ABSTRACT

Single-celled microorganisms are highly diverse and play various roles in the function of natural ecosystems, but it is challenging to recognize how genetic, morphological, and physiological diversity relate to each other in microbial groups. In an exemplary case, micro-algae in the family Symbiodiniaceae comprise many species that can only be distinguished using molecular data. Symbiodinaceae function as symbionts of various marine invertebrates, but are perhaps best known as the key to coral reef ecosystem health and persistence. The team is hosting a virtual workshop in August 2021 to identify and build further consensus among experts regarding the assessment of Symbiodiniaceae diversity. By formalizing consensus approaches and disseminating them broadly, this workshop is creating a more collaborative and welcoming research community and ensuring that all current researchers, as well as those entering the field, feel confident applying for grants, conducting research, and publishing papers that incorporate work on Symbiodiniaceae diversity. This workshop is defining additional forward-thinking research priorities that anticipate methodological advances over the next 5-10 years.

Resolving the molecular diversity of microorganisms is a major technical challenge, particularly for dinoflagellates in the family Symbiodiniaceae. These micro-algal endosymbionts that associate with corals and other marine invertebrates feature massive genomes, high repeat content, and other unique aspects that have hindered their molecular characterization. As sequencing technologies have advanced, so too has our understanding of Symbiodiniaceae diversity, which has grown from the presumption of one widespread species in the 1960s to now encompass fifteen divergent genera and likely hundreds of species. This process of discovery has been fraught with controversy, as the most useful phylogenetic marker?the hyperdiverse internal transcribed spacer 2 (ITS2) region?is frustratingly difficult to interpret in a consistent manner. The ribosomal array is multicopy, it evolves at different rates in different lineages, and it is subject to concerted evolution, all of which complicate phylogenetic reconstructions. Moreover, sampled coral tissues contain populations of symbiont cells that can potentially be from multiple Symbiodiniaceae taxa, complicating efforts to characterize partner specificity. Early debates focused on the extent to which unique ITS2 sequences indicated the presence of multiple symbiont species or simply multiple intraspecific variants. This central conflict has led to different schools of thought about how flexible coral-algal associations can be, and how they might respond to ongoing climate change. Next-generation sequencing has provided more data, but the same foundational interpretive issues remain. This team is gathering pioneering Symbiodiniaceae experts from around the world (ranging from seasoned veterans to newly-minted PhDs) who all collectively agree that a consensus interpretive framework can and must be identified and advanced in order to move the field forward. The participants are generating a ?consensus road map? leading to two workshop products: 1. an NSF white paper; and 2. an open-access, peer-reviewed manuscript. Through pre-workshop surveys, four workshop sessions, post-workshop collaboration, and publication, the team is summarizing current practices and recommending key methods for identifying and analyzing Symbiodiniaceae genetic diversity across three ?umbrellas?: 1. communities, 2. populations, and 3. strains. The written products from this workshop are being distributed to the wider reef and conservation community by leveraging a separate mini-workshop previously developed for the 2022 International Coral Reef Symposium. Workshop participants are also highlighting new technologies and research priorities for the next decade that should help fill some of the remaining knowledge gaps.

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