Acetate as a carbon source for hydrogen production by photosynthetic bacteria

Abstract

Hydrogen is a clean energy alternative to fossil fuels. Photosynthetic bacteria produce hydrogen from organic compounds by an anaerobic light-dependent electron transfer process. In the present study hydrogen production by three photosynthetic bacterial strains (Rhodopseudomonas sp., Rhodopseudomonas palustris and a non-identified strain), from four different short-chain organic acids (lactate, malate, acetate and butyrate) was investigated. The effect of light intensity on hydrogen production was also studied by supplying two different light intensities, using acetate as the electron donor. Hydrogen production rates and light efficiencies were compared. Rhodopseudomonas sp. produced the highest volume of H₂. This strain reached a maximum H₂ production rate of 25 ml H₂ l⁻¹ h⁻¹, under a light intensity of 680 μmol photons m⁻² s⁻¹, and a maximum light efficiency of 6.2% under a light intensity of 43 μmol photons m⁻² s⁻¹. Furthermore, a decrease in acetate concentration from 22 to 11 mM resulted in a decrease in the hydrogen evolved from 214 to 27 ml H₂ per vessel.

Introduction

There are limited reserves of fossil fuels on Earth, and the combustion of the fuels leads to serious problems such as global climate changes. For this reason, much attention is presently being given to the development of clean, sustainable energy systems, with the potential to supplement and even substitute the fossil-fuel-based energy production. One of the alternatives, as an environmentally acceptable fuel, is molecular hydrogen. Hydrogen is a clean fuel yielding only water after combustion.

Photosynthetic bacteria can produce hydrogen at the expense of solar energy and small-chain organic acids as electron donors. The combination of photosynthetic bacteria with anaerobic bacteria can provide a system for hydrogen photoproduction from residual carbohydrates, e.g. from organic wastes (Mao et al., 1986, Sasaki, 1998). In such a system, anaerobic fermentation of organic wastes produces intermediates like low-molecular-weights organic acids in a first step, which are then converted to hydrogen by photosynthetic bacteria at the expense of light energy, in a second step. The conversion efficiency of light energy to hydrogen, with the supply of an appropriate carbon source, are the key factors for hydrogen production by biological systems (Hillmer and Gest, 1977).

The conversion of malate and lactate to hydrogen by photosynthetic bacteria is well documented (Zürrer and Bachofen, 1979, Kim et al., 1981, Kim et al., 1987, Miyake and Kawamura, 1987, Fascetti and Todini, 1995, Sasikala et al., 1997, Ike et al., 1997a, Ike et al., 1997b, Tsygankov et al., 1998). The main products of anaerobic fermentation are acetic and butyric acids (Segers et al., 1981). Little is known about the conversion of acetic and butyric acids to hydrogen by photosynthetic bacteria (Segers and Verstraete, 1983, Sasaki, 1998). The conversion of these acids would be advantageous in order to couple energy production with organic-waste treatment. The few works done, in which hydrogen production rates using different carbon sources were compared, reported lower hydrogen production rates with acetate and butyrate than with malate and/or lactate (Hillmer and Gest, 1977, Kim et al., 1980, Miyake et al., 1984, Sasaki, 1998).

The light efficiency is the other parameter that determines the feasibility of the biological-hydrogen production process. Only in a few studies light efficiencies have been reported (Miyake and Kawamura, 1987, Nakada et al., 1995, Nakada et al., 1996, El-Shishtawy et al., 1997, El-Shishtawy et al., 1998, Otsuki et al., 1998, Yamada et al., 1998). Among these studies, only Otsuki et al. (1998) used carbon sources other than malate or lactate (mixture of acetate, propionate and butyrate) and reported a light efficiency of 0.31%. So far, the highest reported efficiency of light to hydrogen conversion is 7.9%, with light from a solar simulator and lactate as electron donor (Miyake and Kawamura, 1987). However, this value was obtained at a small-scale experiment and with a low light intensity.

The energy conversion efficiencies and H₂ production rates of three different photosynthetic bacteria under different light intensities and with different carbon sources are reported on here.