Comparative time-courses of photoacclimation by Hawaiian native and invasive species of Gracilaria (Rhodophyta)

Abstract

Invasive seaweeds have rapidly increased in global distribution and abundance in recent years, presenting significant threats to nearshore ecosystems. Developing characteristics diagnostic of potential macroalgal invasiveness is an important step in controlling their spread. Studies have identified photoacclimation, the ability of the photosynthetic apparatus to adjust to changes in light, as one important attribute in the success of several invasive terrestrial and marine plants; several studies indicate that the rate of photoacclimation plays a role in the differential success of invasive plants at the expense of natives. In this experiment, we compare two closely related species that occur in Hawaiian coastal waters with similar distribution, habitat, and morphology, the invasive Gracilaria salicornia, and the native G. coronopifolia. Samples were grown in outdoor culture, acclimated to one of two irradiance regimes, and reciprocally transplanted. Photoacclimation rates were assessed using in vivo spectrophotometry and PAM fluorometry. Results indicate that G. salicornia has remarkable tolerance to irradiance extremes and experiences rapid growth when transplanted from low to high photosynthetic photon flux density environments, and that differential rates of overall photoacclimation are insufficient to explain the relative success of these two plants. Photoacclimation is achieved via several complex processes that do not necessarily occur contemporaneously. Future researchers should be wary of assuming its completion based on single indicators.