Utility of Photochemical Traits as Diagnostics of Thermal Tolerance amongst Great Barrier Reef Corals

Abstract

Light availability is considered a key factor regulating the thermal sensitivity of reef building corals, where excessive excitation of photosystem II (PSII) further exacerbates pressure on photochemical pathways already compromised by heat stress. Coral symbionts acclimate to changes in light availability (photoacclimation) by continually fine-tuning the photochemical operating efficiency of PSII. However, how this process adjusts throughout the warmest months in naturally heat-tolerant or sensitive species is unknown, and whether this influences the capacity to tolerate transient heat stress is untested. We therefore examined the PSII photophysiology of 10 coral species (with known thermal tolerances) from shallow reef environments at Heron Island (Great Barrier Reef, Australia), in spring (October-November, 2015) vs. summer (February-March, 2016). Corals were maintained in flow-through aquaria and rapid light curve (RLC) protocols using pulse amplitude modulated (PAM) fluorometry captured changes in the PSII photoacclimation strategy, characterized as the minimum saturating irradiance (E k ), and the extent of photochemical ([1 -C], operating efficiency) vs. non-photochemical ([1 -Q]) energy dissipation. Values of E k across species were >2-fold higher in all coral species in spring, consistent with a climate of higher overall light exposure (i.e., higher PAR from lower cloud cover, rainfall and wind speed) compared with summer. Summer decreases in E k were combined with a shift toward preferential photochemical quenching in all species. All coral species were subsequently subjected to thermal stress assays. An equivalent temperature-ramping profile of 1 • C increase per day and then maintenance at 32 • C was applied in each season. Despite the significant seasonal photoacclimation, the species hierarchy of thermal tolerance [maximum quantum yields of PSII (F v /F m ), monitored at dawn and dusk] did not shift between seasons, except for Pocillopora damicornis (faster declines in summer) and Stylophora pistillata (total mortality in spring). Furthermore, the strategy for dealing with light energy (i.e., preferential photochemical vs. non-photochemical quenching) was unchanged for thermally tolerant species across seasons, whereas thermally sensitive species switched between preferential [1 -Q] and [1 -C] from spring to summer. We discuss how such traits can potentially be used as a diagnostic of thermal tolerance under non-stressed conditions.

Resolving coral photoacclimation dynamics through coupled photophysiological and metabolomic profiling

“…Furthermore, metabolomic shifts were observed within 7 days of treatment, particularly following high light exposure, suggesting a link between metabolite profile and photoacclimation response. The lack of change observed for F q ′/F m ′ (max) contrasts with results from a number of previous studies, which report concurrent shifts in F q ′/F m ′ (max) with shifts in light (Hennige et al, 2008;Warner et al, 2010;Suggett et al, 2012;Nitschke et al, 2018). This result is particularly intriguing, as other metrics (i.e.…”

“…Coral photoacclimation properties and how they vary over space and time have become increasingly well studied (e.g. Anthony and Hoegh-Guldberg, 2003a;Frade et al, 2008), particularly through the introduction of highly resolute bio-optical techniques such as PAM fluorometry (Hennige et al, 2008;Nitschke et al, 2018). However, associated metabolic changes that regulate cellular physiological processes inherent to photoacclimation in the coral holobiont have not been comprehensively explored.…”

“…Endosymbiont photosystem II (PSII) photophysiology was assessed over a period of 21 days using a PAM fluorometer (Diving-PAM, Walz, Effeltrich, Germany) as per Nitschke et al (2018). Immediately after dawn on days 0, 1, 3, 4, 7, 11, 15, 20 and 21 of the experiment, test tube racks containing corals were transported to an adjacent laboratory in a seawater-filled container, and rapid light curves (RLCs) with eight actinic light steps were performed on each Hennige et al, 2008;Nitschke et al, 2018). Briefly, diving-PAM settings were: actinic light factor=1, light curve intensity=3, saturation width=0.8 s, saturation intensity=10, gain=12 and signal damping=3.…”

“…Such fluorometry approaches have proven extremely critical for retrieving highly resolved photophysiological parameterization of corals over space and time (e.g. Hennige et al, 2008;Suggett et al, 2012;Langlois and Hoogenboom, 2014;Warner et al, 2010), but tying these parameters to the underlying metabolic changes that regulate photoacclimation remains challenging and largely unresolved (see Nitschke et al, 2018;Warner and Suggett, 2016).…”

Revealing changes in the microbiome of Symbiodiniaceae under thermal stress

“…In contrast, Durusdinium strain D1a appeared to be the most thermally tolerant strain (as per Karim et al ., ; Goyen et al ., ), yet the temperature stress treatment still resulted in significant declines in F v /F m and growth rates relative to the control by TE. While declines in PSII photochemical efficiency commonly reflects the thermal sensitivity of Symbiodiniaceae both in hospite (e.g., Warner et al ., ; Goyen et al ., ; Nitschke et al ., ) and ex hospite in cultures (e.g. McGinley et al ., ; Levin et al ., ), light stress (e.g.…”