The biochemical composition of Arthrospira (Spirulina) platensis (Nordstedt) Gomont after long-term storage in the state of anhydrobiosis (4 years, 17 years) was determined by the standard methods. It was shown that protein content (55.3–61.2%) and total carbohydrates content (13.0–15.6%) in cyanobacterial cells were in agreement with the data known from the literature and with our results obtained at the onset of storage. The biomass had low content of free nucleotides (1.8–2.6%), lipids (1.3–11.0%), and especially pigments (0.5–1.3%, 0.03–0.12%, 1.4–2.0%, 0.03–0.05% for chlorophyll a, carotenoids, C-phycocyanin, and allophycocyanin, respectively). The content of nucleic acids content was 3.1–24.0 and 0.11–0.16% for RNA and DNA, respectively. Microscopic examination of A. platensis (17 years of storage) showed high numbers of irreversibly damaged and dead cells (34.2 and 65.8%, respectively). To determine the quantitative and morphological parameters of associated microflora, a complex physicochemical treatment (methanol, ultrasound, and centrifugation) of the reactivated cyanobacterial suspension proved the most efficient. Three main groups were distinguished in the morphological structure of the microbiome (rod-shaped, rounded, and convoluted forms). The community was dominated by rod-shaped forms: large and small rods accounted for 60.5 and 14.4%, respectively. Mycelial forms (thin filaments), cocci, and convoluted forms were less common. On average, the volume of a bacterial cell was 0.27 ± 0.04 µm³. The contribution of bacteria to the biomass of A. platensis varied from 3.3 to 11.3% (8.3 ± 4.4% on average) of the dry weight of A. platensis. It has been suggested that the biochemical parameters and viability of cyanobacteria were affected by the accompanying microflora.

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