A new and diverse plastid-bearing microbial eukaryote and its position on the eukaryotic tree of life.

Abstract

Using group-specific environmental clone libraries to target a section of the plastid 16S SSU rDNA gene we have identified a novel plastid-bearing eukaryotic lineage and have named this group “rappemonads” in anticipation of formal taxonomic description. Rappemonad sequences were amplified from aquatic environmental DNA samples collected from a wide variety of marine and freshwater sites suggests the rappemonads display a broad ecophysiology and wide geographical habitat distribution. Phylogenetic analysis was carried out on rappemonad rDNA sequences. Firstly, analyzing plastid SSU rDNA sequences suggesting this group form a diverse, strongly supported monophyletic clade encompassing numerous subclades. Secondly, phylogenetic analysis of a near full-length plastid rDNA gene cluster suggests the rappemonads represent an evolutionarily distinct lineage branching within the cryptomonad/haptophyte radiation, specifically as a sister group to the haptophytes. This suggests that the rappemonad plastid shares common ancestry with the red algal derived secondary plastids of the haptophytes and cryptomonads. Consequently, rappemonad plastid sequences appear to represent the plastid of either a novel highly divergent haptophyte or of an entirely new group. Phylogenetic comparisons suggest that the rappemonad lineage is not representative of the plastid of the biliphytes, a recently discovered microbial eukaryote. The biliphytes have been shown to display a weakly supported relationship with the cryptomonads in nuclear 18S SSU rDNA phylogenies although our analysis based on nuclear-encoded SSU and LSU rRNA genes does not support a biliphyte/cryptomonad relationship. Analyses using qPCR methodology demonstrate that the rappemonads can form transient blooms in the Sargasso Sea. Fluorescence in situ hybridization revealed rappemonad cells were 6.6 ± 1.2 x 5.7 ± 1.0 µm, and appear to contain 2 to 4 plastids. We have shown the rappemonads to be a novel, widespread, microbial algae and potentially an important component in global photosynthetic ecosystems and therefore likely a player in oceanic geochemical cycles. This major algal lineage has so far remained absent