Abstract
BackgroundTriacylglycerol (TAG) from photosynthetic microalgae is a sustainable feedstock for biodiesel production. Physiological stress triggers microalgal TAG accumulation. However excessive physiological stress will impair the photosynthesis system seriously thus decreasing TAG productivity because of the low biomass production. Hence, it is critical to quantitatively and timely monitor the degree of the stress while the microalgal cells growing so that the optimal TAG productivity can be obtained.ResultsThe lack of an on-line monitored indicator has limited our ability to gain knowledge of cellular “health status” information regarding high TAG productivity. Therefore, to monitor the degree of nitrogen stress of the cells, we investigated the correlation between the photosynthetic system II (PS II) quantum yield and the degree of stress based on the high relevancy between photosynthetic reduction and nitrogen stress-induced TAG accumulation in microalgal cells. ΔF/Fm′, which is the chlorophyll fluorescence parameter that reflects the effective capability of PS II, was identified to be a critical factor to indicate the degree of stress of the cells. In addition, the concept of a nitrogen stress index has been defined to quantify the degree of stress. Based on this index and by monitoring ΔF/Fm′ and guiding the supply of nitrogen in culture medium to maintain a stable degree of stress, a stable and efficient semi-continuous process for TAG production has been established.ConclusionThe results indicate that the semi-continuous cultivation process with a controlled degree of stress by monitoring the ΔF/Fm′ indicator will have a significant impact on microalgal TAG production, especially for the outdoor controllable cultivation of microalgae on a large scale.
Highlights
Triacylglycerol (TAG) from photosynthetic microalgae is a sustainable feedstock for biodiesel production
The sufficient supply of NADPH is essential for TAG accumulation [10], whereas excessive electrons will lead to the formation of reactive oxygen species (ROS), which exposes microalgae to oxidative stress and is believed to be a signal trigger for TAG formation [11,12,13]
It should be sensitive to N-stress, including the content of intracellular N and other parameters of physiology or biochemistry, such as the metabolite content, elemental content, photosynthesis activity, and respiration rate
Summary
Triacylglycerol (TAG) from photosynthetic microalgae is a sustainable feedstock for biodiesel production. Protein synthesis in microalgae is immediately suppressed upon nitrogen shortage, which mostly hinders the protein turnover of photosynthetic apparatus, especially the photosystem (PS) II D1 reaction centre protein [8]. This will lead to a decline in the photosynthetic electron transport rate (ETR) and, a reduction in photochemical energy conversion [8, 9]. It is very important to make systematic research on it
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