Screening <Emphasis Type="BoldItalic">Arthrospira</Emphasis> (Spirulina) strains for heterotrophy

Thirty-five clonal, axenic Arthrospira strains were screened for their ability to grow heterotrophically on six carbon sources (20 mM). Glucose (34 strains) and fructose (24 strains) were the only substrates permitting growth in the dark. In some assays, however, not every replicate grew and, in at least one strain (D867), repeat assays over 2 years on material maintained in the light indicated that the ability to use fructose in the dark had become lost. Four further strains from other culture collections were compared, because they are presumed duplicates of three of the main set of strains; in at least three cases the duplicates of a pair differed in their ability to use fructose in the dark. In another comparison, where straight and helical morphotypes of the same strain could be compared, two of the four straight morphotypes (including one duplicate strain) grew with glucose in the dark, whereas none of the helical morphotypes did so. It is suggested that genetic drift has led to losses in the ability to grow heterotrophically in some strains.Ten of the main set of strains were tested for their ability to grow photoheterotrophically with four of the carbon sources (glucose, maltose, fructose, sucrose). All grew with glucose and maltose, but none with fructose or sucrose, in spite of the fact that eight (of this subset) grew with fructose in the dark. Sucrose led to most of a culture lysing, but often with short fragments of trichome surviving and subsequently giving rise to a normal culture. All the surviving cells in the transitory stage showed the presence of large intrathylakoidal granules, which had disappeared by the time that the culture had returned to a healthy state. Bleaching and recovery were more rapid at 70 than 10 μmol photon m⁻² s⁻¹. The presence of DCMU prevented this recovery. There was no bleaching when an inoculum of the recovered material was subcultured to medium with sucrose.