Habitat partitioning among cryptic species of planktonic foraminifera in the Caribbean Sea and its consequences for the calibration and interpretation of paleoceanographic proxies (FOCS)
Final Report Abstract
Planktonic foraminifera are single celled eukaryotes that occur exclusively in the marine realm. They construct calcite shells around their cells, which after the death of the cell sink to the sea floor and accumulate in great numbers in the sediment. Planktonic foraminifera therefore play an important role in the global carbon cycle. In addition, their calcite shells preserved in marine sediments are used to reconstruct past ocean conditions. Such applications of planktonic foraminifera rely on the assumption that species identified by differences in the shape of their fossil shells correspond to biological species with unique environmental requirements and adaptations. However, genetic studies revealed a surprisingly high genetic diversity within species of planktonic foraminifera. These differences appear “cryptic” – they are seen in gene sequences, but not in the shape of the fossils. In order to understand to what degree this cryptic diversity may limit the interpretation of paleoceanographic data derived from fossil foraminifera, we carried out a genetic and morphometric investigation of Caribbean planktonic foraminifera. In parallel project, colleagues from GEOMAR in Kiel investigate the distribution of species in the water column and their geochemical signatures. In combination, these studies will allow us to assess how extensive the genetic diversity in the region is, how it is structured, whether or not it is associated with shape differences and whether or not it causes offsets in geochemical signals. In the course of the project, we have generated large amount of DNA sequences from single foraminifera, which we could unambiguously identify by the shape of their shell. We have used material from the R/V METEOR cruise OPOKA-M78 and supplemented this material with new samples from offshore Puerto Rico where we could extract the DNA whilst preserving the shell intact. The resulting dataset allowed us to conclude that aside from the occurrence of two minor gene variants, none of the genetic types in the Caribbean is endemic to that region. We could show that some morphologically defined species occur globally or in the Caribbean as a single genetic type, highlighting their potential for paleoceanographic studies. Other species were shown to comprise multiple genetic types, which may co-occur in the same samples. Studies based on these species, including culturing studies, may need revision. We have tried to determine whether the occurrence of specific genetic types can be linked to environmental conditions and concluded that at least in the tropical and subtropical realm, the genetic types appear to have unlimited dispersal potential and their occurrence is more due to historical contingency and niche partitioning than ecological exclusion. Finally, we explored the possibility to identify shape differences among genetic types. A post-hoc analysis of shell shape and microstructure revealed a link between genetic differentiation and shape, which could be sued to redefine species concepts and provide a new tool for improved paleoclimatic analyses. However, we noted that the degree of genetic change and shape differences are not correlated and that several of the studied characters evolved in parallel. We are currently investigating to what degree the new combined genetic-morphological species concepts can be transferred on fossil material. In addition, we have taken advantage of the large number of new DNA sequences and constructed a publicly available database for identification of unknown sequences. To this end, we had to curate the names of almost 4000 sequences, harmonise the list of names and detect and correct all identification errors. This new database paves the way for an entirely new approach in plankton diversity studies, where DNA is extracted from filtered water samples and millions of sequences are generated, representing all cells present in the sample. These sequences can then be annotated using our new database. This procedure will allow fast analyses of the occurrence of genetic types in hundreds of samples, allowing us to better constrain their biogeography, ecology and evolution.
Publications
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(2012) Vertical niche partitioning between cryptic sibling species of a cosmopolitan marine planktonic protist. Molecular Ecology, 21(16): 4063-4073
Weiner A, Aurahs R, Kurasawa A, Kitazato H, Kucera M
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(2013) Comparison of Ba/Ca and d18OWATER as freshwater proxies: a multi-species core-top study on planktonic foraminifera from the vicinity of the Orinoco River mouth. Earth and Planetary Science Letters, 383: 45–57
Bahr A, Schönfeld J, Hoffmann J, Voigt S, Aurahs R, Kucera M, Flögel S, Jentzen A, Gerdes A
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(2013) The cryptic and the apparent reversed: lack of genetic differentiation within the morphologically diverse plexus of the planktonic foraminifer Globigerinoides sacculifer. Paleobiology, 39(1): 21-39
André A, Weiner A, Quillévéré F, Aurahs R, Morard R, Douady CJ, de Garidel-Thoron T, Escarguel G, de Vargas C, Kucera M
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(2014) Phylogeography of the tropical planktonic foraminifera lineage Globigerinella reveals isolation inconsistent with passive dispersal by ocean currents. PLoS ONE, 9(3): e92148
Weiner AKM, Weinkauf MFG, Kurasawa A, Darling KF, Kucera M, Grimm GW
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(2015) Genetic and morphometric evidence for parallel evolution of the Globigerinella calida morphotype. Marine Micropaleontology, 114: 19-35
Weiner AKM, Weinkauf MFG, Kurasawa A, Darling KF, Kucera M
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PFR²: a curated database of planktonic Foraminifera 18S ribosomal DNA as a resource for studies of plankton ecology, biogeography, and evolution. Molecular Ecology Resources
Morard, R., Darling, K., Mahé, F., Audic, S., Ujiié, Y., Weiner, A., André, A., Seears, H., Wade, C., Quillévéré, F., Douady, C., Escarguel, G., de Garidel-Thoron, T., Siccha, M., Kucera, M., de Vargas, C.
