Nitrogen fixation in the form of guanine crystals and rearrangements of crystals as an adaptation for various illumination levels of Amphidinium carterae
Abstract
The utilization of biogenic guanine crystals by animals to manipulate light is a well-known phenomenon [1]. Guanine crystal arrays possess a high refraction index and plate-like arrangement, enabling them to function as diffuse scatterers, reflectors, tunable photonic crystals, and image-forming mirrors.
Recently, crystalline inclusions comprising guanine or related purines have been identified in various phylogenetically unrelated photosynthetic microalgae [2]. In Amphidinium carterae, a dinoflagellate, guanine crystals were observed to serve as a long-term, high-capacity store of nitrogen, although their role in light manipulation had been previously speculated for photosynthetic eukaryotes [3].
Confocal Raman microscopy was employed to visualize crystalline guanine within intact A. carterae cells, revealing that the location of guanine crystals was dependent on illumination intensity. Regardless of the light conditions, the initial position of crystal formation is inside and at the periphery of the cell membrane.
However, under low light intensity (ca 3-5 μmol(photons) m-2 s-1), guanine crystals were synthesized closer to the center of the cell, acting as photonic mirrors or diffusers to enhance the efficiency of photosynthesis. Conversely, high light intensity (up to 500 μmol(photons) m-2 s-1), led to the organization of extensive guanine crystal layers between the cell wall and plastids, providing protection against excessive illumination and regulating photosynthetic efficiency.
This study highlights the potential role of guanine crystals in regulating photosynthetic processes and protecting plastids from photo damage in A. carterae.