## 4 Discussion
raciborskii_ at low temperatures is light dependent, with high irradiance hampering its growth [7]. Our study is in line with this finding, additionally showing that the phenomenon of low-temperature/high light-induced photoinhibition is also strain specific. However, the above-mentioned findings do not explain how some _R. raciborskii_ strains have adapted to chill/light. This adaptation requires the development of mechanisms that ensure survival with the combination of two stresses, typical of the temperate climate. The mechanisms of adaptation to chill/light have been explored only accidentally. For example, such conditions may induce the accumulation of carotenoids and phycobilins in _R. raciborskii_ cells [36, 37] which allows better protection from light and results in lower photoinhibition compared to a native species, _Aphanizomenon gracile_[37]. However, neither genetic nor physiological characteristics of this adaptation are well understood. The pattern of gene expression of the selected genes in this study, which are involved in the basic metabolic processes (Figure 5), give insights into this phenomenon. The gene expression results suggest a broad response to chill/light stress on mRNA level (seven of the eight investigated genes upregulated) in AMU-DH-30 strain (Figures 2a, 3 and 5). The biologically relevant upregulation impacting the physiological level can be assumed at least for _HtX_, _nusG_, and _asd_. Furthermore, the increased expression of other genes also suggest their involvement in this type of response. Particularly important seems to be the upregulation of _rbp1_ as it has been documented that RNA-binding proteins (Rbp), involved in many posttranscriptional regulation processes may regulate the cyanobacterial physiology at low temperature. The accumulation of Rbp1 and 2 in _Synechocystis_ sp. during preconditioning [14] was correlated with the increased chill/light tolerance. Our results suggest that the upregulation of _rbp1_ in _R. raciborskii_ may allow the initiation of other processes necessary for adaptation, mainly those responsible for enhanced membrane fluidity and photosynthesis maintenance. In _Synechocystis_, Rbp3 may influence the mRNA levels of _desA_, _desB_, _desD_[38], which has a role in the modifications of membrane fluidity. In our study, the increased level of _desA_ was observed only in AMU-DH-30 strain after 1 h and five days of acclimation to chill/light stress whereas in CS-505 strain this gene was not upregulated. These results suggest that cells of _R. raciborskii_ AMU-DH-30 exposed to chill/light stress were induced to a rapid increase in the desaturation of fatty acids chains and this process is prolonged for several days, indicating the mechanism of chill/light adaptation. Another investigated gene encoding (3R)--hydroxymyristoyl-[acyl-carrier-protein]-dehydratase (FabZ) [39] is involved in a fatty acid elongation cycle that leads the biosynthesis of unsaturated fatty acids. However, _fabZ_ was not upregulated neither in AMU-DH-30 nor CS-505 strain which indicates that this gene does not play any role in chill/light stress. The modification of thylakoid membrane composition and fluidity provides an optimal condition leading to homeostasis of proteins involved in photosynthesis and their functionality under stressful condition [40]. The homeostasis of proteins relies on the mechanisms necessary to remove photodamaged and misfolded proteins. One of the most limiting steps in the photosynthetic repair cycle is the replacement rate of damaged D1, an integral component of PSII, which is particularly sensitive to oxidative stress induced by chill/light. The ATP-dependent zinc metalloprotease (FtsH) plays a crucial role in the control of the photosystem II repair cycle in model cyanobacterium species _Synechocystis_ PCC 6803 [41] and in _Prochlorococcus marinus_ MED 4 [42].