Formation of sphere pellets with relatively homogeneous size could be observed after three days of cultivation period.
In fact,A. niger has been reported as a common filamentous fungus with microscopic morphology of branched hyphae related to the formation of the pellets (Grimm et al., 2004). Based on their morphology,the fungus can be categorized into three groups: the first group with loose pellets in which the centre of the pellets is compact while the outer zone is much looser; the second group with compact smooth pellets in which the whole pellet is compact and the outside of the pellet is smooth; the last group with hollow smooth pellets in which the centre of the pellet is hollow due to autolysis and the outside is smooth (Metz and Kossen, 1977). The pellets were separated from the media and the wet-weight of the pellets were determined prior to harvesting of the microalgae biomass.It was reported that more compact pellets will are advantageous for subsequent cell harvesting (Olaizola, 2003; Zhang and Hu,2012).