Significance: Vast areas of the oceans are N limited, and how microalgae can flourish in these N-poor waters is still not known. Furthermore, mechanisms and sites of N uptake and storage have not been fully determined. We show that crystalline guanine (C ₅ H ₅ N ₅ O) is an important N storage form for phytoplankton and for symbiotic dinoflagellates of corals. The widespread occurrence of guanine reserves among taxonomically distant microalgal species suggests an early evolutionary origin of its function as N storage. Crystalline guanine appears to be a multifunctional biochemical with an important role in the N cycle that remains to be elucidated. In particular, a better knowledge of N-storage metabolism is necessary to understand the impact of eutrophication on coral-symbiont interaction.

DNA concentration has been recently suggested to be the reason why different arrangements are revealed for K⁺-stabilized human telomere quadruplexes by experimental methods requiring DNA concentrations differing by orders of magnitude. As Raman spectroscopy can be applied to DNA samples ranging from those accessible by absorption and CD spectroscopies up to extremely concentrated solutions, gels and even crystals; it has been used here to clarify polymorphism of a core human telomeric sequence G₃(TTAG₃)₃ in the presence of K⁺ and Na⁺ ions throughout wide range of DNA concentrations. We demonstrate that the K⁺-structure of G₃(TTAG₃)₃ at low DNA concentration is close to the antiparallel fold of Na⁺-stabilized quadruplex. On the increase of G₃(TTAG₃)₃ concentration, a gradual transition from antiparallel to intramolecular parallel arrangement was observed, but only for thermodynamically equilibrated K⁺-stabilized samples. The transition is synergically supported by increased K⁺ concentration. However, even for extremely high G₃(TTAG₃)₃ and K⁺ concentrations, an intramolecular antiparallel quadruplex is spontaneously formed from desalted non-quadruplex single-strand after addition of K⁺ ions. Thermal destabilization or long dwell time are necessary to induce interquadruplex transition. On the contrary, Na⁺-stabilized G₃(TTAG₃)₃ retains its antiparallel folding regardless of the extremely high DNA and/or Na⁺ concentrations, thermal destabilization or annealing.