An ultrastructural study of pyrenoids from Chlorella pyrenoidosa.

The fine structure of pyrenoids from Chlorella pyrenoidosa shows a crystalline matrix with an 8-o-nm periodicity. In certain areas of the pyrenoid 2 sets of parallel lines intersect at approximately 8o° to give a criss-crossed appearance to the matrix. The recognition of the crystalline structure depends on the portion of the pyrenoid that is sectioned, as well as the plane of sectioning. A distinct crystal-like body which is closely associated with the thylakoid lamellae that traverse the pyrenoid matrix is observed in one cell. Although 2 pairs of thylakoid lamellae run separately through the pyrenoid matrix of a cell, they appear to be continuous with each other outside the pyrenoid. The fine structure of the pyrenoid, at any given time, seems to be a reflection of its metabolic state. A lack of understanding of the exact function or functions of pyrenoids makes a more rational interpretation of pyrenoid ultrastructure difficult.