Facilitative interactions between the green-tide macroalga Monostroma arctium and the red macroalga Porphyra yezoensis

The interspecific interactions between the green-tide macroalga Monostroma arctium and the red macroalga Porphyra yezoensis were examined in the laboratory. The results showed that M. arctium had a positive effect on the growth and photosynthetic activity of P. yezoensis, regardless of whether the experiment was conducted in fresh thalli co-culture systems, culture filtrate experiments or in dry powder assays (P < 0.05). By contrast, there was no significant effect of P. yezoensis on the same attributes of M. arctium (P > 0.05). A simultaneous nutrient analysis showed that M. arctium had a better nutrient uptake ability compared with P. yezoensis. The addition of M. arctium facilitated nutrient uptake by P. yezoensis in a co-culture system, especially of nitrogen (N). However, when M. arctium was co-cultured with P. yezoensis, the addition of P. yezoensis decreased nutrient uptake compared with the control, although this was insignificant (P > 0.05). Comparative assays between direct and indirect contact assays in semi-continuous algal co-cultures indicated that physical contact was not necessary for the observed result. The secretion of allelopathic substances by the macroalga is the most probable explanation for the observed positive facilitation. The allelopathic substances secreted by M. arctium and P. yezoensis, and the metabolites accumulated in both macroalga were analyzed by high-performance liquid chromatography (HPLC). This analysis indicated that the chemical changes occurred under different co-culture conditions. Our results demonstrated that the interactions among interactive species appear to be variable and context dependent.