Algal-bacterial cooperation improves algal photolysis-mediated hydrogen production

Highlights

• Algal-bacterial cooperation improves the photolysis H₂ production.

• Completely anaerobic culture condition was built by bacterial partner.

• Algal-bacterial cooperation slows chlorophyll reduction and maintains protein.

• Direct water photolysis contributes mainly to the algal H₂ production.

Abstract

In this study, bacterium Pseudomonas sp. strain D was proved to be the main partner assisting Chlamydomonas reinhardtii in improving photolysis-mediated H₂ production and a good partner for promoting H₂ production by the green algae Chlorella and Scenedesmus. In strain D partnered algal-bacterial co-culture, the relative O₂ content in the headspace plus the dissolved oxygen in the culture medium were rapidly consumed by bacterial growth, resulting in a completely anaerobic environment that proved suitable for the activation of algal hydrogenase. Moreover, algal-bacterial cooperation was able to slow the reduction of chlorophyll, enhance starch accumulation, and maintain protein content, which are the potential factors whose control provides an opportunity for improving algal H₂ production. This study systematically analyzed the main pathway responsible for H₂ production by algal-bacterial cooperation and the potential mechanisms for improvement, and proposed an efficient and durable algal-bacterial cooperation system for improving photolysis-mediated H₂ production by green algae.

Introduction

Global warming has become an increasingly serious problem due to the anthropogenic emission of large quantities of greenhouse gases. The increasing demand for carbon emission reduction makes H₂ a more attractive fuel, since it is a clean and high energy-density fuel (Oey et al., 2016). Renewable H₂ production from algal photosynthesis has a great potential to become a next-generation biofuel because algae, (1) have higher productivity, (2) can be grown on non-arable land and, (3) have high solar energy conversion efficiency (Rashid et al., 2013, Oey et al., 2016). Green alga Chlamydomonas reinhardtii is a model algal species that has been used for studying photolysis-mediated H₂ production, because of its high hydrogenase activity, available genetic background and ease of cultivation (Melis, 2007, Oey et al., 2016). In addition to Chlamydomonas, several other green algae genera, including Chlorella, Scenedesmus and Platymonas, have also been reported to produce H₂ (Rashid et al., 2011, Chader et al., 2011, Zhang et al., 2012, Marquez-Reyes et al., 2015).

The H₂ producing activity in green algae is predominantly due [FeFe] hydrogenase, which has a unique active center with an output activity about 10–100-fold higher than [NiFe] hydrogenase, which occurs mainly in cyanobacterial species (Batyrova and Hallenbeck, 2017, Khetkorn et al., 2017). However, [FeFe] hydrogenase is highly O₂ sensitive. This sensitivity was documented to be the main bottleneck for algal photolysis H₂ production. Many solutions have been devised to reduce O₂ level to overcome this problem with algal H₂ production; for example, sulfur deprivation is currently a widely used strategy, because it partially inactivates PSII activity, thus maintaining photolysis O₂ production below the rate of mitochondrial respiration, which in turn results in the required anaerobic condition for H₂ production (Melis et al., 2010, Eroglu and Melis, 2016, Oey et al., 2016, Batyrova and Hallenbeck, 2017). Besides nutrient deprivation, the addition of O₂ scavengers, including chromous chloride, dithionite, and cysteine, or O₂ purge by rinsing the culture atmosphere with inert gases have also been tested to keep the atmosphere around algae cultures free of O₂ (Marquez-Reyes et al., 2015, Batyrova and Hallenbeck, 2017).

Furthermore, co-cultivation of algae and bacteria (such as Azotobacter, Bradyrhizobium, Pseudomonas, and Escherichia) has also been reported as a useful method to achieve anaerobiosis by the respiration of bacterial partners (Wu et al., 2012, Li et al., 2013, Lakatos et al., 2016, Xu et al., 2016, Xu et al., 2017). The cooperation between algal and bacterial cells in co-culture exhibits a co-metabolism pattern: the bacteria removes the O₂ sensitivity from algal hydrogenase, while the alga supplies O₂ and organics for bacterial growth. However, there was very little research work carried concerning deeper reasons for the improvement of algal-bacterial cooperated hydrogen production after the removal of O₂ sensitivity. In this study, we aimed to identify bacterial cooperated effect on algal photolysis-mediated H₂ production, and systematically analyzed the potential mechanisms (such as slowing chlorophyll reduction, enhancing starch accumulation, and maintaining protein content) that mainly responsible for the improvement of H₂ production.