Abstract
Rhizaria are an important component of oceanic plankton communities worldwide. A number of species harbor eukaryotic microalgal symbionts, which are horizontally acquired in the environment at each generation. Although these photosymbioses are determinant for Rhizaria ability to thrive in oceanic ecosystems, the mechanisms for symbiotic interactions are unclear. Using high-throughput sequencing technology (i.e., 454), we generated large Expressed Sequence Tag (EST) datasets from four uncultured Rhizaria, an acantharian (Amphilonche elongata), two polycystines (Collozoum sp. and Spongosphaera streptacantha), and one phaeodarian (Aulacantha scolymantha). We assessed the main genetic features of the host/symbionts consortium (i.e., the holobiont) transcriptomes and found rRNA sequences affiliated to a wide range of bacteria and protists in all samples, suggesting that diverse microbial communities are associated with the holobionts. A particular focus was then carried out to search for genes potentially involved in symbiotic processes such as the presence of c-type lectins-coding genes, which are proteins that play a role in cell recognition among eukaryotes. Unigenes coding putative c-type lectin domains (CTLD) were found in the species bearing photosynthetic symbionts (A. elongata, Collozoum sp., and S. streptacantha) but not in the non-symbiotic one (A. scolymantha). More particularly, phylogenetic analyses group CTLDs from A. elongata and Collozoum sp. on a distinct branch from S. streptacantha CTLDs, which contained carbohydrate-binding motifs typically observed in other marine photosymbiosis. Our data suggest that similarly to other well-known marine photosymbiosis involving metazoans, the interactions of glycans with c-type lectins is likely involved in modulation of the host/symbiont specific recognition in Radiolaria.
Highlights
Marine planktonic microorganisms are major constituents of oceanic ecosystems, being responsible for half of global primary production and participating massively to carbon sequestration to the deep ocean (Falkowski and Raven, 2013)
We assessed the main genetic features of the host/symbionts consortium transcriptomes and found rRNA sequences affiliated to a wide range of bacteria and protists in all samples, suggesting that diverse microbial communities are associated with the holobionts
A. elongata Expressed Sequence Tag (EST) were the best assembled in terms of coverage (8.1X) and proportion of bp assembled (42.4%, Table 1), whereas A. scolymantha and Collozoum sp. holobionts were the most poorly assembled with a coverage of 3.2X and 3.3X and 21.9 and 26.6% of bp assembled, respectively (Table 1)
Summary
Marine planktonic microorganisms are major constituents of oceanic ecosystems, being responsible for half of global primary production and participating massively to carbon sequestration to the deep ocean (Falkowski and Raven, 2013). A number of radiolarian species exhibit a mixotrophic metabolism, hosting eukaryotic photosymbiotic microalgae such as dinoflagellates (Gast and Caron, 1996), chlorophytes (Gast et al, 2000), and haptophytes (Decelle et al, 2012a). The intracellular algae provide photosynthetic products to the host which in turn creates a nutrient-rich micro-environment facilitating algal growth, the symbiotic interactions turning out to be critical for marine food webs and biogeochemical processes (Stoecker et al, 2009). (Decelle et al, 2012b) Marine invertebrates such as corals, anemones and sponges are known to harbor diverse microbial communities along with their photosymbionts (Radax et al, 2012; Sun et al, 2014)
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