Foraminiferal genotype adaptation and their potential to enhance palaeoceanographic proxies

点击翻译按钮获取中文摘要

Planktonic foraminifers (forams) are an important group of single-celled organisms that live in the marine plankton. Their hard shells of calcium carbonate, imprinted with a chemical record of the environment in which they grew, are preserved as microfossils in the sediments where they have accumulated over millions of years. Researchers use them to reconstruct ocean circulation and climate in the past by assuming that all shells of a morphospecies (species defined by shape) were constructed within the same environmental range. Using a unique DNA marker in living forams, I have been able to show that this is not true. Each morphospecies often has several distinct genetic types (genotypes) and many are adapted to different environments. Genotypes therefore build their shells in different environments from one another and scientists are unknowingly using a mixture of different types. This is an unforeseen problem for researches using chemical evidence from fossil shells to understand past climate changes in these regions . During my Advanced Fellowship, my principle task has been to assess genetic variation within important morphospecies and the different adaptations of newly identified genotypes. I have collected thousands of samples to date from the high Arctic to the Antarctic, targeting regions of special interest to scientists who investigate changes in the past climate. Forams can be carried over great distances in the ocean currents and although there seem to be few barriers to dispersal, individual genotypes do have different adaptations and do not live everywhere. Some are more adapted to the heat or cold, others to different amounts or types of food. I have discovered that the same morphospecies living in both polar regions became genetically split around two million years ago when the cold glacial period began. The Antarctic ones also diverged again later and one become adapted to live in the extreme environment of the sea-ice while others have found themselves left behind, isolated in a cold current following the last glacial period. All this knowledge tells us about how life splits up into different species in the ocean and I have alerted scientists to many of these new evolutionary discoveries in leading journals. For past climate reconstruction, pooling several genotypes together must impose error into researcher's calculations and climate models. My evidence on the genetic diversity and ecology of living forams in the cold regions of the high latitude oceans has highlighted and solved some of these problems. I have provided scientists with new ways of improving accuracy in fossil record interpretation by utilising the changing shapes of foram shells in these regions. It is now a high priority to investigate the wind driven upwelling regions of the tropical and subtropical waters which play an equally vital role in earth's climate system to determine the genotypic structure of these quite different ecosystems. I propose to sample these regions during three separate cruises in the Indian and Pacific Oceans to determine which genotypes live there and the specific water column conditions they live in. I will combine genotyping at different depths with high resolution chemical analyses of the different layers of their shell produced during their life cycle. An extensive investigation into genotype shell shape will be carried out to facilitate their identification in the fossil record. Improving the resolution of present day low latitude data on the relationship between shell geochemistry and the physical environment in which genotypes grow is vital for past climate modelling. This work will be done in collaboration with a strong team of experts who work on the reconstruction of past climate both in the UK and abroad. In addition, a new data base of all genotyped specimens and their location is being compiled which will be made available for use by the scientific community when complete.