Evidence of a hormetic effect in growth inhibition algal test.

Elutriation process was applied on sediments of Guanabara Bay (Rio de Janeiro, Brazil), a very polluted coastal ecosystem, in order to determine their potential toxicity. Elutriates were tested using Skeletonema costatum, an eurialin Diatom. Several ratios of elutriate/growth medium (v/v) were assayed (1:10, 1:4, and undiluted) as cultural medium for exponentially growing algae, carrying out a blank control for each replicate. Tests were performed for five days, controlling algal growth by manual cell counting by microscope, in order to verify the status of the organisms every assay’s day. This experimental design was proved able to highlight a particular phenomenon, i.e. hormesis. Regarding some sites in the southern part of the Guanabara Bay growth curves, relative to lower sediment elutriate dilutions, showed, in fact, a so called toxic stimulation typical of hormetic event. On the contrary undiluted elutriates never permitted algal growth. Obtained results could help furthermore to explain the particular eutrophic situation of the water body, in which there is a high superficial eutrophization and absolute life deficiency in the deeper water column.