Kelps (brown seaweeds of the order Laminariales) form dense underwater forests along rocky coastlines of Arctic and cold-temperate regions, providing habitat to a vast number of associated organisms [8]. Depending on their species-specific physiological tolerance levels kelps may populate the subtidal zone along extended latitudinal gradients. In the Northern hemisphere, the impact of climate change (i.e. temperature increase) on kelp will be most effective at the respective Southern distributional limit (or at locations with a similar annual temperature gradient [2]), and potentially result in northward shifts of entire populations. In contrast, kelp species growing in Arctic regions usually thrive under suboptimum temperature conditions and are potentially benefitting from temperature increase. The database on ecophysiological traits of seaweed responses has substantially grown in the last two decades, with a particular increase in knowledge for kelp species from Arctic regions [1, 9]. However, the mechanisms of acclimation patterns and transcriptomic responses occurring at the onset of stress, even before its manifestation by physiology, are understudied in kelp, despite their prime ecological importance [6]. The phenomenon of cross-acclimation implies that the acclimation to one stressor also provides an increased protection towards a second, different stressor. Cross-acclimation has hardly been addressed in seaweeds before, although such studies may provide valuable insights into species physiology allowing the discrimination of stress-specific vs. general stress responses, which should in turn also be reflected by the transcriptome [6]. Recently, preliminary studies on the crossed acclimation to UV-radiation and hydrogen peroxide exposure have been conducted in a MSc project in the laboratory of the main proponent.

 

Saccharina latissima (Linnaeus) C.E.Lane, C.Mayes, Druehl & G.W.Saunders is a habitat-forming kelp of cold-temperate to Arctic distribution. Although its southernmost population in the North-East Atlantic is growing in Galicia (Spain), the population in Southern Brittany may also be regarded as a distributional edge population due to the annual local temperature conditions. The species is stretching northwards up to the western coasts of Spitsbergen. Saccharina latissima has been reported as a species with high phenotypic plasticity exhibiting a remarkably dynamic acclimation capability allowing the species to inhabit a large latitudinal and depth gradient and to respond to short-term stress [3]. Along its latitudinal distribution gradient, ecotype formation in response to prevailing environmental conditions has been suggested [7]. The species is furthermore easy to cultivate, a cDNA library is already available [6], and the proponents have various strains from different locations in stock culture, making it an ideal species for controlled experiments on cross-acclimation and transcriptome analyses. 

 

Scope:

Saccharina latissima has been intensively studied in terms of its acclimatory responses towards single and combined abiotic stress. It is therefore ideally suited as a representative kelp species to study the concept of transcriptomic responses along abiotic stress gradients. Hitherto it is unknown which signaling pathways and which energetic trade-offs are involved in the acclimation response in S. latissima in particular, as in kelp in general. 

 

Questions:

 

To close these gaps in knowledge we propose to focus on a comparison of specimens of S. latissima originating from its Northern (Spitsbergen) and Southern (Brittany, Galicia) distributional limits. Optionally we would like to include strains from the Baltic Sea where the species has to suffer additional salinity stress (all strains are available in the AWI and/or SB Roscoff stock culture). Specimens from the different localities will be exposed to crossed acclimation treatments (temperature vs. high light; temperature vs. UV) and their physiological responses in terms of photosynthetic performance (PAM fluorescence, optode based oxygen measurements), antioxidative strategies (DPPH essay) as well as light protective systems (xanthophyll cycling via HPLC-based pigment analyses) will be analyzed. Samples from these experiments will be subjected to transcriptome analyses following the protocol of Heinrich et al. (2012, [6]). In our previous studies on stress responses in S. latissima we observed that a transcriptomic response will already be visible before a changing phenotype is evident [6]. We will now try to estimate the costs of this “invisible” response by establishing a measure for strength of such response based on more severe treatments and the combination of treatments. We furthermore assume that under optimal conditions the algae will be in a buffered situation enabling them to provide extra energy for a total transcriptomic response above the unstressed level. Our working hypothesis is, that when stress becomes severe total cellular transriptomic response will be limited by metabolic costs, resulting in enhanced transcription of stress genes only at the cost of reducing transcription of other genes, resulting in a shift in metabolism to a more instable status. The whole transcriptome will be addressed not only in relation to a reference control, i.e. interpreting fold changes statistically in terms of processes, but also as absolute transcript distributions across pathways. It is supposed that limits to transcriptome capacity show up first in response to multiple stressors, i.e. synchronous activation of different stress pathways is not probably possible beyond a certain stress level. We will develop a conceptual model to underpin the effects of hidden costs of stress response. The role of recovery in relation to hardening processes will be enlightened in this concept. 

 

Implementation:

The proposing consortium (K. Bischof (1st promotor), S. Frickenhaus, S. Heinrich, K. Valentin, I. Bartsch, E. Corre (2nd promotor), C. T. Tonon, C. Leblanc, P. Potin) from the University of Bremen, the Alfred Wegener Institute and the Biological Station of Roscoff comprises leading experts in the field of ecophysiology [1, 2, 3, 7] and molecular genetics [4, 5, 6,] of seaweeds. All physiological and molecular methods are available to the project and established within the consortium. 

 

References

 

In particular results will provide insight into the molecular mechanisms of (cross-) acclimation in an abundant and ecologically highly relevant species of kelp. Taking S. latissima as a marine model system a more general outcome will be the improved understanding of the concept of transcriptional responses in marine primary producers facing environmental change. 

 

The candidate is expected to publish the results of the project together with the supervisors in peer-reviewed scientific journals. He/she will present the outcomes at two international conferences. Close co-operation and exchange with ongoing international research projects within the consortium will ensure that the outcomes of the Ph.D. project will be recognised by the scientific community. Since environmental impact on kelp is a burning issue of global concern, it is expected that the outcomes of this project will be of high relevance for a wider audience and also gain interest of the general public.