In the present study, a centric diatom Chaetoceros minimus is reported for the first time from the Indian waters. This record will contribute to the phytoplankton checklist of Indian seas. Morphological characteristics and distribution of the species are provided.

Thermal acclimation following changes from 6 °C to 15 °C and from 15 °C to 25 °C was investigated in the diatom Chaetoceros calcitrans. Cell division rate increased with temperature. Cellular carbon showed a slight decrease with an increase in temperature from 6 °C to 15 °C but showed no further change with an increase to 25 °C. Cells grown at 6 °C had low levels of light-harvesting components and a high carotenoid to chlorophyll a (Chla) ratio due to an increase in abundance of photoprotective pigments. The abundance of light-harvesting pigments increased with temperature and this was mirrored by a decrease in carotenoid:Chla ratio. All pigments showed an immediate change in abundance following each temperature shift. However, changes in photoprotective pigments reached a plateau after 12 h, in contrast to the light-harvesting pigments that took 4 days to become acclimated. Light-saturated rates of photosynthesis were at a minimum at 6 °C and increased twofold with temperature between 6 °C and 25 °C. Light-limited photosynthesis showed little change with temperature. It is suggested that, at low temperatures, the rate of carbon fixation is impaired due to a decrease in the activity of photosynthetic enzymes. This imparts an increase in excitation pressure due to an imbalance between light absorption and utilization. Cells respond to this by decreasing their light-harvesting capacity and increasing their ability to dissipate the excess energy. This fairly rapid regulation may be ecologically advantageous in areas where there are transient temperature changes.

Three marine planktonic species of the diatom genus Chaetoceros were studied by light and electron microscopy, from net samples collected from different tropical and subtropical areas: coasts of Baja California, the Gulf of Tehuantepec (both in the Pacific Ocean), the Gulf of Mexico (the Atlantic Ocean) and the Indian Ocean. Two of the species, Chaetoceros pseudodichaeta and C. pseudoaurivilli, were originally described by Ikari in 1926 from Japanese waters. This is the second record of them. C. pseudodichaeta is superficially similar to C. dichaeta but detailed morphology differs, especially that of the intercalary setae, which are four-sided in cross-section, with long spines at the edges, and a pattern of two striae between two costae. Chaetoceros pseudoaurivilli is a distinctive species with a characteristic dome- shaped protuberance on terminal valves and the presence of various rimoportulae on, at least, terminal valves of the chain. Finally, C. pseudosymmetricus is a species only reported twice in the Indian Ocean. Its distinctive character is the heteropolarity of terminal setae: one is coarse, long and strongly curved, and the other is delicate, shorter and smoothly curved. It also has a single rimoportula on terminal valves only. The three species are regarded as rare, and consequently our knowledge of their distribution is rather poor. Additional taxonomic comments are provided for the three species.