Influence of the nature of the exudates released by different marine algae on the growth, trace metal uptake and exudation of Emiliania huxleyi in natural seawater

Marine phytoplankton are known to release metal complexing ligands but little is known about the effect of exudates on the biological behaviour of the different microorganisms, including their toxicity and influence on trace metal availability. In this study, cultures of Emiliania huxleyi grown in filtered seawater, enriched with nitrate and phosphate as well as its own exudates and those of Phaeodactylum tricornutum, Porphyra spp. and Enteromorpha spp., were used to investigate the effects of algal exudates on algal growth, uptake (extracellular adsorption plus intracellular uptake) of Cu, Pb, Cd, Zn, Fe, Mn, Ni and Co, and extent of exudation. Cathodic and anodic stripping voltammetry (CSV and ASV) were used to determine metals, both in the medium and taken up by the algae, and total complexing organic ligands in the medium. Among these ligands, thiol compounds (cysteine-like and glutathione-like) were quantified in the exudates of different origins and during the growth of E. huxleyi in media enriched with them. An improvement of the final cell yield of E. huxleyi was caused by the addition of Enteromorpha exudates (the richest in glutathione-like compounds), and growth inhibition (a decrease of final cell yield and growth rate) was caused by the addition of P. tricornutum exudates (the richest in cysteine-like compounds). The nature and concentration of the organic compounds present in the culture medium also influenced trace metal uptake and the concentration and composition of the exudates produced by E. huxleyi. Therefore, it can be speculated that a bloom of a species of algae that produces large amounts of specific exudates may favour or inhibit the local growth of other algal species and, in an extreme situation, change the biodiversity.