Response of a Benthic Sargassum Population to Increased Temperatures: Decline in Non-Photochemical Quenching of Chlorophyll a Fluorescence (NPQ) Precedes That of Maximum Quantum Yield of PSII

<i>Sargassum</i> is an important primary producer of rocky bottom communities in coastal ecosystems. Like other parts of the planet, benthic populations of <i>S. natans</i> from Ilha Grande Bay (IGB), southeastern Brazil, have been suffering from different forms of natural and anthropogenic disturbances, in particular increasing seawater temperatures. The aim of this study was to understand the effects of temperature on the photosynthetic performance of <i>S. natans</i> using the pulse amplitude modulated (PAM) fluorometry. In the field experiments, the occurrence of photoprotection resulted in a difference between the effective and maximum quantum yields [(<i>ΔF (F'_m - F_s)/F'_m</i> and <i>F_v/F_m</i>, respectively) that was maximized at noon. The stress induced by incubation at 32-35 °C caused a decrease in <i>F_v/F_m</i> by 33% on the first day and approximately 20% on subsequent days. In the laboratory, using two co-occurred species of <i>S. natans</i> and <i>Padina gymnospora</i>, we verified that the photosynthetic apparatus of <i>S. natans</i> collapses at 34 °C. The fate of the energy absorbed by photosystem II (PSII) antenna showed that, in S. natans, photochemical activity and non-photochemical quenching of chlorophyll fluorescence (NPQ) drastically decrease, and only the passive dissipation in the form of heat and fluorescence remains. Our results indicate the disappearance of the NPQ photoprotection at 34 °C before the decline of <i>F_v/F_m</i> as the reason for the collapse of photochemistry of <i>Sargassum</i>.