Assessing the contribution of bacteria to the heat tolerance of experimentally evolved coral photosymbionts

Abstract

Coral reefs are extremely vulnerable to ocean warming, which triggers coral bleaching—the loss of endosymbiotic microalgae (Symbiodiniaceae) from coral tissues, often leading to death. To enhance coral climate resilience, the symbiont, Cladocopium proliferum was experimentally evolved for >10 years under elevated temperatures resulting in increased heat tolerance. Bacterial 16S rRNA gene metabarcoding showed the composition of intra‐ and extracellular bacterial communities of heat‐evolved strains was significantly different from that of wild‐type strains, suggesting bacteria responded to elevated temperatures, and may even play a role in C. proliferum thermal tolerance. To assess whether microbiome transplantation could enhance heat tolerance of the sensitive wild‐type C. proliferum, we transplanted bacterial communities from heat‐evolved to the wild‐type strain and subjected it to acute heat stress. Microbiome transplantation resulted in the incorporation of only 30 low‐abundance strains into the microbiome of wild‐type cultures, while the relative abundance of 14 pre‐existing strains doubled in inoculated versus uninoculated samples. Inoculation with either wild‐type or heat‐evolved bacterial communities boosted C. proliferum growth, although no difference in heat tolerance was observed between the two inoculation treatments. This study provides evidence that Symbiodiniaceae‐associated bacterial communities respond to heat selection and may contribute to coral adaptation to climate change.

Modeling Symbiont Dynamics and Coral Regulation under Changing Temperatures*

“…While this study focuses on coral-algae symbiosis, the principles could extend to coral-microbiome interactions [Ziegler et al, 2017, Voolstra and Ziegler, 2020, Maire et al, 2023a,b, Bourne et al, 2016, Torda et al, 2017, Doering et al, 2021, van Oppen and Blackall, 2019, Marangon et al, 2021, Tran, 2022, Gardner et al, 2023, Voolstra et al, 2024. Coral microbiomes, like algal symbionts, are diverse and can buffer against environmental fluctuations.…”

Exchange or Eliminate: The Secrets of Algal-Bacterial Relationships

“…Coral reefs are very susceptible to ocean warming, and the loss of the dinoflagellate symbiont due to climate change results in coral bleaching [ 170 ]. There is evidence that bacterial communities that are associated with Symbiodiniaceae react to heat selection and may contribute to coral adaptation to altered temperatures [ 175 ]. Increased temperature also affects the bacterial microbiome of the red tide-causing Alexandrium minutum [ 176 ], highlighting that global temperature increases will perturb dinoflagellate ecology significantly.…”

Chlamydiae as symbionts of photosynthetic dinoflagellates

“…Photosynthetic dinoflagellates of the Symbiodiniaceae family associate with a wide range of intra-and extracellular bacteria [7][8][9][10], some of which are beneficial for Symbiodiniaceae physiology and stress tolerance [11][12][13][14][15]. While they can be free-living, Symbiodiniaceae are widely known as endosymbionts of cnidarians, which they provide with organic carbon through photosynthate transfer [16].…”

“…While they can be free-living, Symbiodiniaceae are widely known as endosymbionts of cnidarians, which they provide with organic carbon through photosynthate transfer [16]. Chlamydiae sequences were recently detected in 16S rRNA gene metabarcoding of several Symbiodiniaceae laboratory cultures [7,10,14]. In a Cladocopium sp.…”

“…First, the 'closelyassociated' communities of around 5 × 10 6 Cladocopium cells were sampled as described above. Samples were then bead-beaten for 20 min at 30 Hz with 100 mg of sterile beads (400-600 nm) to open up the Cladocopium cells and stained with SYBR Green as previously described[11,14]. Bacteria were then separated from Cladocopium cells and debris by fluorescence-activated cell sorting on a Aria III/FACS DiVa 9 software (BD Biosciences, Franklin Lakes, NJ) equipped with a 70 μm nozzle and run at 70 psi, as previously described[14].…”