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 N2-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 µM), 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.