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Abstract
Continued studies on diatoms living in the Venice lagoon southern basin sediments have investigated the essential role of the two variables, light and temperature, on their development or their survival capability. Part of the research consisted of transferring lagoon depth sediments to the laboratory, using of special sample-cases designed for this purpose of not disturbing the sediment. The collected material was subjected to different light and temperature conditions. The experimental samples were periodical core over a period of a week and compared with the initial values of samples drawn “in situ”. The results confirmed the constant increase of cells on the sediment surface. The highest increase occurred when the phototactical stimulus was induced by lower luminous intensities, despite the fact that moisture was limited but most likely compensated for by the sediment composition (thin sands of clayey type), which appears to have had a satisfactory water retention. This suggests that the location of these organisms inside the sedimentary matrix is the compromise between water requirements and energy demands (as diatoms that can change feeding system, from autotrophic to heterotrophic). The exposure period (continuous light or alternation of light and dark) doesn't seem to affect the population increase.
Some diatom strains have been isolated from the collected material. In particular, growth tests with different luminous intensities and temperatures have been performed on a strain belonging to the Navicula genus. Apart from the available quantity of light, the results of these tests emphasize that exponential phase is reached in a shorter period of time in strains maintained at higher temperatures. The influence of luminous intensity has proved to be less significant.