Microalgal biomass is attracting attention as a new biological resource in pharmaceuticals, as a source of vegetable proteins and industrial materials, and from the perspective of carbon cycling. Various specialized mass-culture systems have been developed to produce large-scale biomass. However, reducing the costs of microalgal biomass production is necessary. We conducted a long-term culture experiment using a simple mass-culture system for Desmodesmus, which has been extensively studied as a candidate species for microalgal biomass production in recent years. Compared to those on Chlorella and Haematococcus, only a few mass culture studies have been conducted on Desmodesmus, and no such studies have been conducted in Japan. We cultured the original strain of Desmodesmus communis (dSgDes-eco1), isolated from a lake in Saga City, Japan. A maximum biomass productivity of 0.040 ± 0.0017 g/l/day, which is comparable to that obtained in a previous study, was recorded in a simple 100-l culture system that was small in volume, open-culture, and easy to operate. A significant positive correlation was observed between biomass productivity and water temperature. Protein content remained almost constant throughout the incubation period, averaging 47.9%. A significant positive correlation was also observed between the protein content and daily average of accumulated photon flux density. Our culture system is expected to reduce the costs of microalgal biomass and vegetable protein production.

Gut microbes are of crucial in various industrial fields including drug development; therefore, the isolation, cultivation, and classification of difficult-to-culture microorganisms and new species are essential for the development of new live biotherapeutic products. We performed liquid-liquid co-cultures to obtain isolates using a previously unreported method. Selectively filtered fecal samples were anaerobically cultured under the influence of supporting bacteria through a membrane filter. 16S rRNA gene and genomic analyses showed that isolates were identified as either Waltera intestinalis or a new species closely related to W. intestinalis. Isolates in broth medium had short and thin cells during the early stages of growth, but became longer and thicker with longer duration of incubation. Therefore, the cells during the early growth stage may have passed through the filter owing to their small size and elongated at a later stage. Filter-treated isolates grew only in the co-culture, whereas untreated isolates grew better in the mono-culture than in the co-culture. This suggests that the growth of the filter-treated isolates was enhanced by the co-culture, whereas the growth of the elongated cells of the isolates was inhibited by the co-culture. Furthermore, all isolates had genes involved in sporulation and germination. This suggested that the small spores that passed through the filter germinated under the influence of the metabolites produced by the supporting bacteria; however, the growth of the spores was inhibited by the metabolites.

The genus Micromonas (Mamiellales, Mamiellophyceae) is a eukaryotic picoplankton widely distributed in marine environments. Since the establishment of the genus, only one species, M. pusilla, had been recognized for more than half a century. Recently, M. bravo, M. commoda, and M. polaris were described by detailed electron microscopic observation and molecular phylogeny. On the basis of our molecular analysis, we reidentified 14 Micromonas strains (seven M. pusilla strains and seven unidentified Micromonas strains) from Japan that are maintained in the Microbial Culture Collection at the National Institute for Environmental Studies (Tsukuba, Japan). Our molecular phylogenetic analysis of 18S ribosomal RNA gene confirmed that strains NIES-3661 and NIES-3665 were assigned to M. pusilla. However, strains NIES-1413, NIES-3660, and NIES-3663 were identified as M. bravo, and the other nine strains (NIES-1411, NIES-1412, NIES-2527 to NIES-2530, NIES-3662, NIES-3664, and NIES-4096) belonged to M. commoda. Our results highlight the importance of reidentifying strains in culture collections when the species composition of the genus has been revised by using recent molecular approaches.