Consensus thermotolerance ranking for 110 Symbiodinium phylotypes: an exemplar utilization of a novel iterative partial-rank aggregation tool with broad application potential

Corals may reduce the effects of heat-induced bleaching through associations with thermotolerant algal symbionts (Symbiodinium). Although hundreds of Symbiodinium genetic types (phylotypes) are known, thermotolerance has been systematically evaluated for small subsamples within individual reports, making consensus a challenging task. Data on 110 phylotypes were aggregated from 35 reports, each assessing 2–24 phylotypes (median 4). One-third of reports include ties (phylotypes with indistinguishable thermotolerance). Between reports, most phylotypes are unique (74% assessed once), most pairwise comparisons are unknown (relative thermotolerance for 15% of the possible 5995 phylotype pairs are evaluated) and many rankings are inconsistent (52% of 58 phylotype pairs assessed in more than one report are discordantly ranked). Ranking of Symbiodinium phylotypes resulted in 64 thermotolerance cohorts (47 are single phylotypes, 17 contain 2–10 phylotypes with a median of 3 phylotypes) and indicates diverse thermotolerance capabilities within clades and among closely related phylotypes. The iterative partial-rank aggregation method (with error estimation) introduced here is broadly applicable to any quantitative consensus rank building problem where qualitative input ranks are not fully comprehensive nor resolved. Reconstructions based on simulated data (mirroring the empirical data set) indicate robust predictive capabilities for resolving rankings (1·28 approximation of the true solution) and inferring unknown pairwise comparisons (83% correctly predicted pair rankings from only 16% known a priori). Application to Symbiodinium phylotypes represents an additional tool to generate testable hypotheses on the role of symbiont thermotolerance in an ecosystem that is collapsing in the face of climate change and provides the first quantitative index of Symbiodinium thermotolerance and its associated uncertainty. A Lay Summary is available for this article.