Light absorption, photosynthesis and growth of Nannochloris atomus in nutrient-saturated cultures

Abstract

Nannochloris atomus was maintained in exponential growth at photon flux densities (PFD) from 400 to 700 nm, ranging from 10 to 200 μmol m-2 s-1. Growth was lightsaturated at PFDs in excess of 100 μmol m-2 s-1, with a mean light-saturated growth rate at 23 °C of 1.5×10-5s-1 (1.2 d-1). The light-limited growth rates extrapolated to a compensation PFD for growth that was not significantly different from zero, although no changes in cell numbers were observed in a single culture incubated at a PFD of 1.0 μmol m-2s-1. Dark-respiration rates were independent of PFD, averaging 1.7×10-6 mol O2 mol-1 C s-1 (0.14 mol O2 mol-1 C d-1). The maximum photon (quantum) efficiency of photosynthesis was also independent of PFD, with a mean value of 0.12 mol O2 mol-1 photon. The chlorophyll a-specific light absorption cross-section ranged from 3 to 6×10-3 m2 mg-1 chl a and was lowest at low PFDs due to intracellular self-shading of pigments associated with high cell-chlorophyll a contents. The C:chl a ratio increased from 10 to 40 mg C mg-1 chl a between PFDs of 14 and 200 μmol m-2 s-1. These new observations for N. atomus are compared with our previous observations for the diatom Phaeodactylum tricornutum in terms of an energy budget for microalgal growth.