This study reports one of the first detailed analysis of an experimentally-induced association (not occurring naturally) between two unicellular protistans, a slime mold, Physarum polycephalum (Mycetozoa), and a chlorophyte alga, Chorella pyrenoidosa (Chlorophyceae), in laboratory cultures. The host, Physarum, was inoculated with Chlorella resulting in the uptake of algae and "greening" of the plasmodia. Ultrastructural analysis showed that the algae were initially engulfed by Physarum, subsequently retained in perialgal vacuoles, distributed within the plasmodia and formed a stable association lasting for weeks as long as the plasmodia were active. Some of the Chlorella were cyclically exocytized and re-engulfed. The inoculated Physarum-Chlorella system had significantly increased survival, some living up to 31 days, as compared to uninoculated hosts. Fluorometric analysis of chlorophyll a and phaeopigments of the Physarum-Chlorella cultures and exocytized Chlorella resulted in higher acid ratios and chlorophyll a concentrations as compared to unialgal cultures indicating viability of the Chlorella in the host cell. At the end of the association, the Physarum host, which was transformed from a plasmodium phase containing algae to the sclerotia phase, exocytized viable Chorella onto the top of the sclerotia. While not a natural or evolutionary established relationship, the Physarum-Chlorella model system provided notable findings of the initial events during Chlorella engulfment and physiological significance of the experimentally induced association between the two protists. This research provides evidence that new host-algal associations may become established rather rapidly under favorable environmental conditions and yields further evidence of the plasticity of host-symbiotic associations that may have promoted endosymbiotic associations during eukaryotic evolution.
625RCTimes Cited:3Cited References Count:41