A visual key for the categories used in the submission form — read this alongside a paper's figures to standardise how different people call the same features. Organised in the same order as the form.
Draft guide, not a finished standard. These are original schematic diagrams — not reproductions of any published figure — meant to illustrate the general idea of each category, not a real specimen. Descriptions here are a starting point and may not match how every paper in the field uses these terms; check with a curator before treating a borderline call as settled.
01
General class
The pyrenoid's position relative to the rest of the chloroplast — usually the first thing visible at low magnification.
Central
Sits in the middle of the chloroplast body, roughly equidistant from the margins.
Stalked
Connected to the main chloroplast body by a narrow neck, projecting outward from it.
Peripheral
Positioned at or right against the chloroplast margin, rather than embedded centrally.
Other
Doesn't fit the three patterns above — use the free-text field to describe what's actually seen.
Example in the literature: Gibbs, S.P. (1962) illustrates this kind of positional variation across multiple divisions in one paper.
02
Traversing membranes
Whether thylakoid-derived membranes penetrate through the pyrenoid matrix. First call present/absent/unclear on the form; if present, these four shapes describe how.
Sheet
Membranes run through as continuous, unbroken sheets spanning the matrix.
Fenestrated
Sheet-like, but perforated with gaps — "windows" through the membrane.
Curved
Membranes follow a curved rather than straight path through the matrix.
Invaginated
Membrane folds inward from the boundary without necessarily spanning all the way through.
Continuous with external thylakoids? A separate yes/no/unclear call on the form — whether the traversing membrane connects directly to thylakoids outside the pyrenoid, or is disconnected/free-ending within it.
Example in the literature: Griffiths, D.J. (1970) reviews this range of membrane configurations across genera.
03
Diffusion barrier
Whatever structure surrounds the matrix and (presumably) limits CO₂/O₂ diffusion — not every pyrenoid has one, and where present it takes several different forms.
Carbohydrate
Typically a starch sheath — one or more concentric plate-like layers around the matrix.
Membrane
A distinct membrane boundary directly enclosing the matrix, rather than a starch layer.
Shell
A thick, rigid-looking enclosing layer — visibly more substantial than a simple membrane.
Other
Something else entirely, or ambiguous from the figure — describe it in the notes field.
If carbohydrate: the form also asks location (chloroplastic vs extra-chloroplastic/cytosolic) and number of layers (1 / 2 / 3 / 4+) — visible as concentric rings in the diagram above. If membrane: the form asks origin (thylakoid-derived / envelope-derived / other) and morphology (appressed flat against the matrix / curved around it / other).
Example in the literature: Holdsworth, R.H. (1968) describes a crystalline matrix boundary in a diatom pyrenoid — a case worth comparing against the "shell" category above.
04
Plastoglobuli
Small, roughly spherical lipid droplets, sometimes visible in or around the pyrenoid — a simple yes/no call, with an optional free-text location if present.
Present
Small dark globules — often peripheral to the matrix, but note the actual location seen.