Development of free-living sporangial filaments regulated by light and culture density in Neopyropia yezoensis

Neopyropia yezoensis, an important marine crop, reproduces via sporangial filaments. This special sporophyte stage, which directly releases conchospores—the “seed” of gametophytes (thalli)—is poorly understood. Under free-living culture, we found sporangial filaments varied between hollow cells (at high density) and content-enriched bipartite cells (at low density), released as conchospores. Serious intracellular degradation, which might through fusion of a double-membrane coated body with vacuole, was observed in hollow cells but disappeared in newly-formed bipartite cells and reappeared in their subsequent vegetative growth through transmission electron microscope. The bipartite cells can also return to vegetative growth when not released and the conchospores release was obviously inhibited by some growth promoting conditions such as high light (40–100 μmol photons m−2 s−1) and treatment with ampicillin and streptomycin. Based on these results, we summarized that conchopsore release was inhibited by the vegetative growth of bipartite cells and the cell hollowing caused by vacuole-based intracellular degradation in sporangial filaments. These results would increase understanding of sporangial filament development and conchospores release in N. yezoensis.