Iron limitation – A perspective on a growth-restricted cultivation strategy for a H2 production system using the diazotrophic cyanobacterium Nostoc PCC 7120 ΔhupW

Clemens Posten,Christoph Howe,Karin Stensjö,Daniel Becker,Christian Steinweg

doi:10.1016/j.biteb.2020.100508

Clemens Posten, Christoph Howe + Show 3 more

Open Access

https://doi.org/10.1016/j.biteb.2020.100508

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Abstract

H2 is a promising carbon-free alternative to fossil fuels, and can be photoautotrophically produced by cyanobacteria. Despite efforts to increase cyanobacterial H2 production by genetic engineering and technical optimization the yield remains economically non-viable. Recent studies have therefore demanded the impairment of growth to reallocate more carbohydrates into biofuel biosynthesis, and nutrient limitation may be a strategy to achieve that. Here we show that limitation of Fe in the filamentous and heterocystous mutant strain Nostoc PCC 7120 ΔhupW resulted in a 5.3-fold lower chlorophyll a concentration as compared to the controls during four-week cultivation. The Fe-limited filaments contained a heterocyst frequency at ~6% and a 4-fold higher specific carbohydrate concentration as compared to the control cultures. We discuss whether Fe-limiting conditions leading to an accumulation of carbon storage compounds could be used as a heterocyst based H2 production system.

Highlights

In the past decades, cyanobacteria have gained growing interest to be used for renewable hydrogen production based on sunlight, water and a small amount of nutrients (Nagarajan et al, 2017)
These sugars are transported into the heterocysts and catabolized in concert by the oxidative pentose phosphate pathway, glycolysis, respiration and cyclic photosynthesis to feed the nitrogenase with the necessary ATP and electrons (Magnuson, 2019)
This study aims to explore whether iron limitation could be used as a cultivation strategy for H2 production in Nostoc PCC 7120 ΔhupW

Summary

Introduction

Cyanobacteria have gained growing interest to be used for renewable hydrogen production based on sunlight, water and a small amount of nutrients (Nagarajan et al, 2017). Diazotrophic and filamentous cyanobacteria have been intensively studied as a chassis for H2 production. These bacteria mainly possess photosynthetic vegetative cells but under conditions of N-deprivation they can develop heterocysts, a specialized cell type for N2-fixation. Heterocysts are a key element in photobiological hydrogen production as H2 is a byproduct of the N2 reduction reaction to NH3 (Khetkorn et al, 2013) It is the microoxic environment inside the heterocysts that allows the O2-sensitive nitrogenase to function. The electrons for the N2 reduction are originally gained from photosynthetically produced carbohydrates in the vegetative cells These sugars are transported into the heterocysts and catabolized in concert by the oxidative pentose phosphate pathway, glycolysis, respiration and cyclic photosynthesis to feed the nitrogenase with the necessary ATP and electrons (Magnuson, 2019). Due to the spatially separated processes of photosynthesis and hydrogen formation that requires the formation of carbohydrates, the process is referred to as indirect photolysis (Prince and Kheshgi, 2005)

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