Abstract A bacterial strain SSZ01 isolated from a eutrophic lake in Saudi Arabia dominated by
cyanobacterial blooms, showed an antialgal activity against cyanobacteria species. Based on the
analysis of the 16S rDNA gene sequence, the isolated strain (SSZ01) most likely belonged to the
genus Bacillus with a 99% similarity to Bacillus flexus strain EMGA5. The thin layer chromatography
(TLC) analysis of the ethyl acetate extract of this bacterium revealed that this strain can produce
harmine and norharmane compared to different b-carboline analog standards. Harmine and
norharmane were also detected in considerable amounts in bacterial growth medium, indicating a
potential excretion of these compounds into the aquatic environment. The crude extract of Bacillus
flexus as well as pure materials of harmine and norharmane inhibited the growth of tested species of
cyanobacteria. However, the bacterial crude extract has a higher toxicity against tested species of
cyanobacteria than harmine and norharmane. In addition, harmine was more toxic to cyanobacteria
than norharmane. On the other hand, neither pure compounds of harmine and norharmane nor
crude bacterial extract showed any antialgal activity against tested species of green algae. The
results of the present study suggest that B. flexus SSZ01 or its crude extract containing harmine
and norharmane could be a candidate for the selective control of cyanobacterial blooms without
affecting other algal species