In order to explore the abundance and potential environmental functions of green algal laccases, we screened various algae for extracellular laccase-like activities, characterized basic features of these activities in selected species and exemplarily studied the transformation of environmental pollutants and complex natural compounds by the laccase of Tetracystis aeria. Oxidation of the classical laccase substrate ABTS was found to be widespread in chlorophycean algae. The oxidation activity detected in members of the 'Scenedesmus' clade was caused by an unknown thermostable low-molecular-mass compound. In contrast, species of the Moewusinia, including Chlamydomonas moewusii and T. aeria, excreted putative 'true' laccases. Phenolic substrates were oxidized by these enzymes optimally at neutral to alkaline pH. The Tetracystis laccase efficiently transformed bisphenol A, 17α-ethinylestradiol, nonylphenol and triclosan in the presence of ABTS as redox mediator, while anthracene, veratrylalcohol and adlerol were not attacked. Lignosulfonate and humic acid underwent slight (de)polymerization reactions in the presence of the laccase and mediator(s), probably involving the oxidation of phenolic constituents. Possible natural functions of the enzymes, such as the synthesis of complex polymers or detoxification processes, may assist the survival of the algae in adverse environments. In contaminated surface waters, laccase-producing green algae might contribute to the environmental breakdown of phenolic pollutants.