Exploring the biodiversity of acclimation: effects of nutrient limitation and light regime in marine microalgae

Widespread across diverse environments from fresh and marine waters to symbiotes with other organisms, microalgae have adapted to numerous habitats where they form the base of marine food webs and are responsible for roughly half of global carbon fixation. Microalgae derive from a complex evolutionary history characterized by multiple endosymbiotic events through which they acquired their photosynthetic plastids, giving rise to many distinct lineages. As a result, they display broad diversity in the structure and composition of their photosynthetic apparatus, as well as in their physiological adaptations to ever-changing environments, shaped by habitat and phylogeny. In this thesis, we investigate long-term acclimation strategies to different nutrient limitations and light regimes in microalgae belonging to distinct phylogenetic groups and ecological niches. The first introductory chapter covers the state of the art on: (i) the structure and function of the photosynthetic apparatus, (ii) sulphur and iron uptake and assimilatory pathways, (iii) environmental variations in nutrient and light availability and their effects on microalgal physiology, (iv) microalgal species under investigation in the following chapters. In chapter 2 examined the responses to long-term sulphur limitation in three marine microalgae: the chlorophytes Tetraselmis suecica and Dunaliella salina, and the model diatom Phaeodactylum tricornutum. Sulphur is an essential component of proteins, lipids, and other metabolites, and it plays a direct role in photosynthesis through both electron transport and carbon fixation pathways. The three species were grown under sulphur-replete and sulphur-deficient conditions, and we analysed their growth, elemental and pigment composition, in vivo photosynthesis, and the accumulation of proteins related to photosynthesis and sulphur metabolism. Under low sulphate conditions, all species prioritized the allocation of resources to photosynthesis: through modulation of pigment content and stoichiometry of their photosynthetic apparatus, S-limited cells maintained in vivo photosynthetic activity close to that of control cultures while adjusting their growth and cellular composition in species-specific ways. These results were interpreted in an evolutionary framework, considering how changes in oceanic sulphate availability throughout Earth’s history paralleled by shifts in the ecological dominance of different algal groups may have shaped the responses of the species studied. In chapter 3 following the physiological characterization of S limitation, we performed a transcriptomic analysis of D. salina grown under control and growth-limiting sulphur concentrations to obtain a broader view of the metabolic pathways affected during acclimation. We focused on genes related to photosynthesis, including those encoding antenna proteins, as well as genes involved in sulphur metabolism and protein turnover, guided by insights from the physiological data. Although our de novo transcriptome assembly provided good coverage of the targeted pathways, relatively few differentially expressed genes previously reported in the literature were identified. This may reflect differences in gene expression patterns between cells undergoing commonly studied acute stress responses and those acclimated to long-term nutrient limitation. Nevertheless, our results indicated an increase in protein-related metabolic activity, consistent with studies suggesting elevated protein mistranslation under sulphur limitation. In chapter 4 we then assessed the effects of iron limitation on the chlorophytes T. suecica and D. salina. Iron is essential for redox reactions and photosynthesis, particularly as a component of Fe–S clusters in many photosynthetic complexes, and it is one of the major factors limiting marine primary production, with its availability varying across habitats. The two species differed in their ability to reduce cellular Fe requirements: T. suecica exhibited stronger reductions in growth and photosynthesis, while D. salina, adapted to survive in low-Fe environments, maintained its relative Fe content through regulation of cell size and the activation of Fe-sparing strategies. In chapter 5 differences in photosynthetic characteristics identified in earlier experiments prompted a detailed analysis of the acclimation of T. suecica and D. salina to different light regimes. Cultures were acclimated to low light, high light, and fluctuating light conditions. T. suecica displayed relatively low Chl a/b ratios (for chlorophytes), consistent with species possessing large antenna complexes. In line with this, T. suecica showed minimal changes in the Chl a/b under low light. In contrast, D. salina increased its pigment content under low light. The species also differed in their responses to fluctuating light and in their photoprotective mechanisms: T. suecica exhibited NPQ kinetics similar to those of the model species Chlamydomonas reinhardtii, whereas D. salina showed a distinctive pattern characterized by strong NPQ induction at the onset of light, followed by relaxation, and a broader NPQ amplitude across growth conditions, suggesting a fundamentally different mode of NPQ regulation. In chapter 6 in light of the importance of photosynthetic acclimation to light availability, we examined the effects of far-red light on in-hospite coral symbionts, a topic that remains poorly explored. Far red wavelengths may act as cues for regulatory or acclimatory responses in symbiotic algae. We grew the hard coral Cladocora caespitosa and the soft-bodied anemone Anemonia viridis for two months under white light and white light supplemented with infrared light, monitoring growth, symbiont density, and photosynthetic activity. Although no major differences in coral health or growth were detected, some differences in photophysiology emerged. Symbionts of C. caespitosa exhibited higher Chl a/c2 ratios than those of A. viridis, suggesting that the two hosts harbor distinct Symbiodiniaceae lineages. Under IR-enriched light, C. caespitosa symbionts showed increased activation of photoprotective mechanisms, while A. viridis symbionts did not exhibit significant changes in photosynthetic regulation. Although the duration of the experiment may have been too short to reveal effects on host coral growth, the observed differences in symbiont responses suggest that in some algal species, far-red light may serve as an environmental indicator of proximity to the water surface.