Cells are solitary or in 2–16 cell colonies within a thin, hyaline mucilaginous envelope or within an expanded mother cell wall (Figure 1). Cells are round, oval, or slightly reniform, ranging in size from 11.3 to 16.6 μm long and 6.3 to 10.3 μm wide. Cell walls are thick, sometimes stratified (Figure 1b,c), and smooth, without thickening polars. Mother cell walls are extended according to the number and arrangement of daughter cells and are usually an irregularly round shape, with or without tapered thickening ends (Figure 1b,d). The number and shape of chloroplasts are difficult to discern on the microscopic level. When observed with TEM, a horseshoe-shaped chloroplast was observed, which occupied most of the cell volume (Figure 2a,d). Asexual reproduction was by 2–4 autospores released by a rupture of the mother cell wall. These autospores could be propagated again independently inside the mother cell (Figure 1e,g). The asynchronous cell division led to the size difference between the daughter cells (Figure 1i) and the formation of multilayer mother cell walls (Figure 2e,f).

When observed with TEM, 1–4 pyrenoids were observed in the mature cells of Euchlorocystis marina (Figure 2a,d). The pyrenoid was surrounded by a starch sheath composed of 2–4 starch plates, and the pyrenoid matrix was homogenous (Figure 2a,d). However, in some young cells, pyrenoids were not observed (Figure 2b,c).

Sequencing of the 18S rDNA of the new strain produced a 1720 bp sequence, introns were excluded. The final 18S rDNA sequences alignment of 29 taxa with 1633 base positions were used for the phylogenetic tree construction, and Prasiola mexicana (Prasiolaceae) was chosen for the outgroup. The ML and NJ analyses yielded a similar topology and only the ML tree was presented in Figure 3. The 18S rDNA phylogenetic trees consistently recovered that the newly isolated strain and Euchlorocystis subsalina formed an independent clade with high support (99/100 for ML/NJ) (Figure 3). Direct comparison showed that the 18S rDNA of the new strain had 41 differences (97.5% identity) to Euchlorocystis subsalina (Table 1).