Algae are a diverse collection of simple, primarily non‑vascular organisms that carry out photosynthesis but do not belong to the mosses or vascular plants. They range from microscopic green films to vivid orange surface growths and occur on tree bark, rocks, soil, freshwater, and marine habitats. Because algae span several distinct evolutionary lineages, they are grouped here as “other photosynthetic organisms” rather than within the main plant groups.

Classification

• Phylum Chlorophyta — Green Algae Mostly green, chlorophyll‑rich algae found on tree bark, soil, freshwater, and sometimes marine environments; includes many common terrestrial “green film” species.
• Phylum Charophyta — Stoneworts and Relatives Freshwater green algae closely related to land plants, often forming branched, plant‑like structures in ponds and lakes.
• Phylum Rhodophyta — Red Algae Predominantly marine algae with red pigments, ranging from delicate seaweeds to crust‑forming species on rocks.
• Phylum Ochrophyta — Brown Algae and Diatoms A broad group including brown seaweeds, kelps, and diatoms; mostly marine but with some terrestrial and freshwater representatives.
• Phylum Cyanobacteria — Blue‑green Algae Photosynthetic bacteria that form films, mats, or colonies in water and on damp surfaces; included here because of their ecological similarity to algae.
• Phylum Myzozoa — Dinoflagellates Mostly microscopic planktonic algae, some of which cause bioluminescence or algal blooms in marine environments.
• Phylum Euglenozoa — Euglenids Mostly freshwater, single‑celled organisms capable of photosynthesis, often forming green surface films in nutrient‑rich water.

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Phylum Chlorophyta — Green Algae

Phylum Chlorophyta comprises the true green algae, a diverse group characterised by their bright green chlorophyll pigments. They occur in freshwater, marine, and terrestrial environments, and many species form thin green films on tree bark, soil, rocks, and artificial surfaces. Chlorophyta includes both microscopic unicellular algae and simple filamentous or colonial forms. The following Classes are included on iNaturalist:

• Class Chlorodendrophyceae — Small unicellular green algae, mostly marine.
• Class Chlorophyceae — Freshwater green algae including many filamentous and colonial forms.
• Class Chloropicophyceae — Tiny picoplanktonic green algae found in marine environments.
• Class Mamiellophyceae — Marine picoplanktonic algae common in coastal waters.
• Class Nephroselmidophyceae — Small marine flagellates.
• Class Palmophyllophyceae — Deep‑water or shade‑adapted green algae forming simple colonies.
• Class Pedinophyceae — Unicellular freshwater or soil algae.
• Class Picocystophyceae — Marine picoplanktonic algae.
• Class Prasinodermatophyceae — Primitive green algae with simple cell structure.
• Class Prasinophyceae — Mostly marine flagellated green algae.
• Class Pyramimonadophyceae — Marine green flagellates.
• Class Trebouxiophyceae — Terrestrial and freshwater green algae forming crusts or films on bark, rocks, and soil.
• Class Ulvophyceae — Marine and freshwater green algae including sea lettuce and filamentous forms.

My observations

Desmococcus viridis is a bright, vivid green crust‑forming alga that creates smooth patches on bark and shaded surfaces. I’ve seen it as striking green films on tree trunks in the UK.

Class Trebouxiophyceae > Order Prasiolales > Family Prasiolaceae

• Desmococcus viridis

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Phylum Ochrophyta — Brown/Orange Algae

Phylum Ochrophyta includes a wide range of brown, golden, and orange algae found in marine, freshwater, and terrestrial habitats. Many species contain carotenoid pigments that give them distinctive yellow, orange, or brown colours. Although best known for seaweeds and diatoms, this group also includes terrestrial filamentous algae such as Trentepohlia, which form vivid orange coatings on rocks, trees, and man‑made surfaces. The following Classes are included on iNaturalist:

• Class Bacillariophyceae — Diatoms with silica shells, found in water and damp habitats.
• Class Chrysophyceae — Golden algae, mostly freshwater plankton.
• Class Dictyochophyceae — Marine planktonic algae with siliceous skeletons.
• Class Eustigmatophyceae — Small freshwater or soil algae with yellow pigments.
• Class Phaeophyceae — Brown seaweeds including kelps and wracks.
• Class Raphidophyceae — Marine and freshwater algae sometimes forming blooms.
• Class Synurophyceae — Freshwater algae with silica scales.
• Class Ulvophyceae — Mostly marine green seaweeds, but includes the orange terrestrial algae Trentepohlia.

My Observations

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