Sargassum is an important primary producer of rocky bottom communities in coastal ecosystems. Like other parts of the planet, benthic populations of S. natans 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 S. natans 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 [(ΔF (F’_m − F_s)/F’_m and F_v/F_m, respectively) that was maximized at noon. The stress induced by incubation at 32–35 °C caused a decrease in F_v/F_m by 33% on the first day and approximately 20% on subsequent days. In the laboratory, using two co-occurred species of S. natans and Padina gymnospora, we verified that the photosynthetic apparatus of S. natans 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 F_v/F_m as the reason for the collapse of photochemistry of Sargassum.

Sargassum is an important primary producer of rocky bottom communities in coastal ecosystems. Like other parts of the planet, benthic populations of S. natans from Ilha Grande Bay, southeastern Brazil, have been suffering from different forms of natural and anthropogenic disturbances, in particular increasing seawater temperatures. In this study, we focused on understanding the effects of temperature on the photosynthetic performance of S. natans using PAM fluorometry. Measurements in the field revealed the dynamics of effective and maximum quantum yields (ΔF/F’m and Fv/Fm, respectively), as well as the dynamics of photoprotection mechanisms. The stress induced by incubation at 32-35°C, caused a decrease in Fv/Fm by 33% on the first day and approximately 20% on subsequent days. Additionally, direct estimates of the net photosynthesis of S. natans by measurements of dissolved oxygen concentration showed that an increase in temperature inhibited photosynthesis. In the laboratory, using specimens of S. natans and Padina gymnospora, we verified the effect of temperature on the fate of the energy absorbed by the PSII antenna using complementary quantum yields. Exclusively in Sargassum, temperatures in the range of 32 to 35ºC promoted a decline of non-photochemical quenching of chlorophyll fluorescence before the decline of Fv/Fm, indicating that PSII is destroyed in the absence of the NPQ photoprotection.

Marginal Southwestern Atlantic reefs, such as the Abrolhos reefs (Brazil), occur under conditions including high sedimentation, turbid waters, and elevated nutrient inputs, while also being exposed to local and climate-related stressors. Long-term standardized assessments of net calcium carbonate (CaCO₃) accretion on settlement substrates remain scarce for marginal reefs. Here, we assessed annual net CaCO₃ accretion and early successional community dynamics from 2012 to 2023 using standardized Calcification Accretion Unit (CAU) tiles deployed at two reef sites in the Abrolhos Bank, Brazil: Pedra de Leste (PL), located in the nearshore reef arc, and Parcel dos Abrolhos (PA), located in the offshore reef arc. Mean net CaCO₃ accretion was 329 ± 94 g m⁻² yr⁻¹, reaching a maximum in 2019−2020 (433 ± 82 g m⁻² yr⁻¹) and declining in 2022-2023 (255 ± 75 g m⁻² yr⁻¹). At PA, net CaCO₃ accretion in 2022−2023 was 52% lower than the decadal mean. The relative cover of calcifying and non-calcifying functional groups also varied through time at both study sites. At PL, non-calcifying cover reached 36% in 2017−2018 and 50% in 2022−2023, driven mainly by algal turfs and ascidians, while CCA cover declined. In 2022−2023, this shift coincided with a marked reduction in net carbonate accretion. In contrast, crustose coralline algae (CCA) dominated CAU tile communities at PL in 2019−2020 and coincided with the highest net carbonate accretion observed at that site. These patterns indicate that variation in early successional community composition, particularly in the relative abundance of CCA and turf algae, was associated with temporal variation in net CaCO₃ accretion on settlement tiles. The recent decline observed in PA highlights the value of continued long-term monitoring to determine whether similar changes are reflected in established reef communities.