A possible chlorophycean affinity of some Neoproterozoic acritarchs

Two taxa of Neoproterozoic acritarchs of unknown affinity, Multifronsphaeridium pelorium and Species A, are analysed by electron microscopic (SEM, TEM) and chemical (micro-FTIR, pyrolysis GC-MS, thermal desorption-MS) methods. Both acritarch species are characterised by multi-branched processes and a remnant trilaminar sheath (TLS) structure. The TLS-bearing wall structures in these acritarchs suggest a possible biological affinity to chlorophyte algaenan. The molecular data obtained from the two acritarchs were generally similar and also consistent with a chlorophycean affinity. A significant aliphatic moiety is evident in these acritarchs as a short-chain series of n-alkene/alkane pyrolysates and prominent aliphatic IR bands. The restricted molecular-weight range (<C₂₀) of the n-alkene/alkane doublets and the lack of isoprenoid and other branched alkanes in the pyrolysates suggest a low degree of branching in the aliphatic component of these acritarch macromolecules. The significant methyl (CH₃) IR signal was attributed to the terminal groups of short n-alkyl moieties. Alkylbenzenes, alkylphenols and alkylindoles were also significant pyrolysis products, indicating an aromatic component, although the latter two components may be attributed to artificially- and/or diagenetically-formed melanoidin moieties. The macromolecular structure of Multifronsphaeridium sp. and Species A consists of short n-alkylpolymethylenic chains, probably linked via ether/ester bonds, with possibly a small aromatic content. This study presents ultrastructural and molecular evidence of a genetic relationship between Neoproterozoic acritarchs and Chlorophyceae.