Technical note: The silicon isotopic composition of choanoflagellates: implications for a mechanistic understanding of isotopic fractionation during biosilicification

The marine silicon cycle is intrinsically linked with carbon cycling in the oceans via biological production of silica by a wide range of organisms. The stable silicon isotopic composition (denoted by δ30Si) of siliceous microfossils extracted from sediment cores can be used as an archive of past oceanic silicon cycling. However, the silicon isotopic composition of biogenic silica has only been measured in diatoms, sponges and radiolarians, and isotopic fractionation relative to seawater is entirely unknown for many other silicifiers. Furthermore, the biochemical pathways and mechanisms that determine isotopic fractionation during biosilicification remain poorly understood. Here, we present the first measurements of the silicon isotopic fractionation during biosilicification by loricate choanoflagellates, a group of protists closely related to animals. We cultured two species of choanoflagellates, Diaphanoeca grandis and Stephanoeca diplocostata, which showed consistently greater isotopic fractionation (approximately −5 ‰ to −7 ‰) than cultured diatoms (−0.5 ‰ to −2.1 ‰). Instead, choanoflagellate silicon isotopic fractionation appears to be more similar to sponges grown under similar dissolved silica concentrations. Our results highlight that there is a taxonomic component to silicon isotope fractionation during biosilicification, possibly via a shared or related biochemical transport pathway. These findings have implications for the use of biogenic silica δ30Si produced by different silicifiers as proxies for past oceanic change.

The silicon isotopic composition of choanoflagellates: implications for a mechanistic understanding of isotopic fractionation during biosilicification

The marine silicon cycle is intrinsically linked with carbon cycling in the oceans via biological production of silica by a wide range of organisms. The stable silicon isotopic composition (denoted by δ30Si) of siliceous microfossils extracted from sediment cores can be used as an archive of past oceanic silicon cycling. However, the silicon isotopic composition of biogenic silica has only been measured in diatoms, sponges and radiolarians, and isotopic fractionation relative to seawater is entirely unknown for many other silicifiers. Furthermore, the biochemical pathways and mechanisms that determine isotopic fractionation during biosilicification remain poorly understood. Here, we present the first measurements of the silicon isotopic fractionation during biosilicification by loricate choanoflagellates, a group of protists closely related to animals. We cultured two species of choanoflagellates, Diaphanoeca grandis and Stephanoeca diplocostata, which showed consistently greater isotopic fractionation (approximately −5 to −7 ‰) than cultured diatoms (−0.5 to −2 ‰). Instead, choanoflagellate silicon isotopic fractionation appears to be more similar to sponges grown under similar DSi concentrations. Our results highlight that there is a taxonomic component to silicon isotope fractionation during biosilicification, possibly via a shared or related biochemical transport pathway. These findings have implications for the use of biogenic silica δ30Si produced by different silicifiers as proxies for past oceanic change.

Loricate Choanoflagellates from the South Atlantic coastal zone (~32 °S) including the description of Diplotheca tricyclica sp. nov.

The biodiversity of marine heterotrophic protists is poorly known in the South Atlantic coastal zone (~32 °S) especially regarding the nanoflagellates. The presence of loricate choanoflagellates was reported for the first time in the Patos Lagoon estuary and the adjacent coastal zone. Seventeen species of eleven genera of loricate choanoflagellates were observed between October 1998 and May 2000 in fixed water samples (lugol's solution + glutaraldehyde) in a JEM 100-SX transmission electron microscope. Most species were recorded in euhaline and mixopolyhaline waters during the spring and summer, none in autumn and a few (four) in winter. The absence of choanoflagellates at the more sheltered inshore stations is due freshwater influence, and at the beach station, probably due the strong wave action. The probably cosmopolitan species Pleurasiga minima, Cosmoeca norvegica, C. ventricosa and Parvicorbicula circularis were present in spring or summer in the estuary channel and coastal area while Stephanoeca diplocostata which apparently prefers lower temperature, was recorded in winter. Calotheca alata and Campyloacantha spinifera are mainly temperate species and were present in spring. The new species Diplotheca tricyclica was recorded at the estuary channel in the summer 1999, in high salinity water.

Phylogeny of Opisthokonta and the evolution of multicellularity and complexity in Fungi and Metazoa

While early eukaryotic life must have been unicellular, multicellular lifeforms evolved multiple times from protistan ancestors in diverse eukaryotic lineages. The origins of multicellularity are of special interest because they require evolutionary transitions towards increased levels of complexity. We have generated new sequence data from the nuclear large subunit ribosomal DNA gene (LSU rDNA) and the SSU rDNA gene of several unicellular opisthokont protists – a nucleariid amoeba (Nuclearia simplex) and four choanoflagellates (Codosiga gracilis, Choanoeca perplexa, Proterospongia choanojuncta and Stephanoeca diplocostata) to provide the basis for re-examining relationships among several unicellular lineages and their multicellular relatives (animals and fungi). Our data indicate that: (1) choanoflagellates are a monophyletic rather than a paraphyletic assemblage that independently gave rise to animals and fungi as suggested by some authors and (2) the nucleariid filose amoebae are the likely sister group to Fungi. We also review published information regarding the origin of multicellularity in the opisthokonts.

Silicification of 'cell walls’ of certain protistan flagellates

Silica deposition is described for two protistan flagellates, Synura petersenii (Chrysophyceae, algae) and Stephanoeca diplocostata (Choanoflagellida, Protozoa). Both taxa produce silica units intracellularly and subsequently assemble them outside the protoplast to form a ‘cell wall’. In Synura the cell wall consists of a scale case to which scales are added throughout the cell cycle. In Stephanoeca individual siliceous, costal strips are accumulated outside the protoplast and assembled into a lorica once sufficient strips have been produced. In both taxa silica is laid down within silica deposition vesicles (s.d.vs) of uncertain origin. Microtubules are involved in the orientation and support of s.d.vs during early stages of silica unit biogenesis. Detailed comparisons of silica deposition are made between Synura and Stephanoeca and between these and other silica-depositing protistans.