Characterising nutrient-induced fluorescence transients (NIFTs) in nitrogen-stressed Chlorella emersonii (Chlorophyta)

Although determining the nutrient status of algae is highly desirable for water quality management, conventional methods for assessing nutrient limitation are confounded by a range of theoretical and practical limitations. The current paper examines the effectiveness of a novel rapid fluorometric technique, based on observations of nutrient-induced transients in chl a fluorescence (NIFTs), in providing accurate measurements of algal nutrient limitation. The progress of N-starvation in batch Cultures of the freshwater chlorophyte Chlorella emersonii was followed and the NIFT responses of cells to N-starvation and additions of both NH4+ and NO3- were characterised using a number of techniques including NIFTs, conventional pigment analysis using UV/Visible spectrophotometry and Fourier transform infrared (FT-IR) and Raman spectroscopy. Results showed that the addition of NH4+ and NO3- elicited distinct changes in in vivo chl a fluorescence in N-limited Chlorella cells. The nature of the fluorescence change was dependent on the form of nitrogen supplied, with NH4+ additions producing a more complex effect on fluorescence than NO3- additions. Interestingly, the greatest fluorescence response following NH4+ injection occurred prior to cells becoming extremely N-starved. Despite Chlorella exhibiting many of the characteristic features of N-deficient cells such as lowered capacity for photosynthetic electron transport (rETR(max)), reduced Chl a : beta-carotene ratios and increased levels of carbohydrate accumulation relative to protein, values for the maximum effective quantum yield of Photosystem II ((phi PSIIe-max) remained high over the course of N-starvation.