Morphological variability of the planktonic diatom Thalassiosira delicatula Ostenfeld emend. Hasle from the Mexican Pacific, in culture conditions

Abstract

Cultures of marine microalgae have greatly contributed to the better understanding of the morphology, phylogeny, life cycles, physiology and ecology of these species. In this paper, the planktonic diatom Thalassiosira delicatula Ostenfeld emend. Hasle, originally isolated and cultured (in f/2 medium, a light-darkness cycle of 12:12, and at temperature of 20 °C ± 2 °C) from material obtained from the tropical, southern Mexican Pacific, was studied by LM, SEM and TEM. General morphology of the species agrees well with previous descriptions, including the formation of chains and colonies embedded in mucilage, the chitin threads connecting cells in the chain, the shape of the cells (in girdle view rectangular with a concavity in the valve centre), and processes (with a large rimoportula between valve face and mantle, a marginal ring of occluded ones, and two or three marginal rings of fultoportulae) and areolae arrangement. However the shape and distribution of chloroplasts and threads produced by marginal fultoportulae in living cells, and the presence of more than one central fultoportula, close to a larger areola (two and three fultoportulae were detected in this study) had never been described. Urn-shaped marginal fultoportulae were also very common in most specimens studied, whereas in the literature this character was found only once for the species. Additionally, two rimoportulae were detected in at least one valve. This morphological variability is discussed. This species is a new record for the Mexican Pacific and also the first record of the northern Pacific Ocean.