Effects of light intensity on the selectivity of lipid and chitin nanofiber production during photobioreactor cultivation of the marine diatom Cyclotella sp.

Diatoms are single-celled algae that make cell walls of nanopatterned biogenic silica through metabolic uptake of dissolved silicon. The centric marine diatom Cyclotella sp. produces intracellular lipids for biofuels, and nanofibers of the N-acetyl glucosamine biopolymer β-chitin, extruded from pores in the biosilica cell wall. The effects of light delivery on the dynamics of CO2 consumption and its assimilation into lipid and chitin were studied in a bubble-column photobioreactor through a two-stage cultivation process under silicon-limited and nitrate-replete conditions. The mean light intensity spanned the linear and saturation regimes of photosynthesis-irradiance (P-I) curve (12-376 μE/m2-s). Lipids and chitin were produced following silicon uptake under nitrogen-replete conditions. Although the final cell number density was fixed by silicon delivery, net CO2 consumption and total biomass carbon increased within increasing light intensity according to the profile defined by the P-I curve. Chitin production on a per cell basis was not sensitive to light intensity, and averaged 44.7 ± 2.3 mg chitin/109 cells (1.0 S.E). Increasing the light intensity from the linear to saturation regime enhanced lipid production on top of this fixed chitin production, with peak productivities of 22 ± 2 mg/L-day (1.0 S.E.) for chitin, and 74 ± 9 mg/L-day (1.0 S.E.) for lipid. At low light intensity corresponding to the linear regime of the P-I curve, product yields were 34% ± 2 wt% chitin and 5.1 ± 0.6 wt% lipid (6.4 ± 0.7 mg/109 cells, 1.0 S.E.). At light saturation, the chitin yield within the biomass was15 ± 1 wt% (1.0 S.E.), whereas the lipid yield was 38 ± 4 wt% (110 ± 2 mg/109 cells, 1.0 S.E.). This study demonstrates that the control of both silicon and light delivery tunes the relative chitin and lipid production by the diatom Cyclotella.