Description of four Millepora spp. transcriptomes and their potential to delimit the Caribbean fire coral species.

Millepora is a relatively species-rich genus of hydrocorals, with 16 species distributed around the globe. It is considered an important reef building cnidarian. The current diversity of Caribbean Millepora species consists of Millepora complanata, M. alcicornis, M. squarrosa and M. striata. Here, we report the de novo transcriptome assembly and phylotranscriptomic analysis of M. alcicornis, M. complanata, M. squarrosa and a undescribed morphotype (Millepora sp.) found in exposed Thalassia beds and mangrove areas in southwest Puerto Rico. Over 345 million sequence reads were obtained for the analysis of the Millepora transcriptomes (Illumina HiSeq4000; 2x150bp). The analysis pipeline consisted of assembly with Trinity, BUSCO, RSEM and ORFs calling for each transcriptome, followed by ontology (Blast2GO) and phylogenetic analysis. The phylogenetic analysis was performed after selecting homologous genes among the transcriptomes, resulting in 10,596 sequences. Concatenation analysis (Maximum Likelihood and Bayesian inference) and a coalescence-based analysis were performed to the dataset too. Concatenation analysis yielded a topology supporting a clade of M. complanata and M. alcicornis, with Millepora sp. outside this clade and M. squarrosa as an outgroup. The coalescence-based tree estimation analysis (ASTRAL-II), presented a different topology placing M. alcicornis and Millepora sp. as sister taxa, rather than grouping with M. alcicornis with M. complanata. Our coalescence analysis indicated that there is a high degree of incomplete lineage sorting, suggesting a very recent time of species emergence among three out of the four Caribbean Millepora species. Calculations of ABBA-BABA statistics derived from transcriptome-wide SNP data indicate the possible presence of introgression between Millepora complanata and M. alcicornis.

Peroxynitrite Generation and Increased Heterotrophic Capacity Are Linked to the Disruption of the Coral-Dinoflagellate Symbiosis in a Scleractinian and Hydrocoral Species.

Ocean warming is one of the greatest global threats to coral reef ecosystems; it leads to the disruption of the coral-dinoflagellate symbiosis (bleaching) and to nutrient starvation, because corals mostly rely on autotrophy (i.e., the supply of photosynthates from the dinoflagellate symbionts) for their energy requirements. Although coral bleaching has been well studied, the early warning signs of bleaching, as well as the capacity of corals to shift from autotrophy to heterotrophy, are still under investigation. In this study, we evaluated the bleaching occurrence of the scleractinian coral Mussismillia harttii and the hydrocoral Millepora alcicornis during a natural thermal stress event, under the 2015-2016 El Niño influence in three reef sites of the South Atlantic. We focused on the link between peroxynitrite (ONOO-) generation and coral bleaching, as ONOO- has been very poorly investigated in corals and never during a natural bleaching event. We also investigated the natural trophic plasticity of the two corals through the use of new lipid biomarkers. The results obtained first demonstrate that ONOO- is linked to the onset and intensity of bleaching in both scleractinian corals and hydrocorals. Indeed, ONOO- concentrations were correlated with bleaching intensity, with the highest levels preceding the highest bleaching intensity. The time lag between bleaching and ONOO- peak was, however, species-specific. In addition, we observed that elevated temperatures forced heterotrophy in scleractinian corals, as Mu. harttii presented high heterotrophic activity 15 to 30 days prior bleaching occurrence. On the contrary, a lower heterotrophic activity was monitored for the hydrocoral Mi. alicornis, which also experienced higher bleaching levels compared to Mu. hartii. Overall, we showed that the levels of ONOO- in coral tissue, combined to the heterotrophic capacity, are two good proxies explaining the intensity of coral bleaching.

Diversity of fatty acid composition of symbiotic dinoflagellates in corals: evidence for the transfer of host PUFAs to the symbionts.

High diversity of fatty acid (FA) composition of endosymbiotic dinoflagellates of the Symbiodinium group (zooxanthellae) isolated from different cnidarian groups has been found. To explain this diversity, FA composition of the total lipids of pure symbiont fractions (SF) and host cell tissue fractions (HF) isolated from one hydrocoral, two soft coral, and seven hard coral species inhabiting the shallow waters of the South China Sea (Vietnam) were compared. Symbiodinium phylogenetic clade designation for each SF was also determined, however, the relationship between the clade designation and FA composition of Symbiodinium was not found. The profiles of marker polyunsaturated FAs (PUFAs) of symbionts (18:4n-3, 18:5n-3, 20:5n-3) did not depend on taxonomic designation of the host and reflected only a specimen-specific diversity of the SF lipids. Several FAs such as 20:0, C24 PUFAs, 22:5n-6, and 18:2n-7 concentrated in HF lipids but were also found in SF lipids. For ten cnidarian species studied, the principal components analysis of total FAs (27 variables) of the symbiotic fractions was performed. The clear division of the symbiotic dinoflagellates according to the host systematic identity was found on a subclass level. This division was mainly caused by the FAs specific for the host lipids of each cnidarian subclasses such as hard corals, soft corals, and hydrocorals. Thus, the coral hosts affect the FA profile of their symbionts and cause the diversity of FA composition of Symbiodinium. The transfer of FAs from the coral host to their symbiotic dinoflagellates and modulation of PUFA biosynthesis in symbionts by the host are considered as possible reasons of the diversity studied.