GROWTH OF THE EDIBLE MICROALGA ARTHROSPIRA PLATENSIS IN RELATION TO BORON SUPPLY

Abstract

Arthrospira (Spirulina) platensis is an edible cyanobacterium that has been consumed worldwide as a nutrient source under the name spirulina. When preparing synthetic media for this microalga, boron is usually added to them. However, whereas boron is necessary for the N-2-fixation-dependent growth of heterocystous cyanobacteria, boron requirement by A. platensis, which is non-heterocystous, has not yet been carefully examined. To examine the effect of boron on A. platensis, we prepared a boron-depleted medium in which borate concentration was below a detection limit (0.2 mu M), as determined by the spectrophotometric quantitation with H-resorcinol. Using this boron-depleted medium, A. platensis NIES-39 was analyzed for changes in growth, dry biomass weight and the protein, chlorophyll a and C-phycocyanin contents. Experimental results showed that removal of boron from the growth medium had no detectable effect on them. A control experiment with a heterocystous cyanobacterium Anabaena sp. UTEX 2576 showed that the growth of this cyanobacterium was suppressed in a boron-depleted medium. These experiments demonstrated that boron was not required for the growth of A. platensis. Our results indicate that boron supply is not necessary for the propagation of A. platensis even in the regions where boron deficiency in the soil deteriorates the growth of common crops. Use of boron-free media would also be helpful to improve the reproducibility of sensitive physiological experiments such as metabolome analysis, because precipitation of the insoluble salt manganese borate, which may reduce the concentration of Mn2+ in the medium, does not occur in boron-free media.