Post Date: 19 August 2010

Cadmium toxicity explanation and prediction in marine phytoplankton

The applicability of the newly proposed subcellular partitioning model (SPM) in Cd toxicity explanation and prediction as well as the importance of phytochelatins (PCs) in Cd detoxification are not well known for marine phytoplankton. Therefore, a series of experiments were conducted to investigate differential Cd toxicity, using Cd accumulation, subcellular distribution and phytochelatin synthesis as major tools. These included Cd toxicity experiments under different irradiance and temperature conditions, after exposure to and recovery from different levels of environmental Cd stress for various durations for a Hong Kong marine diatom Thalassiosira nordenskioeldii. Another experiment investigating the species differential Cd sensitivity was also conducted for the marine diatom Thalassiosira pseudonana, green algae Chlorella autotrophica, and dinoflagellate Prorocentrum minimum.

Generally, increased irradiance and temperature led to an elevation in Cd sensitivity by exerting great influence on the diatom’s biochemical composition and physiological processes. However, the calculated median inhibition concentration (IC50) based on MSF-Cd or organelle-Cd exhibited the least difference, strongly meriting MSF-Cd and organelle-Cd being the best predicators of Cd toxicity for marine diatom at different environmental conditions. A similar result was also obtained for the species differential Cd sensitivity experiment. Therefore, the subcellular partitioning model (SPM), which incorporates the subcellular fates of metal, may provide a better means to predict metal toxicity as compared to other models.

Exposure history also exerted great influence on the subsequent Cd sensitivity. The Cd sensitivity increased with the pre-exposed [Cd²⁺] and period, due to the physical accumulation of Cd concentration in MSF and the possible functional damage on MSF. After the environmental Cd stress was alleviated, the Cd sensitivity initially increased followed by a gradual restoration, the extent of which depending on the pre-exposed [Cd²⁺] and recovery period. Irreversible toxic effects on the diatoms during the exposure period may prevent a further and complete restoration of Cd tolerance even after long time of recovery.

The intracellular PC pool was tightly and dynamically controlled by the cell. Therefore the responses of PCs were quick not only to the elevation of environmental [Cd²⁺], but also to the alleviation of [Cd²⁺] stress. When putting together the data obtained from different Cd toxicity experiments for T. nordenskioeldii (under different temperature condition, after Cd exposure and recovery), a strong relationship between [Cd²⁺] or intra-Cd and the relative change in PC-SH was observed, providing a strong and quantitative support of the role of PC-SH in phytoplankton as a good indicator of environmental and intracellular Cd stress. The intra-Cd/PC-SH ratio (or intra-Cd to PC-SH difference), however, could not always explain the differential Cd toxicity as expected. The possible reasons included other detoxification mechanisms, the turnover of PCs and the function of PCs other than detoxification.