Occurrence of the thermotolerant zoochlorellae Symbiochlorum hainanense associated with hydrocorals in the Southwestern Atlantic Ocean

The monospecific <i>Symbiochlorum</i> genus (Ulvophyceae) was recently described and found to be associated with bleached corals in the South China Sea. A new microchlorophyte strain was isolated from the hydrocoral <i>Millepora alcicornis</i> collected in a reef system in the Southwestern Atlantic Ocean. Cells undergo multiple fission to release quadriflagellated spores. The strain’s phenotypical traits are consistent with a benthic lifestyle. Growth rates were equally high (0.3 d<sup>-1</sup>) at 24°C and 32°C, confirming its thermotolerance. Bayesian and Maximum likelihood phylogenetic reconstruc tions concatenating 18S rDNA, <i>tuf</i>A, and <i>rbc</i>L gene sequences placed the new strain in the <i>Symbiochlorum</i> genus, highly distinct from its former representatives, Ignatius tetrasporus and <i>Pseudocharacium americanum</i>, but as a sister strain to Pacific isolates of <i>Symbiochlorum hainanense</i>. This is the first report of the occurrence of <i>S. hainanense</i> in the Atlantic Ocean. Repeated and more frequent heat waves oceanwide may favor the spread and increase of thermotolerant organ isms such as <i>S. hainanense</i> in corals, with unforeseeable consequences for coral reefs' resilience.