Interactive effects of ultraviolet radiation and thermal stress on two reef‐building corals

Abstract

In shallow tropical waters, ultraviolet radiation (UVR) occurs at high intensity simultaneously with high water temperatures, and both stressors are predicted to increase in the future and to have a major impact on reef coral survival. The poor knowledge of the interactive effects of those two factors, however, prevents a good estimation of the coral resistance to climate change pressure. The results obtained in this study on two genetically distant scleractinian coral species, Pocillopora damicornis and Turbinaria reniformis, highlight an interaction of temperature and UVR on the corals' physiology as well as a species‐specific response. Overall, increase in UVR level exacerbated the negative impact of thermal stress. Whereas stressors did not reduce the autotrophic capacity (symbiont density, net photosynthetic rates) of T. reniformis, they significantly lowered it in P. damicornis, which initially contained twice more symbionts in its tissue. Only grazing rates (heterotrophy) were significantly decreased in T. reniformis under thermal stress. For both corals, calcification was slowed by the combination of thermal and UVR stress. Calcification was impaired likely due to a decrease in autotrophic energy supply in P. damicornis, and in heterotrophic energy supply in T. reniformis. This study confirms that the response of corals to global change needs to be studied using multifactorial approaches and a combination of different environmental factors.

Effect of feeding and thermal stress on photosynthesis, respiration and the carbon budget of the scleractinian coral Pocillopora damicornis

“…In such conditions, heterotrophy increases resilience to bleaching by storage of long 58 lasting energy deposits (Hughes & Grottoli, 2013), and sustains basic coral host metabolism and the 59 recruitment and re-colonization of zooxanthellae when more favorable conditions are present (Grottoli 60 et al, 2006;Rodrigues & Grottoli, 2007;Hughes et al, 2010). Therefore, investigations of the proportional 61 input from heterotrophy compared to autotrophy during coral bleaching reveal a potentially very high (up 62 to 100%) contribution of heterotrophically acquired carbon to daily animal respiration (CHAR) in certain 63 species (Grottoli et al, 2006;Courtial et al, 2017). This implies the existence of species-specific adaptions 64 among corals to utilize active feeding as a primary nutrient source under stressed conditions.…”

“…to cope with ROS accumulation are not met, causing ingestion or expulsion of excess zooxanthellae ( Figure 350 1c-d, Figure 2c Earlier studies on thermally stressed P. damicornis revealed increased feeding rates of up to 127 % 352 compared to fed controls (Courtial et al, 2017), implying an increased demand of nitrogen to withstand 353 stress factors by synthesizing protective compounds (e.g. antioxidants and heat-stress proteins) to avoid 354 oxidative stress (Downs et al, 2002).…”

Ultra-Violet Radiation Has a Limited Impact on Seasonal Differences in the Acropora Muricata Holobiont

“…Although UVR has been shown to decrease calcification rates in several coral species (Jokiel and York, 1982;Gleason and Wellington, 1993;Torres-Pérez and Armstrong, 2012), calcification rates of Acropora muricata in this study as well as Acropora validata and Porites compressa in previous studies (Glynn et al, 1993;Kuffner, 2002) were not impacted by UVR. The different effects of UVR on coral skeletal growth may be linked to the amount of energy acquired by the coral via its algal endosymbiont Symbiodinium, and dedicated to calcification (Courtial et al, 2017). Overall, our data suggest that A. muricata has two seasonally driven phenotypes: thin tissue holobionts with faster calcification rates in summer, and thicker tissue but slower calcifying holobionts in winter.…”

Peer Review #2 of "Borneo coral reefs subject to high sediment loads show evidence of resilience to various environmental stressors (v0.2)"

“…Manuscript to be reviewed Guest et al, 2016;Morgan et al, 2017). It may also be due to lower UV light penetration that can exacerbate temperature stress (Courtial et al, 2017), or potentially from higher heterotrophy, which increases the supply of essential metals to the symbionts thus sustaining them through elevated temperatures (Ferrier-Pagès et al, 2018). This study further suggests that while turbid reefs are potentially more resilient to elevated SST, the mechanism/s responsible for this resilience remain unclear.…”