Clade-Specific Sterol Metabolites in Dinoflagellate Endosymbionts Are Associated with Coral Bleaching in Response to Environmental Cues

Abstract

Cnidarians cannot synthesize sterols (which play essential roles in growth and development) de novo but often use sterols acquired from endosymbiotic dinoflagellates. While sterol availability can impact the mutualistic interaction between coral host and algal symbiont, the biosynthetic pathways (in the dinoflagellate endosymbionts) and functional roles of sterols in these symbioses are poorly understood. In this study, we found that itraconazole, which perturbs sterol metabolism by inhibiting the sterol 14-demethylase CYP51 in dinoflagellates, induces bleaching of the anemone Heteractis crispa and that bleaching perturbs sterol metabolism of the dinoflagellate. While Symbiodiniaceae have clade-specific sterol metabolites, they share features of the common sterol biosynthetic pathway but with distinct architecture and substrate specificity features of participating enzymes. Tracking sterol profiles and transcripts of enzymes involved in sterol biosynthesis across time in response to different environmental cues revealed similarities and idiosyncratic features of sterol synthesis in the endosymbiont Breviolum minutum. Exposure of algal cultures to high levels of light, heat, and acidification led to alterations in sterol synthesis, including blocks through downregulation of squalene synthase transcript levels accompanied by marked growth reductions. IMPORTANCE These results indicate that sterol metabolites in Symbiodiniaceae are clade specific, that their biosynthetic pathways share architectural and substrate specificity features with those of animals and plants, and that environmental stress-specific perturbation of sterol biosynthesis in dinoflagellates can impair a key mutualistic partnership for healthy reefs.

Phytosterol Profiles, Genomes and Enzymes – An Overview

“…Sterols contributing less than 2% of a total were not considered for clarity of this schematic representation. 1, Leblond et al (2011) ; 2, Miller et al (2012) ; 3, Akihisa et al (1992) ; 4, Raederstorff and Rohmer (1987) ; 5, Chiu et al (1985) ; 6, Morikawa et al (2009) ; 7, Schaller et al (1998) ; 8, Schaeffer et al (2000) ; 9, Mitova et al (1999) ; 10, Bhatt and Bhatt (1984) ; 11, Tasende (2000) ; 12, Lu et al (2014) ; 13, Mercer et al (1974) ; 14, Lu et al (2020) ; 15, Conner et al (1982) ; 16, Rampen et al (2010) ; 17, Patterson and Nes (1991) ; 18, Mikami et al (2018) ; 19, Giner et al (2009) ; 20, Lamacka and Sajbidor (1997) ; and 21, Chakrabarti et al (2017) . Tetrahymena thermophila , Saccharomyces cerevisiae , and Homo sapiens are shown as single pathway end-product species.…”

“…Dinoflagellates from the SAR group (comprising stramenopila, alveolate, and rhizaria) shown in the new tree of eukaryotes ( Burki et al, 2020 ) exhibit very striking sterol biosynthetic peculiarities. This group is widespread in aquatic ecosystems and is most importantly hosting symbiotic species required for the remarkable biology of cnidaria and especially reef-building corals, which depend on their symbionts for nutrients and sterols ( Lu et al, 2020 ). The sterol supply to an anemone from its symbiodinaceae symbionts was shown to be associated with a symbiosome enriched with a Niemann-Pick type C2 (NPC2) transporter protein ( Hambleton et al, 2019 ).…”

“…Dinoflagellates produce a specific 4-methylsterol called dinosterol and some other particular sterols like gorgosterol (C 30 sterol products). These organisms use lanosterol as a sterol precursor ( Shimizu et al, 1976 ; Alam, 1979 ; Volkman, 2016 ; Lu et al, 2020 ), and have evolved a so-called alternative squalene epoxidase to produce (3 S )-2,3-oxidosqualene ( Pollier et al, 2019 ). Dinosterol amounts are highly variable between dinoflagellates species in the genera Symbiodinium , Gymnodinium , Fragilidium , and Gonyaulax ( Piretti et al, 1997 ; Amo et al, 2010 ).…”

“…4-SMT has distinct specificity and substrate recognition characteristics when compared to SMT1 and SMT2 that catalyze methyl transfers on olefinic sterol side chains ( Darnet et al, 2020 ). An orthologous 4-SMT sequence from Symbiodinium is reported in databases suggesting that 4-methylsterols in dinoflagellates originate from the same biosynthetic process functionally described in the case of round worms ( Lu et al, 2020 ). The occurrence of 4-methylsterols produced through C4-methylation in very distant taxa suggest a convergent evolutionary process at play to gain biological functions, yet unclear in the case of dinoflagellates.…”

“…The occurrence of 4-methylsterols produced through C4-methylation in very distant taxa suggest a convergent evolutionary process at play to gain biological functions, yet unclear in the case of dinoflagellates. Fugacium kawagutii exhibits a sterol profile with all compounds methylated at C4, and dinosterol and 24-desmethyldinosterol accounting for sixty percent of the total ( Lu et al, 2020 ). Dinosterol has also two exocyclic carbon atoms in its side chain, at C24 and at the unusual C23 position for a sterol side chain alkylation, and a double bond at C22.…”