Biological identification and determination of optimum growth conditions for four species of Navicula

Four species in the genus Navicula were isolated using the serial dilution method. Based on scanning electron microscopy (SEM) and sequence comparisons of two segments of genes (small ribosomal subunit and large subunit of Rubisco), the species were identified as Navicula perminuta, N. pseudacceptata, N. vara, and N. rhynchocephala. Based on phylogenetic analysis and culture trials, there was a close relationship between N. perminuta and N. vara. Growth of these species was evaluated using measurements of optical density at 680 nm (OD₆₈₀) under various environmental factors. Results showed that the optimum culture conditions were 25℃, 50–100 μmol photons m-2 s-1, pH 8.0, and salinities from 25 to 30. However, the favorable salinity for N. perminuta was surprisingly high at 35. Nutrient requirement analysis demonstrated that growth of Navicula depended on the availability of SiO₃2-. Their relative growth rates (RGR) peaked at the highest tested level (0.25 mmol/L). The optimal concentrations of NO₃- and PO₄3- were 3.6 mmol/L and 0.18 mmol/L, respectively. Culture of these Navicula species for abalone or sea cucumber aquaculture should take these factors into consideration.