Which one of the following is a motile unicellular organism with a single cilium and can exist as an autotroph as well as a saprotroph?

Euglena is a motile unicellular protist that exhibits mixotrophy, allowing it to function as both an autotroph and a saprotroph [t1][t4]. In the presence of sunlight, it utilizes chloroplasts containing chlorophyll a and b for photosynthesis [t5]. When light is unavailable, it becomes heterotrophic (specifically saprotrophic or osmotrophic), absorbing organic nutrients from its environment [t4][t5]. While the question mentions a 'single cilium', in biological contexts, the terms flagella and cilia are structurally similar; Euglena typically possesses a primary emergent flagellum used for locomotion [t4][t6]. In contrast, Chlamydomonas is a unicellular green alga with two flagella [t8]. Nostoc and Oscillatoria are filamentous cyanobacteria; Nostoc is known for nitrogen fixation in heterocytes [t2], and Oscillatoria moves via gliding rather than flagellar or ciliary action [t9].

Sources

1. [1] https://pmc.ncbi.nlm.nih.gov/articles/PMC12423646/
2. [2] https://www.sciencedirect.com/topics/agricultural-and-biological-sciences/nostocales
3. [3] https://en.wikipedia.org/wiki/Cyanobacteria