Holothuria polii, the common black sea cucumber (Echinodermata, Holothuroidea), is abundant in the Mediterranean Sea along the Alexandria coast. This species was given great value commercially and for human consumption. The present study was objective to assess the biological effect of marine pollution on the sea cucumber, Holothuria polii collected from two locations in Alexandria coast; Abo- Qir station as an industrial area compared with Miami station as a reference area, using the oxidative stress biomarkers approach, heavy metals bio-accumulation, and Rickettsia-like organism (RLOs) infection. Sea cucumber samples were collected from both stations in Alexandria, during two seasons; winter (January) and summer (July) 2016. Atomic absorption spectrophotometer was used to detect Zn, Mn, Pb, Cu, and Cd in the body wall of the animal as well as in water samples.
The results showed a significant decrease in the antioxidant parameters; catalase (CAT), superoxide dismutase (SOD), Glutathione reductase (GSH), and glutathione transferase(GST) activities, while the Malondialdehyde (MDA) level was significantly increased in the body wall tissue of sea cucumber collected from Abo-Qir station as compared with Miami station. The concentrations of heavy metals (Zn, Mn, Pb, Cu, and Cd) in the body wall of sea cucumber and seawater samples at the two stations differed significantly, showed an increase in the industrial area (Abo-Qir station) compared with the Miami station. As parasites are ubiquitous in the marine pollutant environment, so the present study proved that the sea cucumber samples collected from the Abo-Qir station were infected by Rickettsia-like organism (RLOs), found in the digestive tract and the respiratory tree of this marine animal. The present results of pollutant biomarkers indicated that the degree of pollution increased in summer as compared with the winter season in the two locations.

 
This study also suggested that the sea cucumber was an ideal bioindicator animal for marine pollution. The suppression of antioxidant biomarkers in the tissue of sea cucumber compared with high values of MDA confirmed the presence of anthropogenic contaminants in the Abo-Qir station, which essentially due to the industrial rejections. Also, Rickettsia-like organism (RLOs) infection and the bio-accumulation of the heavy metals in the tissues of sea cucumber obstructed the antioxidant defense responses in the animal.