Studies on a marine diatom Cosinodiscus pavillardii, Forti

Abstract

A large centric diatom, tentatively identified as Coscinodiscus pavillardii Forti, was isolated from a collection of samples taken in the central area of the Sierra Leone River estuary on 23rd December, 1964. Its morphological characters are examined, and taxonomic problems involved in its identification are discussed. Laboratory studies were made with the diatom using unialgal but not axenic cultures. The effects of light intensity, temperature, salinity, different cell sizes and vitamins on the growth rate of the diatom were exmined. The saturating light intensity was 6,000 lux and there was little or no inhibition at 15,000 lux. The growth (division) rate was virtually unchanged over the salinity range of 13-33 /oo (one-third to about full strength sea water). The diatom survived at salinities below 13º/oo but not at 6.7º/oo and below. The smaller cells (73-164 μ) divided faster than the larger ones (327-345 μ), but their division rates were independent of the relative growth constant. Of the three vitamins - biotin, cobalamin and thiamine - only cobalamin medium was lower than in the complete medium (containing all the vitamins). In the absence of biotin in the complete medium the growth rate was better than in the complete medium. Whilst in culture, spore (i.e. male gemete and auxospore) formation occurred in cells of about 20-60% of the mean maximum cell diameter (464 ± 44 μ). Spore formation lowered the vegetative growth rate and occurred during the exponential phase of growth. The extent to which the growth rate was reduced was dependent on the proportion of cells involved in the activity. The effects of light, temperature, salinity and vitamins on spore formation were also examined. The favourable conditions were - (i) light intensity of 5,500 - 8,000 lux, used either as continuous light or as an alternating twelve hour light/dark cycle; (ii) salinities of 19 - 33º/oo, (iii) temperatures of 23 - 28 C. A light intensity of 11,000 - 12,000 lux; temperatures above and below the stated range; and salinities of 16.5º/oo and below, inhibited their formation. The alternating light dark cycle promoted better spore formation than continuous light, Cobalamin was found to be required for spore formation although its action seemed to be linked with the promotion of vegetative growth. When biotin was omitted from the complete medium there was a noticeable increase in the number of spores produced. Light is considered to play the most significant role in sex determination. Moreover, it appeared that spore formation in the diatom is a result of a short-day quantitative photoperiodic response complicated by an additional ionic effect, The methods of spore formation in the living material, as well as a probable case of Isogamy are described and illustrated by photomicrographs.