A golden pennate diatom and a green euglenid, Phacus sp., industriously photosynthesising before the slide dries out. Perhaps unexpectedly for its appearance, the diatom can actually move, by secreting mucus to glide on through the raphe (a slit), and often quickly enough to screw up your photography. Luckily, this one was paused at that moment. The colour comes from its plastid, of the “brown” algal category.
The euglenid’s surface is covered by proteinaceous strips making up its pellicle — in many species, they slide against each other and enable metaboly, a pattern of movement that makes the euglenid feel ‘squishy’. This genus, on the other hand, lacks the sliding ability of its pellicle strips, and sits rigidly in one shape. Inside are a big clear doughnut-shaped starch globule, a red eyespot used for seeking light, and numerous green plastids that were once inherited through secondary endosymbiosis of a green alga. Only a single clade of euglenids is photosynthetic — the rest have never seen a plastid as anything other than a food item.
(a wallpaper version can be found here, hopefully big enough to crop or shrink as necessary)
Secrets of the Universe: Past, Present, FutureX