Alternative Photosynthetic Electron Transfers and Bleaching Phenotypes Upon Acute Heat Stress in Symbiodinium and Breviolum spp. (Symbiodiniaceae) in Culture

Abstract

The breakdown of the symbiosis between cnidarians and Symbiodiniaceae often occurs upon periods of elevated sea surface temperature and gives rise to bleaching events that affect coral reefs worldwide. In this respect, an impairment of photosynthesis would be responsible for light-dependent generation of toxic reactive oxygen species putatively contributing to death of symbionts and/or host cells. In some Symbiodiniaceae species, alternative photosynthetic electron flows (AEF) have been documented to occur upon a shift to high temperature, possibly contributing to photoprotection and to the balance of energetic ratio between photoproduced ATP and NADPH. By using a combination of in vivo spectrofluorimetric and oximetric techniques, we studied the capacity for electron rerouting toward oxygen and cyclic electron flow (CEF) around photosystem I in eight Symbiodiniaceae in culture belonging to Symbiodinium and Breviolum genera upon an acute shift from 25 to 33 • C. CEF capacity was determined as the kinetic of PSI primary donor P700 re-reduction in the presence of DCMU, a PSII inhibitor. An active oxygen uptake in light was estimated by comparing net oxygen evolution and relative electron transport rate of PSII at different light intensities. Among strains that showed elevated capacity for both AEF, some were thermotolerant while others were thermosensitive. Conversely, in some thermotolerant strains, capacities for these AEF were low upon acute heat stress. A principal component analysis of these results indicates that the long-term heat tolerant/bleached phenotype of cultured Symbiodinium and Breviolum spp. is not correlated with a capacity for different AEF across isolates during early onset of acute heat stress.

Coral Productivity Is Co-Limited by Bicarbonate and Ammonium Availability

“…In the present study, rETR PSII and E O both saturated at light intensities above 300 µmol photons m −2 •s −1 and 600 µmol photons m −2 •s −1 at the beginning of the experiment and after 3 weeks of bicarbonate and ammonium supplementation, respectively ( Figure S6). However the shape of the relationship between both parameters (i.e., linear) differed from those previously reported in [53] and more recently in [52] (i.e., non-linear with net VO 2 saturating more rapidly than rETR PSII ). Finally, the calcification rate measured in the iC & iN condition was also higher than in the control condition (P = 0.006; Figure S5) but similar to the rates measured in the other experimental treatments.…”

“…. ) in various species of Symbiodiniaceae isolated in culture [52,53]. In the present study, rETR PSII and E O both saturated at light intensities above 300 µmol photons m −2 •s −1 and 600 µmol photons m −2 •s −1 at the beginning of the experiment and after 3 weeks of bicarbonate and ammonium supplementation, respectively ( Figure S6).…”

In vivo assessment of mitochondrial respiratory alternative oxidase activity and cyclic electron flow around photosystem I on small coral fragments

“…Besides, in several species of Symbiodiniaceae in culture, a high light-dependent oxygen uptake was found to occur at the acceptor side of PSI through a Mehler-type reaction 37,38 . As illustrated in Fig.…”

“…in culture activates a light-enhanced dark respiration during photosynthesis 8 . A more detailed characterization through the combination of various techniques (O 2 exchange, PSII chlorophyll a fluorescence emission, PSI primary electron donor [P700] absorbance change in the far-red spectrum, and analysis of the thylakoid transmembrane electric field), revealed that an oxygen-dependent reduction at the acceptor side of PSI occurs as photoprotective mechanism in Symbiodinium isolates 37 , 38 .…”

Opportunistic bacteria use quorum sensing to disturb coral symbiotic communities and mediate the occurrence of coral bleaching

“…Over the last two decades, mounting evidence has shown that bleaching was strongly correlated with the disrupted dynamic equilibrium of host–symbiont relationships (Rosenberg et al ., ; Bourne et al ., ; Ricci et al ., ), which favour opportunistic and potentially pathogenic bacteria coupled to the physiological impairment of the holobiont (Cárdenas et al ., ; Roder et al ., ). Thus, the dysbiosis of coral holobiont microbiome has increasingly become an important factor leading to bleaching (Mao‐Jones et al ., ; Dang et al ., ).…”