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.

Highlights

  • While no difference in the mean values of φPSII was detected in R. cirrhosa (Swedish) (Figure 3E), a significant change was found in R. maritima (Thai)

  • The results demonstrate distinguished photosynthetic responses to warming above ambient levels between temperate Ruppia cirrhosa and tropical R. maritima, most likely depending on how close to their thermal tipping point the plants are growing in their natural habitat

  • Analysis of ITS (ITS1-5.8S-ITS2) and rbcL DNA sequences indicated that samples of Ruppia from Sweden was close to Ruppia cirrhosa, while samples from Thailand was close to Ruppia maritima

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Summary

Introduction

Licensee MDPI, Basel, Switzerland.Attribution (CC BY) license (https://creativecommons.org/licenses/by/ 4.0/).Global climate change is putting immense pressure on organisms and whole ecosystems [1,2,3,4,5]. Resilience and adaptation to such stress are key factors determining future biodiversity dispersal in terrestrial and marine environments. In the world’s oceans, average mean temperatures are predicted to increase by 1.3 ◦ C before 2065 [1]. Even more alarming may be the acute effects of specific climate change-induced events such as marine heatwaves [2,3], especially influencing shallow coastal waters that get warmed rapidly and where temperatures may fluctuate greatly on a diel basis. Reportedly, such events have

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