Compared photophysiology of native seagrasses with an invasive macroalga in Sydney Harbour, Australia

Abstract

The comparative photophysiology of the seagrasses Zostera capricorni and Halophila ovalis and the invasive macroalgae Caulerpa taxifolia was investigated in a shallow mixed meadow at Chowder Bay (Sydney, Australia), where the three species coexist. This study was developed under the auspices of the COST Action “Seagrasses: from genes to ecosystems”, targeting one of its objectives, the development of innovative devices for the continuous measurement of seagrass photosynthesis. Automated multi-channel chlorophyll fluorometers were deployed for 24-hour periods to examine and compare the changes in the photosynthetic efficiency and energy quenching mechanisms of the 3 species. Tissue samples were collected at predawn and noon, frozen and analysed by HPLC for detailed pigment analysis. The invasive C. taxifolia showed generally higher photosynthetic efficiency than both Z. capricorni and H. ovalis. Both seagrass species showed down-regulation of photosynthesis at noon, evidenced by the low quantum yield and the significant reduction of the antennae pigments between pre-dawn and noon. In contrast, C. taxifolia showed no reduction in antennae pigments or total photosynthetic pigments along the day. While both seagrasses showed 3- to 7-fold increases in the epoxidation state of xanthophyll cycle pigments between pre-dawn and noon, in C. taxifolia there was no significant change. Our results show that C. taxifolia is better adapted to high light conditions than Z. capricorni and H. ovalis, which means that, from the photophysiological point of view, this invasive macroalgae constitutes a serious competitor for seagrasses in shallow areas with high irradiance.