Physiological stress response of foraminifera

The 70-kDa heat-shock protein (Hsp70) is member of a set of proteins, which undergo increased synthesis in response to a variety of physical and chemical stress. Heat shock proteins as chaperones are important for the synthesis and folding of proteins or the denaturation or removal of damaged (denatured) proteins. The phylogenetic highly conserved Hsp70 has been found in most investigated organisms so far, and recently for the first time in benthic foraminifera, a diverse and abundant group of protists in marine sediments of present and past times. The aim of this project is the application of this recent finding to quantitatively characterize the physiological response of foraminifera to different stress factors, simulating changing natural environmental conditions. Foraminifera have been extensively used for monitoring modern and particularly past environmental conditions, yet their ecological preferences and tolerance ranges have mostly been determined indirectly, by assessing vitality and mortality. Using foraminiferal Hsp70 expression as a biomarker for stress, we aim to investigate and determine directly the tolerance ranges of different species towards oxygen concentration, salinity, temperature, and environmental toxins (cadmium and lead). The focus of this project is on recent foraminifera from the marine intertidal and shelf area. Benthic organisms in these oceanic regimes not only have to cope with diurnal and seasonal changes of environment conditions but also with anthropogenic induced influences such as pollution. An additional aim of this project is the study of the influence of hardening on the stress response. Comparing the response of pre-stressed and unstressed specimens will provide further information on how foraminifera adapt to different stressors. The results will be used to define ecological optima and tolerance ranges of foraminifera for the tested parameters. Knowledge on ecological demands and physiological limitations of recent species is essential for interpreting fossil foraminiferal assemblages and the reconstruction of past environmental conditions.

In this project, benthic foraminifera (a species-rich group of protozoa colonizing the seabed) were kept in laboratory cultures to study their physiological response to various stress factors. Investigations concentrated on large benthic foraminifera of the species Heterostegina depressa, which occur in tropical to subtropical shallow water areas and bear photosynthetic algae as symbionts inside their cells. The two environmental parameters temperature and salinity were tested. Both factors can have a strong influence on the health of the foraminifera and their symbionts during large fluctuations. Due to anthropogenic influence and climate change, temperature and salinity in many shallow water areas will presumably rise sharply in the future and will trigger stress reactions or even death in the organisms living there. In the project, temperatures along a gradient of 25C to 38C and different seawater salt contents along a gradient of 2.6% to 4.0% were tested. The initiated stress reactions were analysed by the amount of 70 kDA heat shock protein (Hsp70) present in the biomass of the large benthic foraminifera. The total protein (= protein of the holobiont) was investigated, which contained both, foraminiferal and symbiont proteins. For this purpose, an extraction protocol was adapted for the isolation of proteins from large benthic foraminifera. The project results show for the first time at which exact temperature level or salt content the Hsp70 protein expression starts to raise and thus increased physiological stress begins in the tested large benthic foraminifera. The results make an important contribution to the definition of ecological optima and tolerance ranges of this group.