Effects of different photoperiods and concentrations of phosphate on the growth of the cyanobacterium <i>Cylindrospermopsis raciborskii</i> (Woloszynska)

Abstract

The freshwater cyanobacterium, Cylindrospermopsis raciborskii is potentially toxic and widely distributed in tropical and sub-tropical regions. This species is highly abundant throughout the Anzali Lagoon during the months of July and August. The effects of four different photoperiods (L/D 12:12, 10:14, 14:10, 8:16) as well as three different concentrations of phosphate (2 fold, 3 fold and half of the main culture) on growth rates of C. raciborskii were studied. C. raciborskii, isolated from water samples collected from the Anzali Lagoon, was cultured in Zehnder 8 media (using Z₈ without a nitrogen source) under light intensity of 3000 Lux at 25+2°C. A control group cultured with the main culture medium was also used. This experiment was carried out for 25 days. Inoculants and C. raciborskii cells were counted every other day throughout the experiment period using a Neubauer chamber and specific growth rates (SGR) and cell division rates (G) were estimated. The maximum growth rate (8x10⁶ ± 5 cells ml^-1) was observed at 12 L:12 D photoperiod followed by 14 L:10 D, 10 L:14 D and 8 L:16 D. The results obtained in the present study especially with 14 L:10 D and 12 L:12 D were in complete agreement with general growth pattern of C. raciborskii observed in warmer months with rather longer daytime in tropical and subtropical areas. The maximum growth rates were observed in the group treated with 2 fold PO₄ concentration. The strategy of growth in this species may depend on nitrogen fixation, when other algae were nitrogen limited and fast PO₄ uptake even if very low concentration is available.

Key words: Cyanobacteria, Cylindrospermopsis raciborskii, Anzali Lagoon, photoperiod, phosphate, growthrate.