Depth Distribution of Ammonia Genotypes in the NE Atlantic

Ammonia is one of the most abundant genera of benthic foraminifera in the world and occurs mainly in sheltered, shallow marine and estuarine environments, avoiding the higher latitudes. Approximately 40 species and sub-species have been described over the years. However, few of these are now currently recognised and their taxonomy, based on test morphology, is in a confused state.

More recently, taxonomical studies have been made based on phylogenetic analyses of a fragment of the rDNA large sub-unit (LSU). These studies have revealed 14 different molecular types of Ammonia worldwide, with LSU genotypes T1, T2, T3, and T6 being present in the NE Atlantic, the area of the present study. However, partial rDNA small sub-unit (SSU) sequences are more commonly used in foraminiferal studies than those of the rDNA LSU, since a much more extensive data base of foraminiferal SSU sequences are readily available in GenBank for comparative analysis. Consequently, we used partial SSU sequences in this study, coupled with Scanning Electron Microscopy (SEM), to characterise molecular types of Ammonia species. Using this method, we have identified six SSU genotypes in the NE Atlantic (SSU1-6), with one of these (SSU5), further subdivided into two. For each SSU genotype we have determined the LSU genotype as assigned by Hayward et al. (2004), in order to further establish the distributions of our identified genotypes. The SSU and LSU genotypes are as follows: SSU1=T6; SSU2=T2; SSU3=T2; SSU4=T1; SSU5=T3; SSU5b=T3; SSU6 has no equivalent LSU genotype.

Foraminiferal samples were collected from 25 sites in the UK and mainland Europe. Out of these, Ammonia species were absent from 12 sites (the majority were northern sites above 57°N) and present at 13 sites. Of these, a single genotype was found at seven sites, with eight sites containing two genotypes. All six genotypes were found around the British Isles in a pattern suggesting they may be distributed according to depth. SSU1 was found alone or with SSU2 in intertidal samples. SSU2 was found with SSU5 at samples taken at extreme low tide and SSU5 was found with SSU6 in subtidal samples, suggesting that the population may be changing from SSU1<SSU2<SSU5<SSU6 with increasing depth. This is corroborated in mainland European samples, where SSU5 and SSU6 were found together in subtidal samples from the Portuguese margin, and the Rhone prodelta. To date, SSU3 and SSU4 have only been found in intertidal samples from Cork.