Effects of Sedimentation on the Photobiology of Mesophotic Rhodoliths From the Southwestern Atlantic Ocean

Abstract

Mesophotic rhodolith beds are free-living nongeniculate coralline algae (NGCA) bioconstructions with slow growth rates and massive associated diversity, distributed worldwide along continental shelves. They are subjected to deleterious impacts of sedimentation from natural and anthropogenic sources, which can bury the thalli, causing photosynthetic reduction and impaired gas exchange, among other effects. There have been only a few studies exploring the consequences of smothering and burial, which limit our knowledge of these type of impact. In this study, we evaluated the photosynthetic response and recovery of NGCA under different sedimentation conditions in the laboratory. Experiments were performed considering different loads of natural sediment (medium sand) during continuous burial periods. The Potential (Fv/Fm) and Effective Quantum Yield (ΔF/Fm′) of photosystem II (PSII) and the relative Electron Transport Rate (rETR) were determined using pulse-amplitude modulated fluorometry as indicators of PSII integrity and photosynthetic activity. PSII functionality (Fv/Fm) was impaired over time under the highest sediment load tested (burial under 100 g of sediment per thallus). However, only 2 days was necessary to recover at least half of the photosynthetic capacity observed in the control samples. This recovery pattern was also supported by ΔF/Fm′ and rETR. Our results illuminated the limits of the resilient behaviour of NGCA to sediment smothering and burial, where long burial periods (2 months) associated with heavy sediment load could cause heavy damage to the photosynthetic apparatus, which can lead to death. However, these organisms could recover after sediment removal. These results provide important information for setting up management strategies to assess the smothering and burial impacts of sediments on rhodoliths.