Does light quality affect the sinking rates of marine diatoms?

Although the spectral quality of light in the ocean varies considerably with depth, the effect of light quality on different physiological processes in marine phytoplankton remains largely unknown. In cases where experiments are performed under full spectral ir radiance, the meaning of these experiments in situ is thus unclear. In this study, we determined whether variations in spectral quality affected the sinking rates of marine diatoms. Semicontinuous batch cultures of Thalassiosira weissflogii (Gru.) Fryxell et Hasle and Ditylum brightwellii (t. West) Grunow in Van Huerk were grown under continuous red, white, or blue light. For T. weissflogii, sinking rates (SETCOL method) were twice as high (approximate to 0.2 m . d(-1)) for cells grown under red light as for cells grown under white or blue light (approximate to 0.08 m . d(-1)), but there were no significant differences in carbohydrate content (approximate to 105 fg .mu m(-3)) or silica content (approximate to 17 fg .mu m(-3)) to account for the difference in sinking rates. Thalassiosira weissflogii grown under blue light was significantly smaller (495 mu m(3)) than cells grown under red light (661 mu m(3)) which could contribute to its reduced sinking rate. However, cells grown under white light were similar in size to those grown under red light but had sinking rates not different from those of cells grown under blue light, indicating the involvement of factors other than size. There were no significant differences in sinking rate (approximate to 0.054 m . d(-1)) or silica content (approximate to 20 fg .mu m(-3)) in D. brightwellii grown under red white, or blue light, but cells grown under red light were significantly (20%) larger and contained significantly (20%) more carbohydrate per mu m(3) than cells grown under white or blue light. Spectral quality had no consistent effect on sinking rate, biochemical composition (carbohydrate or silica content), or cell volume in the two diatoms studied. The similarity in sinking rate of cells grown under white light compared to those grown under blue light supports the ecological validity of sinking rate studies done under white light.