Photoautotrophic organisms globally fix 111-117x1015 grams of carbon per year and around half of this global net primary production is aquatic (Behrenfeld et al., 2001; Field et al., 1998), with green algae a major contributor to this global carbon fixation. However, aquatic environments have some limitations The concentration of CO2 is often 2,200 times lower in water than in air, and diffusion is also 8,000 times slower. In addition, Rubisco, which catalyses the first major step of carbon fixation, converting atmospheric CO2 into precursors of energy-rich molecules, exhibits slow catalytic rates, low affinity for CO2 and competition from O2 for the active sites. Therefore, most green algae have developed a Carbon Concentrating Mechanism (CCM). In eukaryotic micro-algae, the Rubisco micro-compartment is called the pyrenoid and together with active inorganic carbon transporters and strategically located carbonic anhydrases, elevated CO2 within the pyrenoid improves photosynthetic efficiency. Most photosynthetic organisms have an hexadecameric Rubisco holoenzyme (L8S8), composed of eight ~55-kDa large subunit (LSU), encoded by a chloroplast gene (rbcL) and eight ~15-kDa small subunit (SSU), encoded by a nuclear gene family (RbcS) in Form I Rubisco. The CCM has been particularly well-defined in the model unicellular chlorophyte Chlamydomonas reinhardtii and recent studies showed that for full CCM induction, a key protein linker EPYC1 and its interaction with Rubisco SSU were necessary. The overall goal of this study was to use a phylogenetic approach, firstly to investigate SSU structure across the green algal phylogeny, and also to explore CCM diversity in two specific groups of species. This study used a variety of methodologies combining physiological experiments, biochemistry, imaging and bioinformatic analyses. The results firstly showed the presence of two different Rubisco SSU structures within the green algae. Secondly, the Rubisco catalytic properties found in streptophyte algae closely related to land plants (streptophytes) reflect the strength of any CCM and pyrenoid leakiness, whereas Rubisco in extant land plants reflects more recent selective pressures associated with the terrestrial atmospheric environment. This research also provides evidence for diversity of CCM expression in two closely related genera (Chlamydomonas and Chloromonas), ranging from species expressing a CCM and pyrenoid, or a CCM without a pyrenoid, to neither pyrenoid or CCM. This study provides the first preliminary analyses of five different genomes confirming multiple independent origins of the pyrenoid in green algae but has also allowed an initial comparison of the molecular components essential for pyrenoid formation across these species.