteria R. sphaeroides over the spectral range from 300 to 1300 nm, where 80% of the solar radiation is concentrated.
Section snippets
Current state of knowledge
A comprehensive review of the experimental techniques for measuring the radiation characteristics necessary to solve Eq. (1) has been reported by Agrawal and Mengüç [20]. The absorption and extinction coefficients are linearly proportional to the microorganism concentration if single scattering prevails. Then, they can be used to recover their mass extinction and absorption cross-sections, and , respectively, expressed in m2/kg and defined as [21]where X is
Microorganism cultivation and sample preparation
First, A. variabilis ATCC 29413-U was purchased from the American Type Culture Collection (ATCC) and received in freeze dried form. The culture was activated with 10 mL of sterilized milli-Q water. It was cultivated under aerobic conditions in ATCC medium 616 with irradiance of 2000–3000 lux provided by fluorescent light bulbs (Ecologic by Sylvania, USA). One liter of ATCC medium 616 contained 1.5 g NaNO3, 0.04 g K2HPO4, 0.075 g , , 6.0 mg citric acid, 6.0 mg ferric ammonium
Results
First, the extinction and absorption coefficients of A. variabilis were measured at three different microorganism concentrations, namely , and 0.1251 kg/m3 in the spectral region from 300 to 1300 nm. The measurements were performed twice and the arithmetic mean of the results is reported. The relative difference between measurements was less than 2% over the entire spectral range of interest. Similarly, the extinction and absorption coefficients of R. sphaeroides were determined at
Conclusions
This paper presents the complete set of radiation characteristics of two of the most promising hydrogen producing microorganisms. For the first time, a consistent set of radiation characteristics of both randomly oriented filamentous cyanobacteria A. variabilis and unicellular purple bacteria R. sphaeroides have been measured over the spectral range from 300 to 1300 nm. The reported radiation characteristics can be used to model photobioreactors or natural environments containing these
Acknowledgments
The authors gratefully acknowledge the support of the California Energy Commission through the Energy Innovation Small Grant (EISG 53723A/03-29; Project Manager: Michelle McGraw). They are indebted to Prof. J. Jay, Dr. T. Semenic and Dr. I. Martini for their helpful discussions and exchanges of information.
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