Physiological stress modulates epiphyte ( Rhizoclonium sp.)-basiphyte ( Agarophyton chilense ) interaction in co-culture under different light regimes

Epiphytism is a common phenomenon in macroalgal aquaculture worldwide. This problem can affect the productivity and quality of the farmed species which may have socioeconomic consequences for the human communities that depend on this activity. In southern Chile, the aquaculture of the red macroalga Agarophyton chilense in the area of Maullín River has suffered losses in local production (> 90%) due to the proliferation of the filamentous green epiphyte Rhizoclonium sp. This epiphyte becomes entangled with A. chilense fronds, diminishing its quality and preventing its sale as raw material. Thus, the aim of this study was to investigate the effects of light intensity (90 and 180 μmol photons m −2 s −1 ) on the competitive interaction between the epiphyte Rhizoclonium sp. and the basiphyte A. chilense co-cultured at different initial biomass proportions in a replace series experiment. At the end of the experiment, the presence of Rhizoclonium sp. did not affect negatively the growth of A. chilense , but both species showed stress responses (i.e. reduced F v /F m and C:N ratio, suggesting nutrient limitation) in co-culture compared with monocultures. The epiphyte-basiphyte interaction was not modulated by light availability at any initial co-culture proportion and could have been related to nutrient availability during experimentation. Using the replacement series approach, an interspecific competition was observed at all initial co-culture proportion under 180 μmol photons m −2 s −1 while competition was found at the initial co-culture 1:1 under 90 μmol photons m −2 s −1 . Our results suggest that the interaction in co-culture between both A. chilense and Rhizoclonium sp. seems to be regulated by nutrient availability as well as it demands utilization rather than light availability.