A Nostoc-like cyanobacterium, strain SAG 2306, was isolated from a clay soil sample at Assiut region (Egypt). Morphology and analysis of 16S rDNA identified it as a member of the heterocyteous cyanobacteria (Nostocales). Unexpectedly, chlorophyll a and dry weight were higher in manganese deprived (Mn-) or in manganese double deprived (Mn-) than in replete (Mn+) cells. The latter phase of Mn- diluted the cellular content in daughter cells to a critical minimal value of about 1.5% relative to the original 100% in Mn+. Similar to growth, Nostoc sp. exhibited a significant age-dependent biphasic impact on hydrogenases activity. Older cells of 120 h age displayed higher hydrogen evolution rates than younger ones. The results suggest inhibitory effects of the 12 mu M manganese contained in BG11 medium (Mn+) not only on growth but also on hydrogen production during the first 72 h of growth. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.