Influence of temperature and light on growth and photosynthetic physiology of Fucus evanescens (Phaeophyta) embryos

Abstract

The influence of temperature and light on growth and photosynthetic physiology were investigated in embryos of Fucus evanescens grown at 5 or 20°C under irradiances of 15 or 150 μmol photons m⁻² s⁻¹ for 7–10 days. Growth was light-independent, but high-temperature embryos were always significantly larger than those grown at low temperature. Photosynthesis-irradiance responses were measured at growth temperature and a standard temperature (20°C) to isolate instantaneous effects of temperature from acclimation responses. Our data indicate that growth and photosynthesis are uncoupled during the early development of Fucus, and that acclimation of the photosynthetic light-harvesting apparatus occurred. Light-limited net photosynthesis (P_sub-sat) responded similarly to high temperature and low light. Rates of P_sub-sat were similar in embryos grown at 20°C (regardless of light) and at 5°C in low (c. 1.2 nmol O₂ mm⁻³ min⁻¹), whereas those of 5°C high-light embryos were lower (c. −0.04 nmol O₂ mm⁻³ min⁻¹). Changes in P_sub-sat were associated with changes in initial slope of the photosynthesis-irradiance curve (α) and dark respiration. Differences in α were attributed to increased absorption due to increased chlorophyll a content and PSII reaction centre densities. Changes in α were also correlated with changes in fluorescence induction kinetics, with high-temperature and/or low-light embryos exhibiting higher ratios of variable: maximum fluorescence (F_v/F_m) than 5°C high-light embryos (c. 0.5 vs. 0.19). In contrast to P_sub-sat, changes in light-saturated photosynthesis (P_max) in response to growth under different temperature/light regimes did not confer metabolic compensation. Rates of P_max were highest in 20°C high-light embryos (7.3 nmol O₂ mm⁻³ min⁻¹), lower in 20°C low-light and 5°C low-light embryos (c. 2.6 nmol O₂ mm⁻³ min⁻¹) and lowest in 5°C high-light embryos (2.3 nmol O₂ mm⁻³ min⁻¹). We suggest that the ability to achieve temperature-independent rates of P_sub-sat may be important for fucoid embryos that recruit in intertidal microhabitats where photosynthesis is often light-limited.