Interplay between carbon concentrating mechanism and osmoregulation / ion-regulation in Charophytes: chance or obstacle for adaptation to terrestrial conditions? (ICOR)

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Charophytes are submerged macrophytes belonging to the Streptophyta, a clade which also include all land plants. Together with the Zygnematophyceae and the Coleochaetophycae, Charophyceae form the ZCC clade, from which the common ancestor of land plants evolved. Charophytes are the most complex streptophyte algae, which exhibit features thought to be mandatory for successful colonization of the terrestrial habitat (e.g. functional rhizoids, gametangia envelopes, cortication). Hence, they represent an important stepping stone worth being investigated in detail in order to understand the mechanisms of terrestrialization. The drastic decline in availability of inorganic carbon (Ci) was probably one of the most striking challenge during terrestrialization. Passive diffusion of CO2 is unable to meet the photosynthetic demand of complex aquatic eukaryotic photoautotrophs at the recent low atmospheric pCO2 conditions. Therefore, eukaryotic algae usually employ sophisticated carbon-concentrating mechanisms (CCMs) to enrich CO2 in the vicinity of Rubisco. However, the existence and function of a CCM in Charophytes is uncertain. Moreover, s strong interference between pH and ion acclimation and Ci acquisition is anticipated.In preliminary experiments we established cultivation systems for Charophytes under defined conditions. Selected species are able to grow under a wide range of pCO2, salinity and pH levels. For the project we will concentrate on Chara braunii, because its available well-annotated genome sequence provides a solid base for the planned molecular work. Searches in the C. braunii genome resulted in the detection of many genes that potentially encode proteins involved in the CCM of eukaryotic model algae such as Chlamydomonas reinhardtii.The initial aim of this proposal is to unravel whether or not C. braunii performs a CCM. For this purpose, we will compare the CO2 affinities of intact C. braunii and its carboxylating enzyme Rubisco. The alga will be cultivated under different CO2 and pH levels to investigate if it can respond to different Ci levels by changing the photosynthetic CO2 affinity. The expression of selected genes putatively involved in Chara CCM will be analyzed by qPCR. Finally, the interaction of Ci and ion acclimation will be investigated in C. braunii after cultivation at different salinities. Collectively, we aim to find out if Charophytes perform a CCM, which genes may be involved in this process, and, how far it is interacting to different ionic relations in thesurrounding waters.