. 2017 Mar 6;8:14511. doi: 10.1038/ncomms14511

Table 1. Assumptions and conclusions of significant recent works modelling carbon fluxes in single-celled phytoplankton.

Study	Model set-up and assumptions	Conclusions
Holtz et al.⁴⁷,⁴⁸ (E. huxleyi)	4 compartments (PY=pyrenoid, CP=chloroplast, CV=coccolith vesicle, CY=cytosol)	is used at low [CO₂]
	Carbonate chemistry consists of , , CO₂ and H⁺.	Intracellular pH gradients allow concentration of CO₂ around RuBisCO without up-gradient movement of carbon.
	Hypothesized Ca²⁺/ CY-to-CV transporter coupled to an H⁺-ATPase, with no leakage of from CV	pHs: PY=5.0, CY=7.0, CP=8.0, CV=8.3–8.6
	Hypothesized upregulation of down-gradient flux into cell with decreased [CO₂] in PY.	A net efflux of CO₂ is not necessary to remove δ¹³C from cell
	Passive CO₂ and fluxes. No flux from CP to PY.
	CA assumed in CP and PY but not in CY and CV
	Isotope model consists of 2 compartments (CY and CP) and does not consider isotopes of calcite.
	Membrane permeabilities to CO₂ and assumed, but highly heterogeneous; different for all compartments.
Bolton & Stoll,² (Coccolithophores)	3 compartments (CP, CV and CY)	active transport to CP increases at low [CO₂], at the expense of transport to CV
	Carbonate chemistry consists of and CO₂.	This effect is greatest in large cells.
	fluxes are all active and independent of []	Difference in vital effects (δ¹³C calcite—δ¹³C_medium) between small and large cells greatest at low [CO₂]
	Passive CO₂ fluxes.
	Membrane permeabilities and CA activities assumed from Hopkinson et al.⁴⁶.
	assumed (−27‰).
Hopkinson et al.⁴⁶ (Diatoms)	1, 2 & 3 compartments (PY, CP, CY)	Membranes are highly permeable to CO₂ (1.5 × 10⁻⁴–5.6 × 10⁻⁴ m s⁻¹)
	Carbonate chemistry consists of and CO₂.	Membranes are highly impermeable to (2.5 × 10⁻⁸–2.9 × 10⁻⁷ m s⁻¹)
	Used ¹⁸O labelled DIC to track temporal evolution of carbonate system.	δ¹³C org is a function of passive diffusion of CO₂, active movement of , kinetic fractionation factors associated with CA-catalysed hydration and dehydration, and the kinetic isotopic fractionation associated with RuBisCO.
	Passive CO₂ fluxes.
	Passive and active fluxes.
	assumed (−29‰).
Schulz et al.,²⁶ (E. huxleyi)	2 compartments (CP and CY)	Carbon concentrating mechanism relies upon active (ATP driven) uptake of CO₂ and
	Carbonate chemistry consists of and CO₂.	Reduction in with increased uptake into CP
	Active uptake of and CO₂ independent of concentration	is larger when there is a greater degree of intracellular carbon leakage from the chloroplast.
	Passive CO₂ fluxes (membrane permeability to CO₂=1.8 × 10⁻⁵ from⁷⁶- green algae)
	No efflux of HCO₃ ⁻
	assumed (−29‰).
Cassar et al.,²⁵ (Diatom—Phaeodactylum tricornutum)	2 compartments	is not a unique function of
	Active and diffusive uptake of CO₂	depends on leakiness of CP
	No uptake
	assumed (−27‰).
	Inferred fluxes based on an energy minimization approach.
Keller & Morel,²⁴ (General phytoplankton)	1 compartment	Downward curvature of against is consistent with active uptake contrary to³²
	No diffusion or efflux	is a poor indicator of carbon substrate
	Active uptake scales with growth rate
	inferred from model.

For earlier work and the evolving appreciation of the importance of cell size, shape and growth rate see Laws et al.¹⁵ and references therein.