Light-path length and population density in photoacclimation of Nannochloropsis sp (Eustigmatophyceae)

Photoacclimation in the marine eustigmatophyte Nannochlropsis sp., used extensively as a food chain component in aquaculture, was studied both in the laboratory and outdoors. Cell-chlorophyll and carotenoids were used as markers to assess photoacclimation to strong light, as well as to decreasing growth irradiance due to cell proliferation. Focusing on practical aspects involved in mass cultivation, three different approaches were used as follows: (a) cultures initially exposed to low light (150 mu mol photon m(-2) s(-1)) then transferred to strong light (1000 to 3000 mu mol photon m(-2) s(-1)); (b) initially low cell density cultures grown in reactors of different light-paths, exposed to strong PFD, in the laboratory and outdoors; (c) initially low or high cell density cultures exposed to strong light. As has already been established in many reports, cell-chlorophyll represented a sensitive parameter in assessing cell response to changes in the intensity of the light source as well as to modifications in the light regime to which the cells were exposed. Cell-chlorophyll concentration sharply decreased initially upon transferring the culture from low PFD cell(-1) to high PFD cell(-1) due to either culture dilution (i.e. decrease in cell density and mutual shading) or to an increase in PFD. After some 7 days of photoacclimating to 2000 and 3000 mu mol photon m(-2) s(-1), chlorophyll a content began to rise at a much faster rate than cell number, which also increased in response to the higher irradiance. Cell-chlorophyll in the culture exposed to 2000 mu mol photon m(-2) s(-1) increased after acclimation earlier and at a faster rate than in the culture exposed to 3000 mu mol photon m(-2) s(-1), indicating the later irradiance affected a stronger stress. The length of the reactor's light path exerted a decisive effect on cell response to strong light through its influence on the light regime in the culture. Upon a sharp increase in PFD, carotenoids in the 1-cm reactor increased in much higher rate than chlorophyll, compared with the 3-cm light path reactors. This marked difference in cell response to a shift-up in light was attributed to the vast variations in the light regime associated with differences in the length of the light path and areal density. Growth of low cell density cultures ceased temporarily upon transfer to strong light, in contrast with high cell density cultures transferred to strong light, which continued growth without a lag.