Functional Shell Morphology of Larger Benthic Foraminifera

Larger symbiont-bearing benthic foraminifera (LBF) as indicative elements in oligotrophic tropical seas are important elements of shallow marine environments during distinct periods in Earth History, first in the Late Palaeozoic, afterwards from the Cretaceous to Recent. The function of their large tests, basically to provide their endosymbiotic algae with light to obtain net photosynthetic rates together with the need to resist extreme hydrodynamics in shallow water, has been intensively studied. However, this research was yet restricted to a 2- dimensional representation of the inner shell and hampered by test destruction due to thin sections. The newly developed technique of X-ray micro-Computer-Tomography (microCT) with resolutions less than 1 micrometer allows its usage in the investigation of the complex tests of larger benthic foraminifera without destruction. Growth studies on living and fossil species could be performed, because the stepwise growth of the foraminiferal cell constructs tests by a sequence of chambers, where a growth step is represented by the construction of a single chamber. In their sequence, the volumes of chamber lumina represent cell growth, thus demonstrating interruptions, increase/decrease or oscillations in growth rates caused by external factors depending on time (e.g. seasons). Beside chamber volumes, characters of chamber form like height, width, basal septal distance and backbend of the chamber margin, can be measured by microCT, thus their correlation with cell growth can be investigated. Additionally, the position, density and form of external test structures like pores, piles, septal elevations and papillae can be measured and their relations with cell growth can be shown. Internal test structures such as septal undulations, chamber partitions by septulae, stolons, canal system and densities of egg-holders, where the symbiotic algae are protected from the cell plasma of the host simultaneously getting the best position for collecting light, can be determined. The combination of all measurements allows interpretation of different biological and environmental dependencies of LBF. Varying growth rates determine environmental abiotic (e.g. seasons, instantaneous events) and biotic influence (e.g. predation), leading to life time estimation for the fossil forms by comparison with their living relatives. Especially the reaction to environmental gradients like water depth, substrate or temperature is expressed in test form together with external and internal test structures (increase in knots or papillae with water depth, test flattening, compartimentation, wall thinning, etc.). Additionally to these factors that affect or have affected the life of LBFs, the determination of test densities during growth allows the reconstruction of palaeoenvironmental factors like water energy, where tests become entrained due to orbital wave energy, then transported by currents, where transport width and depth depends on slope topography. Furthermore, the accumulation of larger foraminifera in the past leading to large carbonate buildups could be explained by this detailed investigation of shape, size and densities of LBF-tests. Finally, in living species the detailed investigation of growth between different species expressed in test shape can be compared with molecular genetic relations for finding coincidence or difference between morphological and molecular genetic relations. If there is high coincidence, then the compound morphological investigation in fossil forms will help to differentiate species on a biological base and investigate their phylogenetic relationships.

Project P23459-B17 named "Functional Morphology of Larger Benthic Foraminifera (LBF)" has been granted on March 16, 2011 by the Austrian Science Foundation and it was designed to be completed after three years time by the employee after he finished his PhD. During these three years, a number of scientific discoveries have been made. Some of these have been published in international journals, others are still under review process, other manuscripts are still under preparation and the last portion of data must be still partially increased prior to submission. This project dealt with large unicellular protists called foraminifera, which build their shell as a sequence of small cavities (i.e., chambers) containing the growing cell cytoplasm. Target of this project was to check if shape and size of each single chamber might reflect environmental influence. To approach this task, we had the possibility to use the computed tomography technique which permits to extract, visualize and measure whatever part of the scanned object on a three dimensional basis. By applying this technique on a large spectrum of specimen (approximately 950 specimens scanned) and after having measured a large variety of parameters on most of the scanned material, we had the opportunity to obtain an incredibly large data set but at the same time we realized that there are too many open questions we still have and want to address. We found out in which way foraminifera modify their internal geometry according to their needs for life without disturbing the general morphology. We have developed a solid mathematical approach to investigate the growth pattern of LBF. We have observed an unexpected variety in morphological modifications and/or aberrations. We have learned how to obtain high qualitative scans of multiple specimens by scanning at reduced time (and price) and resources; now we can scan with extreme efficiency. We have also observed that larger benthic foraminifera grow in different ways depending on their test geometry, but all fit size-limited growth models. We have observed that the growth of LBF is characterized by constant oscillations and, recently, we had the opportunity to confirm that such oscillations are perfectly matching lunar cycles and depend also on tidal intensity. During the last three years we produced more than 15 peer -reviewed papers, we attended 12 congresses to present our latest results and we supported 3 BSc, 1 MSc and 2 PhD students.