Cell wall thickening in two Ulva species in response to heavy metal marine pollution

Abstract

The present work was aimed at evaluating the responses of two green algae, Ulva rigida and Ulva intestinalis, to heavy metal pollution in their natural environment. These species were harvested in the Atlantic shorelines of El Jadida (Morocco) at two different stations: first, a non-polluted site (S1) which served as a control, and a second one (S2), which is affected by industrial effluents, mainly from a phosphate facility. Except for iron, the heavy metal concentrations in seawater were higher in station S2 than in station S1 (notably cadmium and lead levels). Consequently, the algal samples collected from S2 presented higher levels of heavy metals compared with those collected in S1. Pollution in station S2 resulted in a reduction in size of thalli of both algal species. Optical microscopy of algal slices showed that the polluted environment induced the thickening of the cell walls along with an enlargement of cell areas. Elevated cell wall yields, higher content of polysaccharides (mainly ulvans), sulfate, and protein were also detected in algae from S2. However, the monosaccharide distribution in cell wall polysaccharides was quite identical in the two species, regardless of the station. Our results describe an adaptation mechanism mainly based on cell wall thickening which restrains the entry of toxic elements into the cells, allowing these algae to enhance their tolerance to heavy metal stress.