Editing Pyrenoid (section)

=== Gross morphology and ultrastructure ===
There is substantial diversity in pyrenoid morphology and ultrastructure between algal species. The common feature of all pyrenoids is a spheroidal matrix, composed primarily of RuBisCO.<ref name = bazinga /> In most pyrenoid-containing organisms, the pyrenoid matrix is traversed by thylakoid membranes, which are in continuity with stromal thylakoids. In the unicellular red alga ''Porphyridium purpureum'', individual thylakoid membranes appear to traverse the pyrenoid;<ref>Brody, M., & Vatter, A. E. (1959). Observations on cellular structures of ''Porphyridium cruentum''. The Journal of Biophysical and Biochemical Cytology,5(2), 289-294. {{PMID|13654450}}</ref> in the green alga ''[[Chlamydomonas reinhardtii]]'', multiple thylakoids merge at the periphery of the pyrenoid to form larger tubules that traverse the matrix.<ref>Sager, R., & Palade, G. E. (1957). Structure and development of the chloroplast in ''Chlamydomonas'' I. The normal green cell. The Journal of Biophysical and Biochemical Cytology, 3(3), 463-488.{{PMID|13438931}}</ref><ref>{{Cite journal|last1=Engel|first1=Benjamin D|last2=Schaffer|first2=Miroslava|last3=Kuhn Cuellar|first3=Luis|last4=Villa|first4=Elizabeth|author-link4=Elizabeth Villa|last5=Plitzko|first5=Jürgen M|last6=Baumeister|first6=Wolfgang|date=2015-01-13|title=Native architecture of the Chlamydomonas chloroplast revealed by in situ cryo-electron tomography|journal=eLife|language=en|volume=4|pages=e04889|doi=10.7554/eLife.04889|issn=2050-084X|pmc=4292175|pmid=25584625 |doi-access=free }}</ref> Unlike carboxysomes, pyrenoids are not delineated by a protein shell (or membrane). A starch sheath is often formed or deposited at the periphery of pyrenoids, even when that starch is synthesised in the cytosol rather than in the chloroplast.<ref>Wilson, S., West, J., Pickett‐Heaps, J., Yokoyama, A., & Hara, Y. (2002). Chloroplast rotation and morphological plasticity of the unicellular alga Rhodosorus (Rhodophyta, Stylonematales). Phycological research, 50(3), 183-191.</ref>

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''<ref>{{Cite journal|last=Holdsworth|first=Robert H.|title=The Presence of a Crystalline Matrix in Pyrenoids of the Diatom, Achnanthes Brevipes|date=1968-06-01|url=https://rupress.org/jcb/article/37/3/831/12498/THE-PRESENCE-OF-A-CRYSTALLINE-MATRIX-IN-PYRENOIDS|journal=Journal of Cell Biology|language=en|volume=37|issue=3|pages=831–837|doi=10.1083/jcb.37.3.831|issn=0021-9525|pmc=2107439|pmid=11905213}}</ref> and the dinoflagellate ''Prorocentrum micans''.<ref>{{Cite journal|last=Kowallik|first=K.|date=1969-07-01|title=The Crystal Lattice of the Pyrenoid Matrix of Prorocentrum Micans|url=https://jcs.biologists.org/content/5/1/251|journal=Journal of Cell Science|language=en|volume=5|issue=1|pages=251–269|doi=10.1242/jcs.5.1.251|issn=0021-9533|pmid=5353655|url-access=subscription}}</ref> 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.<ref name=":0">{{Cite journal|last1=Freeman Rosenzweig|first1=Elizabeth S.|last2=Xu|first2=Bin|last3=Kuhn Cuellar|first3=Luis|last4=Martinez-Sanchez|first4=Antonio|last5=Schaffer|first5=Miroslava|last6=Strauss|first6=Mike|last7=Cartwright|first7=Heather N.|last8=Ronceray|first8=Pierre|last9=Plitzko|first9=Jürgen M.|last10=Förster|first10=Friedrich|last11=Wingreen|first11=Ned S.|last12=Engel|first12=Benjamin D.|last13=Mackinder|first13=Luke C. M.|last14=Jonikas|first14=Martin C.|date=September 2017|title=The Eukaryotic CO2-Concentrating Organelle Is Liquid-like and Exhibits Dynamic Reorganization|url=https://www.cell.com/cell/fulltext/S0092-8674(17)30933-9|journal=Cell|volume=171|issue=1|pages=148–162.e19|doi=10.1016/j.cell.2017.08.008|issn=0092-8674|pmc=5671343|pmid=28938114}}</ref>

In ''Porphyridium'' and in ''Chlamydomonas'', there is a single highly conspicuous pyrenoid in a single chloroplast, visible using light microscopy. By contrast, in diatoms and dinoflagellates, there can be multiple pyrenoids. The ''Chlamydomonas'' pyrenoid has been observed to divide by fission during chloroplast division.<ref>{{Cite journal|last=Goodenough|first=Ursula W.|date=1970|title=Chloroplast Division and Pyrenoid Formation in Chlamydomonas Reinhardi1|url=https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1529-8817.1970.tb02348.x|journal=Journal of Phycology|language=en|volume=6|issue=1|pages=1–6|doi=10.1111/j.1529-8817.1970.tb02348.x|bibcode=1970JPcgy...6....1G |s2cid=84245338|issn=1529-8817|url-access=subscription}}</ref><ref name=":0" /> In rare cases where fission did not occur, a pyrenoid appeared to form de novo.<ref name=":0" /> Pyrenoids partially dissolved into the chloroplast stroma during every cell division, and this pool of dissolved components may condense into a new pyrenoid in cases where one is not inherited by fission.