Photosynthesis and nitrogen uptake of the giant kelp Macrocystis pyrifera (Ochrophyta) grown close to salmon farms

Finfish aquaculture is an activity that has experienced an explosive global development, but presents several environmental risks, such as high nitrogen outputs with potential eutrophication consequences. Therefore, the integration of seaweed aquaculture with the aim of decreasing nitrogen emissions associated with intensive salmon farming has been proposed as a bioremediation solution. Ecophysiological knowledge about seaweeds cultured close to farming cages is, however, still rudimentary. We experimentally studied the growth and physiological responses of Macrocystis pyrifera (Linnaeus) C. Agardh in a suspended culture system near a commercial salmon farm at three culture depths in order to understand its productivity performance. The results showed maximum growth responses at intermediate depths (3 m) as opposed to near the surface (1 m) or at a deeper culture level (6 m). At 6 m depth, light limitations were detected, whereas the sporophytes growing at 1 m depth responded to high irradiances, especially in late spring and summer, where they were more intensely exposed to decay of photosynthesis than individuals from other depths. Accordingly, photosynthetic pigment concentrations (chlorophyll a and c, and fucoxonthin) were higher during low-light seasons (winter and early spring) but decreased during the summer. On the other hand, although both nitrogen uptake and Nitrate Reductase (NR) activity varied seasonally, increasing significantly in spring and summer, these variables were not affected by culture depth. Therefore, the optimal culture depth of M. pyrifera near salmon farms appears to be a physiological integration between nitrogen supply and demand, which is modulated by plant acclimation to the seasonal change in light and temperature. The results allow to discuss about the environmental constrains of M. pyrifera in an ecophysiological context to improve the understanding of its aquaculture, and to contribute relevant information on the use of this species in bioremediation.