Abstract
Microalgae have great potential for the production of biofuels due to the ability of the organism to accumulate large quantities of storage lipids under stress conditions. Mitogen activated protein kinase (MAPK) signaling cascades are widely recognized for their role in stress response signal transduction in eukaryotes. To assess the correlation between MAPK activation and lipid productivity, Chlamydomonas reinhardtii was studied under various concentrations of NaCl. The results demonstrated that C. reinhardtii exhibits elevated levels of extracellular-signal regulated kinase (ERK) and c-Jun N-terminal kinase (JNK) activities after undergoing osmotic stress, as well as an increase in cellular lipid content. To establish a more direct causal link between both kinases and lipid productivity, C. reinhardtii was subjected to biochemically induced regulation of ERK and JNK pathways. Activating the MEK-ERK pathway via C6 ceramide treatment increased ERK activation and lipid production simultaneously, while PD98059 mediated inhibition of the pathway yielded opposite results. Interestingly, suppression of the JNK pathway with SP600125 resulted in a substantial decrease in cell viability under osmotic stress. These results suggest that ERK and JNK MAP kinases have important roles in microalgal lipid accumulation and cell growth under osmotic stress, respectively.
Highlights
Concerns regarding unsustainable consumption of fossil based energy sources as well as the inevitably dwindling reserves have resulted in an increasing interest in renewable fuel sources
In order to verify whether mitogen activated protein kinase (MAPK) is activated under osmotic stress, western blot analysis of cultures grown under increasing concentrations of NaCl was done for three different types of MAP kinases, extracellular-signal regulated kinase (ERK), Jun N-terminal kinase (JNK), and p38
To further scrutinize how the MEK-ERK pathway is affected by osmotic stress, both the MEK1/2 and ERK1/2 activities were analyzed after the cells were subjected to various levels of osmotic stress for 5 days
Summary
Concerns regarding unsustainable consumption of fossil based energy sources as well as the inevitably dwindling reserves have resulted in an increasing interest in renewable fuel sources. Even though subjecting the microalgae to a stress environment can greatly improve the overall lipid productivity, such a process requires greater complexity in the cultivation protocols. This is because often times a two-step cultivation process, where the cells must be harvested and transferred from one environment optimized for growth to another environment which is optimized for lipid accumulation, is necessary[19]. Adding salts to the culture would preclude the reusability of the growth medium, and this would result in a significant increase in the water footprint[20] This is not feasible, especially in large scale open outdoor cultivation systems, due to a large amount of energy consumption associated with the harvesting. The mechanism that triggers lipid biosynthesis under stress conditions is unknown
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