chloroplast.

When examined with transmission electron microscopy, the pyrenoid matrix appears as a roughly circular electron dense granular structure within the chloroplast. Early studies suggested that RuBisCO is arranged in crystalline arrays in the pyrenoids of the diatom Achnanthes brevipes and the dinoflagellate Prorocentrum micans. However, recent work has shown that RuBisCO in the pyrenoid matrix of the green alga Chlamydomonas is not in a crystalline lattice and instead the matrix behaves as a phase-separated, liquid-like organelle.