Physiological acclimation of floating Macrocystis pyrifera to temperature and irradiance ensures long-term persistence at the sea surface at mid-latitudes

Large numbers of giant kelp rafts are floating along temperate coasts of the southern hemisphere, carrying a wide diversity of associated organisms with them. During voyages, floating kelps are exposed to strong variations in environmental conditions such as UV-radiation (UVR) and temperature that affect algal physiology, growth, and reproductive output. Consequently, it was predicted that the interactive effects of high temperature and UVR suppress algal persistence and reproductive output at the sea surface. This hypothesis was tested by exposing Macrocystis pyrifera (Linnaeus) C. Agardh sporophytes to two irradiance (PAR, PAR + UV) and three temperature (cool, ambient, warm) conditions. An outdoor-tank experiment with two consecutive runs (1st and 2nd) was conducted in northern-central Chile (30 S) to assess growth and physiological responses (pigment contents and photosynthesis) of floating M. pyrifera. Results showed that after being afloat for 15 days, algae physiologically acclimated efficiently to changing abiotic conditions by a decrease in pigment contents and dynamic photoinhibition. However, in kelps exposed to 20 degrees C these acclimation processes operated at the expense of growth, resulting in reduced biomass gains, lower blade elongation rates, and diminished reproduction. Overall, floating M. pyrifera responded with high physiological plasticity to the tested UV regimes (UVB 30-100 kJm(-2); UVA 300-2000 kJm(-2)), but under stressful temperature conditions (similar to 20 degrees C) photoacclimation processes are costly and an important fraction of the energy gained via photosynthesis becomes unavailable for algal growth. We suggest that at mid latitudes (25 degrees S-40 degrees S) detached sporophytes of M. pyrifera have the capacity to float for long time periods and over large distances at water temperatures <20 degrees C. (C) 2011 Elsevier B.V. All rights reserved.