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.

Summary: In natural environments, plants are exposed to rapidly changing light. Maintaining photosynthetic efficiency while avoiding photodamage requires equally rapid regulation of photoprotective mechanisms. We asked what the operation frequency range of regulation is in which plants can efficiently respond to varying light. Chlorophyll fluorescence, P700, plastocyanin, and ferredoxin responses of wild‐types Arabidopsis thaliana were measured in oscillating light of various frequencies. We also investigated the npq1 mutant lacking violaxanthin de‐epoxidase, the npq4 mutant lacking PsbS protein, and the mutants crr2‐2, and pgrl1ab impaired in different pathways of the cyclic electron transport. The fastest was the PsbS‐regulation responding to oscillation periods longer than 10 s. Processes involving violaxanthin de‐epoxidase dampened changes in chlorophyll fluorescence in oscillation periods of 2 min or longer. Knocking out the PGR5/PGRL1 pathway strongly reduced variations of all monitored parameters, probably due to congestion in the electron transport. Incapacitating the NDH‐like pathway only slightly changed the photosynthetic dynamics. Our observations are consistent with the hypothesis that nonphotochemical quenching in slow light oscillations involves violaxanthin de‐epoxidase to produce, presumably, a largely stationary level of zeaxanthin. We interpret the observed dynamics of photosystem I components as being formed in slow light oscillations partially by thylakoid remodeling that modulates the redox rates.

The unicellular cyanobacterium Cyanothece sp. American Type Culture Collection (ATCC) 51142 is capable of performing oxygenic photosynthesis during the day and microoxic nitrogen fixation at night. These mutually exclusive processes are possible only by temporal separation by circadian clock or another cellular program. We report identification of a temperature-dependent ultradian metabolic rhythm that controls the alternating oxygenic and microoxic processes of Cyanothece sp. ATCC 51142 under continuous high irradiance and in high CO ₂ concentration. During the oxygenic photosynthesis phase, nitrate deficiency limited protein synthesis and CO ₂ assimilation was directed toward glycogen synthesis. The carbohydrate accumulation reduced overexcitation of the photosynthetic reactions until a respiration burst initiated a transition to microoxic N ₂ fixation. In contrast to the circadian clock, this ultradian period is strongly temperature-dependent: 17 h at 27 °C, which continuously decreased to 10 h at 39 °C. The cycle was expressed by an oscillatory modulation of net O ₂ evolution, CO ₂ uptake, pH, fluorescence emission, glycogen content, cell division, and culture optical density. The corresponding ultradian modulation was also observed in the transcription of nitrogenase-related nifB and nifH genes and in nitrogenase activities. We propose that the control by the newly identified metabolic cycle adds another rhythmic component to the circadian clock that reflects the true metabolic state depending on the actual temperature, irradiance, and CO ₂ availability.