Temperature- and light-dependent growth and metabolism of the invasive red algae Gracilaria vermiculophylla - a comparison with two native macroalgae.
We conducted two temperature experiments to investigate the invasion success of the coarsely branched red algae, Gracilaria vermiculophylla. Our working hypothesis was that the coarsely branched G. vermiculophylla, with well-known broad environmental tolerances, would have physiological traits in-between the typical r and K strategies. A factorial experiment provided light-dependent models of growth rate at six temperatures, with maximum growth of 0.045 day-1 at 20°C. Light-saturated growth and maximum light utilization efficiency both displayed a bell-shaped temperature dependency, with optima at 21°C and 23°C respectively. The minimum light required to maintain growth was low (<1 µmol photons m-2 s-1) for lower temperatures (5-20°C) and increased exponentially to 7 µmol photons m-2 s-1 at 30°C, documenting that G. vermiculophylla has a wide tolerance to low light levels under temperature ranges occurring in the upper littoral zone in the Baltic Sea. A second experiment investigated the metabolic acclimation of G. vermiculophylla to four temperatures, while comparing its physiological responses to those of two native species, Fucus vesiculosus and Ulva lactuca. This experiment showed that the optimum temperature for light-saturated photosynthesis increased for all three species as they became long-term acclimated to higher temperatures. Short-term incubation at high temperature (30°C) was suboptimal for all three algal species when grown at low temperatures (10-15°C) but the algae were unaffected when cultured at higher temperatures (20-25°C). Finally we evaluated the capacity of each of the three species for metabolic homeostasis and found that F. vesiculosus and G. vermiculophylla had an almost identical metabolic performance regardless of acclimation temperature, whereas net photosynthesis of U. lactuca increased significantly with growth temperature. Our results show that G. vermiculophylla shares traits with both the slow-growing, leathery F. vesiculosus (K-strategy) and the fast-growing, sheet-like U. lactuca (r-strategy), by combining relatively high growth rates with a robust metabolic response to changing temperatures. In conclusion, we suggest that having both K and r metabolic traits explains, in part, the invasion success of G. vermiculophylla in temperate estuaries.