Oxidative stress tolerance in the filamentous green algae Cladophora glomerata and Enteromorpha ahlneriana

Cladophora glomerata (L.) Kütz. and Enteromorpha ahlneriana Bliding are morphologically similar filamentous green algae that are dominants in the upper littoral zone of the brackish Baltic Sea. As these two species co-exist in a continuously fluctuating environment, we hypothesised that they may have different strategies to cope with oxidative stress. This was tested in laboratory experiments through stressing the algae by high irradiance (600 μmol photons PAR m⁻² s⁻¹) at two different temperatures (15 and 26 °C) in a closed system. Thus, oxidative stress was created by high irradiance (photo-oxidative stress) and/or carbon depletion. The extent of lipid oxidative damage, antioxidant enzyme activities and the amount of hydrogen peroxide excreted by the algae to the surrounding seawater medium were measured. The results suggest that the two species have different strategies: the annual C. glomerata could be classified as a more stress-tolerant species and the ephemeral E. ahlneriana as a more stress-susceptible species. Low temperature in combination with high irradiance created less lipid oxidative damage in C. glomerata than in E. ahlneriana, which was probably related to the higher regular activities of the hydrogen peroxide scavenging enzymes catalase and ascorbate peroxidase in C. glomerata, whereas in E. ahlneriana high activities of these enzymes were only obtained after the induction of oxidative stress. Superoxide dismutase activities were similar in both species, but the mechanisms to remove the hydrogen peroxide produced by the action of this enzyme were different: more through scavenging enzymes in C. glomerata and more through excretion to the seawater medium in E. ahlneriana. The high excretion of hydrogen peroxide, possibly in combination with brominated volatile halocarbons, by E. ahlneriana may have a negative effect on epiphytes and may partly explain why this alga is usually remarkably devoid of epiphytes and grazers compared to C. glomerata.