Assessment of acute toxicity of water soluble fraction of diesel on two species of marine microalgae

Abstract

Acute toxicity of water soluble fraction (WSF) of diesel fuel was assessed by evaluating its effects on growth of two marine microalgae, Isochrysis and Chaetoceros. Pure cultures of each of the two microalgae were exposed to concentrations of 0% (controls), 5%, 10%, 15% and 20% of diesel WSF (in triplicates) and allowed to grow for 96 hours. Cell density was determined in each of the culture vessels every 24 hour. Cell density growth rate in every treatment, percentage growth inhibition by various concentrations and IC₅₀ of diesel WSF on the two microalgae were determined following standard methods. While control cultures recorded exponential growth within 96 hour, the four WSF concentrations resulted in inhibition of cell growth of the two microalgae species every 24 hour. Cell growth inhibition effect increased significantly (p<0.05) from 5% to 20% WSF exposure in Isochrysis and Chaetoceros with 100% and 98% growth inhibition in the two species by 20% WSF after 96 hour. Diesel WSF had IC₅₀ of 7.59% and 8.08% against Isochrysis and Chaetoceros respectively. Growth inhibition effects of WSF against the two species were not significantly different (p>0.05). Ecological implications of the growth inhibition effects of diesel WSF microalgae were discussed.

Key Words: Cell density, water soluble fraction (WSF), concentration, growth inhibition, 96 hour IC₅₀, Isochrysis, Chaetoceros