Auxosporulation and cell enlargement in thalassinoid diatoms

Abstract

Our understanding of diatom reproduction is limited, especially for marine planktonic centric diatoms. This project aimed to observe sexual behavior and auxospore development in the polar centric group, Thalassiosirales, that maintains the morphology of a nonpolar centric diatom. Auxosporulation is the main means of restoring large cell sizes in the majority of diatom species examined. Monoclonal cultures of T. weissflogii (CCMP1587), T. nordenskioeldii (CCMP997) and Thalassiosira cf. oceanica were subjected to sexual induction protocols, resulting in non-typical cells. Spermatogenesis was not observed and presumed absent in all inductions. Spherical cells with non-siliceous cell walls were observed, occurring in about 12% of the population for T. nordenskioeldii, and 17% in T. weissflogii. However, these spherical cells were not eggs or auxospores and further observations using light microscopy (LM) suggest that these cells were arrested in this stage of enlargement and eventually died. Spherical and irregular shaped cells were observed in T. weissflogii, as well as cells in general appearance similar to auxospores. The nuclear behavior observed in T. weissflogii indicates that this species could reproduce via autogamy, but mature initial cells enclosed within such “auxospores” were never observed. Inductions were not successful in Thalassiosira cf. oceanica. In the absence of gametogenesis and mature auxospores, the larger typical vegetative cells encountered in cultures following induction protocols in T. nordenskioeldii and T. weissflogii are therefore attributed to vegetative cell enlargement.