The focus of this study is to elucidate the photosynthetic activity of microalga Chlorella vulgaris using pulse amplitude modulation (PAM) fluorometry. C. vulgaris is a species of microalga that is currently being studied for its use in biodiesel production. It has many of the advantages of other algal-based biofuel feedstocks such as not competing for arable land, its ability to grow on waste water, and the production of large amounts of short-chain fatty acids. C. vulgaris also has the ability to grow mixotrophically (simultaneous heterotrophic and phototrophic growth), however, the extent of heterotrophic versus phototrophic activity in C. vulgaris grown in light conditions has not yet been determined. PAM fluorometry is a reliable and sensitive method of measuring photosynthetic activity in whole cells. By measuring the fluorescence emitted by the algae when the cells are subjected to continuous illumination or flashes of actinic light, the mode of photosynthesis can be determined (linear vs. cyclic). This investigation uses PAM fluorometry on C. vulgaris, grown in both light (photomixotrophically) and dark conditions (heterotrophically), in order to evaluate the OJIP transient (the fluorescence profile of the early steps of photosynthesis) of the algae in both conditions throughout their life cycle. Cells grown in the light ultimately yield the same biomass as cells grown in the dark while consuming equivalent amounts of glucose; however, they have a higher abundance of lipids. One hypothesis is that the cells use cyclic electron transfer instead of linear photosynthesis. The purpose of this experiment is to determine which mode of photosynthesis the cells are using under the different growth conditions. Furthermore, fluorometry parameters (e.g. quantum efficiency, maximum variable fluorescence, etc.) will be evaluated in order to compare the rate of photosynthesis during different points in the alga’s life cycle. Without light, cells cannot perform photoassembly, the light-dependent process through which the OEC is constructed. The rate of photoassembly will be evaluated by exposing heterotrophic cells to light for varying periods of time and analyzing the appearance of the OJIP transient. The results may give insight into the extent of the mixotrophy exhibited, as well as the fluctuations in photosynthetic activity in mixotrophic organisms’ life cycles, which will assist in optimizing the photosynthetic efficiency of microalgae in general.

Pulse amplitude modulation fluorometry as a means to determine photosynthetic modes of Chlorella vulgaris

255th National Meeting (2018)

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