G5 Doctoral dissertation (article)
Acclimation responses of hydrogen producing cyanobacteria




List of Authors: Murukesan. Gayathri
Publisher: University of Turku
Place: Turku
Publication year: 2018
ISBN: 978-951-29-7373-6
eISBN: 978-951-29-7374-3

Abstract

During recent years, cyanobacteria have been gaining popularity rapidly as a platform for CO2 sequestration and production of a wide range of industrially attractive products. Factors such as simple nutritional requirements, flexibility to genetic manipulations and ability to adapt to fluctuating environmental conditions make these organisms suitable for bioindustrial processes that could be leveraged to address earthbound challenges like climate change and extraterrestrial ambitions like long‐term manned missions to space. The main objective of this thesis was to improve our understanding of the growth and cellular acclimation of cyanobacteria in response to specific environmental conditions such as N deficiency leading to the improved H2 photoproduction yield and simulated Martian atmosphere. The research activities extended from cyanobacterial cultivation for biomass accumulation, CO2 sequestration and production of some valuable metabolites, such as carbohydrates and carotenoids to conversion of solar energy into energy of hydrogen biofuel by alginate‐entrapped cultures. 

Major part of my research was dedicated to optimization of a biohydrogen production platform using heterocystous cyanobacteria. I evaluated possible routes to prolong H2 photoproduction in native (Calothrix 336/3) and model (Anabaena PCC 7120, ΔhupL) strains of cyanobacteria entrapped in Ca2+‐ alginate films. Periodic supplementation of nitrogen through addition of air, or air + 6% CO2 was shown to restore the photosynthetic activity of the entrapped cells and increased the H2 production yields in Calothrix 336/3 and ΔhupL cells (excluding air + 6% CO2). Despite obvious recovery of the photosynthetic activity, the H2 photoproduction yields did not alter post air‐treatments of the wild‐type Anabaena PCC 7120, which could be linked to the presence of active uptake hydrogenase recycling H2. In general, Calothrix showed a more stable photosynthetic apparatus and resilience to H2 photoproducing conditions. Such robustness is most probably determined by an efficient reactive oxygen species scavenging network. Indeed, characterization of carotenogenesis pathway in Calothrix 336/3 showed high content of hydroxycarotenoids that are efficient antioxidants. Research revealed that alginate‐entrapped cyanobacteria under H2 photoproducing conditions tend to employ strain‐specific strategies to counteract the C/N imbalance and oxidative stress, especially when exposed over extended periods. Further, my research proposed the prominent role of the uptake hydrogenase enzyme in photoprotection of the filaments during stress conditions such as the long‐term N‐deprivation. Part of my research was focused on the growth and acclimation of unicellular and heterocystous filamentous cyanobacteria under a low‐pressure atmosphere simulating Martian (< 1 atm, N‐limitation, high CO2) conditions. Here, the availability of CO2 and N2, and the presence or absence of O2 showed an effect on the growth and heterocyst formation in cyanobacteria, in an interdependent manner. The tested strains were able to tolerate 100% CO2 in atmospheric pressures as low as 100 mbars. 

To summarize, my results show that the acclimation responses of H2 producing cyanobacteria to various stress‐inducing conditions are indeed strain specific. In depth understanding of such behavior is especially important to consider when designing a commercially inclined platform incorporating cyanobacteria.



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Last updated on 2019-29-01 at 22:15