P. roundii is a pelagic, marine diatom.
From Hernández-Becerril & Díaz-Almeyda 2006:
The cells are arranged in stepped chains (2-6 cells per chain were found), linked by the apices of the valves, overlapping approximately one seventh to one eighth (12-14%) of the valve length; there are two chloroplasts per cell. The valves are lightly sillicified, characteristically lanceolate to sigmoid and clearly asymmetrical. One margin of the valve is convex, the other almost straight, whereas the apices are somewhat pointed, slightly rostrate in the convex margin. The marginal raphe carries a central nodule at the central interspace and the fibulae, discernible under LM, are spaced regularly. The interstriae are invisible with LM and appear irregular and very lightly silicified in EM. The striae consist of a single row of large, round to square poroids and the hymen of poroids is divided into four or more usually two sectors. The copulae of the cingulum have striae and 38-42 interstriae in 10μm, the valvocopula is wider than the adjacent copula.
28-32 interstriae in 10 μm, 18-20 fibulae in 10 μm, 3-4 poroids in 1 μm
From Hernández-Becerril & Díaz-Almeyda 2006:
The species was found in few stations of a single cruise from the Gulf of Tehuantepec in 1989, where it was common but not relatively abundant (less than 10%).
P. roundii is a chain-forming pennate diatom found in the Gulf of Tehuantepec, Mexico.
Marine, coastal waters
From Hernández-Becerril & Díaz-Almeyda 2006:
This species is almost identical to Pseudo-nitzschia subpacifica (Hasle) Hasle from which it differes in the number and structure of the poroids between interstriae, the structure of the interstriae, and the copulae of the cingulum.
51-67 μm in length, 4.6-6.5 μm in width
photosynthetic
