A family of diatom-like silicon transporters in the siliceous loricate choanoflagellates

Abstract

Biosilicification is widespread across the eukaryotes and requires concentration of silicon in intracellular vesicles. Knowledge of the molecular mechanisms underlying this process remains limited, with unrelated silicontransporting proteins found in the eukaryotic clades previously studied. Here, we report the identification of silicon transporter (SIT)-type genes from the siliceous loricate choanoflagellates Stephanoeca diplocostata and Diaphanoeca grandis. Until now, the SIT gene family has been identified only in diatoms and other siliceous stramenopiles, which are distantly related to choanoflagellates among the eukaryotes. This is the first evidence of similarity between SITs from different eukaryotic supergroups. Phylogenetic analysis indicates that choanoflagellate and stramenopile SITs form distinct monophyletic groups. The absence of putative SIT genes in any other eukaryotic groups, including non-siliceous choanoflagellates, leads us to propose that SIT genes underwent a lateral gene transfer event between stramenopiles and loricate choanoflagellates. We suggest that the incorporation of a foreign SIT gene into the stramenopile or choanoflagellate genome resulted in a major metabolic change: the acquisition of biomineralized silica structures. This hypothesis implies that biosilicification has evolved multiple times independently in the eukaryotes, and paves the way for a better understanding of the biochemical basis of silicon transport through identification of conserved sequence motifs.

Biosilicification Drives a Decline of Dissolved Si in the Oceans through Geologic Time

“…This is exemplified by the distribution of Si transporters in the metazoans and their sister group, the choanoflagellates (Marron et al, 2013). SIT-Ls were identified from all three of the main bilaterian groups: lophotrochozoans (polychaete annelid worms), ecdysozoans (copepods) and deuterostomes (tunicates).…”

Molecular Diversity of Sea Spray Aerosol Particles: Impact of Ocean Biology on Particle Composition and Hygroscopicity

“…Diatoms are known to have rigid cell walls (frustules) composed of amorphous silica (SiO2) which is released upon cell lysis. 24,25 The SSA spectrum categorized as polysaccharides (Figure 1f) features skeletal C-C, C-O and stretching C-C-O bands at 991 and 1044 cm -1 , CH2 bending vibration that typically appears at 1459 cm -1 in lipids, a peak at 1642 cm -1 that can be either C=O stretching in carbohydrates or C=C stretching vibration of unsaturated fatty acid chains and a broad peak at 2929 cm -1 identified as the C-H stretching modes of -CH-, methylene (-CH2-) and terminal methyl (-CH3) groups of fatty acid chains. 26,27 This spectrum obtained from SSA was compared with the spectrum of commercially available standards representative of cellular polysaccharides: lipopolysaccharide (LPS) from E. coli, lamanarin, inulin, sodium alginate and peptidoglycan.…”

“…[4][5][6]9,14,22 The OM fraction of SSA depends on size and varies in composition; field studies have reported that the majority of OM in the bulk submicrometer marine aerosol is relatively water insoluble and that in supermicrometer SSA is mostly water soluble. [12][13][14][23][24][25][26] Spectroscopic measurements of SSA particles collected in the field have shown that the oxygen-rich organic fraction of individual particles contains molecules with spectral signatures that are characteristic of saccharides, 24 and signatures for carboxylic acids 26 and alkanes 27 have also been observed.…”

Protistan Skeletons: A Geologic History of Evolution and Constraint

“…Silicification in sponges involves a different set of organic constituents (references in Wallace et al 2012), demonstrating that distinct proteins and polysaccharides have been recruited for silica skeletogenesis in different silicifying clades. In this regard, it is curious that preliminary molecular research on silica precipitation in choanoflagellate loricae (e.g., Gong et al 2010) indicates that silicon transporters in this group are related to those in diatoms, implying lateral genetic transfer (Marron et al 2013). Not much is known about the molecular biology of silification in radiolaria or other groups.…”