Sensitivity of Photosynthesis to Warming in Two Similar Species of the Aquatic Angiosperm Ruppia from Tropical and Temperate Habitats

Abstract

Climate change-related events, such as marine heatwaves, are increasing seawater temperatures, thereby putting pressure on marine biota. The cosmopolitan distribution and significant contribution to marine primary production by the genus Ruppia makes them interesting organisms to study thermal tolerance and local adaptation. In this study, we investigated the photosynthetic responses in Ruppia to the predicted future warming in two contrasting bioregions, temperate Sweden and tropical Thailand. Through DNA barcoding, specimens were determined to Ruppia cirrhosa for Sweden and Ruppia maritima for Thailand. Photosynthetic responses were assessed using pulse amplitude-modulated fluorometry, firstly in short time incubations at 18, 23, 28, and 33 °C in the Swedish set-up and 28, 33, 38, and 43 °C in the Thai set-up. Subsequent experiments were conducted to compare the short time effects to longer, five-day incubations in 28 °C for Swedish plants and 40 °C for Thai plants. Swedish R. cirrhosa displayed minor response, while Thai R. maritima was more sensitive to both direct and prolonged temperature stress with a drastic decrease in the photosynthetic parameters leading to mortality. The results indicate that in predicted warming scenarios, Swedish R. cirrhosa may sustain an efficient photosynthesis and potentially outcompete more heat-sensitive species. However, populations of the similar R. maritima in tropical environments may suffer a decline as their productivity will be highly reduced.

Experimental Assessment of Vulnerability to Warming in Tropical Shallow-Water Marine Organisms

“…Our results suggested that heat stress threshold lies between 37 and 42 • C in seagrasses and macroalgae and between 32 and 34.5 • C in corals, where the photosynthetic performance and growth were significantly affected. Our results are comparable to the thermal optima and threshold in tropical shallow-water organisms reported in previous studies (Mayfield et al, 2013;Buerger et al, 2015;Kong et al, 2019;George et al, 2020;Rasmusson et al, 2020Rasmusson et al, , 2021Keng et al, 2021).…”

“…As this study aims to evaluate vulnerability to warming in shallow-water marine organisms, we focused on their stress responses to an increase in temperature within a range recorded in their natural settings encompassing the future warming scenarios. The testing temperatures were chosen based on past SST (27.45 ± 0.02 • C to 32.00 ± 0.02 • C, Figure 1B), records of temperature profiles of the sampling sites (28.84-30.25 • C in shallow water coral reefs and 26.96-36.70 • C in tidal flat areas at the respective sampling periods, Supplementary Table 1), our previous investigations (Rasmusson et al, 2020(Rasmusson et al, , 2021unpublished data) and other studies from the tropical shallow waters (Sutthacheep et al, 2013a;Pedersen et al, 2016;George et al, 2018;Kong et al, 2019). Extreme temperatures, exceeding 40 • C, have been reported in tropical seagrass meadows Pedersen et al, 2016;George et al, 2018) whereas the highest temperature reported in shallow water coral reefs in Thailand was 32.7 • C (Sutthacheep et al, 2013a).…”

“…Tropical shallow-water marine organisms experience great temporal variability in temperature that is largely influenced by solar radiation and tidal cycles in their natural habitats; temperatures above 40 • C have been recorded during spring tides in seagrass meadows George et al, 2018), whereas shallow-water coral reefs may experience a temperature up to 34 • C (Palumbi et al, 2014). As their surrounding temperature is already reaching their upper thermal threshold, these organisms are particularly susceptible to heat stress due to climate warming and ECEs (Stuart-Smith et al, 2017;Vinagre et al, 2019;Rasmusson et al, 2021). Although acclimation is possible, it may not be fast enough to keep up with the accelerating pace of climate change for most organisms (Collier et al, 2017;Hoegh-Guldberg et al, 2018).…”

“…Photosynthesis, a fundamental energy process, is highly sensitive to temperature. Therefore, photosynthetic parameters have been widely used to detect thermal stress (Robison and Warner, 2006;Sinutok et al, 2012;Rasmusson et al, 2020Rasmusson et al, , 2021Danaraj et al, 2021). Accumulating evidence has revealed that heat stress lowers the overall photosynthetic efficiency of marine photosynthetic organisms by inhibiting the photosystems, electron transport chain, and carbon fixation (Robison and Warner, 2006;Sinutok et al, 2012;Rasmusson et al, 2020;Danaraj et al, 2021).…”

Comparative study on anatomical traits and gas exchange responses due to belowground hypoxic stress and thermal stress in three tropical seagrasses

“…Increasing temperature affects seagrass energy balance and primary production ( George et al, 2018 ; Kong et al, 2019 ; Rasmusson et al, 2020 , 2021 ). Both photosynthesis and respiration are controlled by temperature; however, photosynthesis often shows lower optimal temperature than respiration ( Marín-Guirao et al, 2016 ; Pedersen et al, 2016 ; George et al, 2018 ; Rasmusson et al, 2020 , 2021 ).…”