New methods for hydrogen production by marine microalga Chlorella pyrenoidosa in natural seawater

Highlights

• Chlorella pyrenoidosa produced 45 ml/l (183 μM) of H₂ in N- limited condition.

• H₂ volume reduced by 20% under 3.5% initial O₂, and by 50% under 9.7%–21% of O₂.

• N-limited and P-deprived conditions together decrease by 85% of H₂ volume.

Abstract

In this study, we demonstrated a new method of achieving long-term photodependent hydrogen production under nitrogen limiting conditions in a medium with natural seawater. Moreover, centrifugation stage in pretreatment of culture was avoided. Chlorella pyrenoidosa passed into deprivation conditions after exhaustion of nitrogen source. Microalgae began to release hydrogen at 144 h of cultivation after stages of evolution and consumption of oxygen. C. pyrenoidosa produced 45 ml (183 μM) of hydrogen per liter of culture. A significant effect of the initial oxygen concentration in the gas phase on the release of hydrogen was found. When initial oxygen content was 3.5%, the final H₂ yields reduced by 20%, while under 9.7%–21% of O₂, it reduced by 50% (compared to argon gas phase). Perhaps this was due to the low content of cells. We also demonstrated a decrease in the release of hydrogen by C. pyrenoidosa by 85% when using double stress conditions: phosphate-deprivation and limiting nitrogen source.

Introduction

Exhaustion of traditional fossil energy sources and air pollution by combustion products is a serious problem of modern society. One promising alternative energy carrier is molecular hydrogen [1]. Among various ways for hydrogen production the biological method is the most environmentally friendly, since it is held at atmospheric pressure and room temperature. Molecular hydrogen can be produced by many microorganisms [2], among which green microalgae are of a particular interest, since they are deriving molecular hydrogen from water using solar energy [3], [4]. Long-term evolution of hydrogen by green microalgae in nutrient-deprived condition is widely known [5]. However, most of the studies were carried out with freshwater cultures [6]. Upscaling fresh water-based process of hydrogen evolution can lead to competition for fresh water sources. Therefore, a promising area of research is the production of hydrogen by marine cultures.

The first publications aiming to produce hydrogen by marine microalgae applied method of sulfur-deprivation by using of centrifugation [7], [8]. Cultivation in 0.3 l bioreactor was separated in two steps: in the first step, dark hydrogen evolution was performed, on the second step bioreactor was placed in photoconditions. It was shown that after the phase change culture produced hydrogen with the maximum rate of 11.7 nl/h followed by a sharp decrease in hydrogen evolution, which was probably due to activation of PSII and following inhibition of hydrogenase [7]. Other researchers have shown photoproduction of hydrogen in sulfur-deprived condition using inoculum grown on modified medium (low sulfur content). Cultures passed through stages typical for sulfur-deprivation: oxygen evolution, followed by its absorption and evolution of hydrogen during 96 h of cultivation. However, the amount of produced hydrogen was also very low (1 ml/l) [8]. Such low evolution of hydrogen may be related to the presence of significant of amounts of sulfates in seawater (0.028 M) [9]. There are many publications studying effect of protonophores and electron transport chain inhibitors on photosynthesis and hydrogen production. Application of inhibitors leads to PSII inactivation and decrease of oxygen production and hydrogen evolution. Zhang Y. and coauthors have shown that two-stage cultivation with carbonyl cyanide m-chlorophenylhydrazone (CCCP) protonophor (dark, and then light phase) may increase hydrogen production up to 5 ml/l in long-term or 11 ml/l in short-term experiments [10]. Ji et al. cultured Tetraselmis subcordiformis in a bioreactor with N-, P- and S-deficient media. The culture samples were collected at different phases of growth and evolution of hydrogen by the samples was tested after 12 h of dark adaptation and addition of CCCP. Maximum yield was obtained in case of N-deprivation (55.8 ml/l) on day 6, while for P- and S- deprived cultures, hydrogen yield did not exceed 16 ml/l [11]. Thus, CCCP application failed to achieve significant long-term evolution of hydrogen. Microalgae have a large amount of intracellular nitrogen and phosphorus. Li and coauthors [12] showed that use of inoculate grown on nitrogen-poor medium gave a 34% increase in hydrogen evolution by marine Chlorella pyrenoidosa, and it could produce up to 30 ml/l H₂ without dark incubation and inhibitors [13]. Low (0.1–2 mg/l chlorophyll) initial concentration of washed Сhlorella sp. cells (as sole phosphorus source) transferred to phosphorous-deprived conditions produced hydrogen up to 40 ml/l, due to the exhaustion of internal phosphorous source during growth [14]. The combination of several types of deprivations, such as sulfur and nitrogen, can simultaneously increase the evolution of hydrogen by 21% compared to nitrogen deprivation for fresh water Chlorella protothecoide [15]. Thus, we speculate that the use of combined phosphorus and nitrogen deprivation might allow greater hydrogen production in natural seawater.

Some problems arise when scaling up hydrogen production process by microalgae [16], [17]. One of them is the need for production of microalgal biomass. It can be solved by a system of two bioreactors: first is used for the growing of biomass, second is applied for the evolution of hydrogen [16]. However, concentrating the cells by filtration or centrifugation will be required. Both processes are not only energy-consuming [18] and traumatic for microalgae, but they can also lead to contamination of the culture by exogenous microflora. Laurinavichene et al. [19] showed that it is possible to obtain sulfur-deprivation culture by using low concentration of sulphates in the medium or using a large volume of inoculum (10%) for sowing into medium without sulfur source. Moreover, both methods made it possible to obtain 61% and 78% of hydrogen vs the standard washing method, respectively. However, such studies have not been performed for marine microalgae. In this study, we attempted to develop a new method for obtaining long-term light-dependent hydrogen evolution by marine microalgae in natural seawater without using centrifugation and to investigate the effect of various factors on the amount of hydrogen production.