Abstract
The physiology and biochemistry of Phaeodactylum tricornutum have been extensively studied, and some aspects of its biology are well known. Phenotypic plasticity is probably one of its most remarkable features. Two basic morphotypes, fusiform and triradiate, can be found in natural liquid environments. Although the transformation from one morphotype to the other has been previously reported, ecological causes and consequences of such transformation remain unknown. Here, we report changes in the relative abundance of the two morphotypes of Phaeodactylum associated with changes in the inorganic carbon system. The relative abundance of the triradiate morphotype increased in cultures grown at either high pH or high dissolved inorganic carbon, and markedly so under subsaturating illumination for growth. At high pH, this increase was strongly correlated with a decrease in the overall culture growth rate. At the individual (cell) level, there was a larger increase in the surface area of the triradiate morphotype (resulting in a higher surface-to-volume ratio) than in the fusiform morph under subsaturating illumination for growth. These results provide new insights into the biology of this diatom, mainly related to the performance (fitness) of the two basic morphotypes under contrasting illumination for growth and different inorganic carbon conditions.