Which of the following is a parasitic green Algae ?

Question

Accepted Answer

The correct option is 4: Cephaleuros

Cephaleuros is the classic and most important example of a parasitic alga in the class Chlorophyceae, which encompasses the green algae. This genus is known globally for causing the highly destructive "Red Rust" disease, particularly on important perennial cash crops like tea (*Camellia sinensis*), coffee (*Coffea arabica*), citrus, and mango, making it a significant phytopathogen in tropical and subtropical regions. Unlike typical free-living aquatic algae, *Cephaleuros* species penetrate the host plant's tissues, usually the leaves and young stems, using specialized, root-like structures called haustoria to draw nutrients. Although it possesses chlorophyll and can photosynthesize, its pathogenic action severely damages the host tissue, leading to chlorotic spots, necrosis, and ultimately reduced yield and vigour of the crop. Its filamentous, terrestrial, and parasitic habit clearly distinguishes it from most aquatic green algae, confirming its status as a true parasitic green alga.

The incorrect option is 1: Porphyra

*Porphyra* is a genus of red algae, belonging to the Phylum Rhodophyta, not the green algae (Chlorophyta). It is a multicellular, marine macroalga commonly known as "nori" and is one of the most commercially important seaweeds globally, primarily used for food. The alga is typically free-living and performs photosynthesis as an autotroph, obtaining its energy through standard means without being parasitic on other organisms. Therefore, because it is a red alga and non-parasitic, it cannot be the correct answer for a parasitic *green* alga.

The incorrect option is 2: Harveyella

*Harveyella* is a genus of red algae, belonging to the Phylum Rhodophyta, and it is indeed a well-known example of a parasitic alga. However, it is an **alloparasite**, meaning it specifically parasitizes other species of red algae, such as *Rhodomela* and *Odonthalia*, rather than being a parasite of vascular plants. The parasite lacks photosynthetic pigments and depends entirely on its algal host for nutrition. Crucially, the question asks for a parasitic *green* alga, and since *Harveyella* is a red alga, its classification excludes it from being the correct option in this context.

The incorrect option is 3: Trebouxia

*Trebouxia* is a genus of green algae, but its ecological role is overwhelmingly defined by its symbiotic relationship as the **phycobiont** (algal partner) in the vast majority of lichens. In this classic mutualistic relationship, the algal cells are housed and protected by a fungus (the mycobiont). The *Trebouxia* performs photosynthesis and provides the fungal partner with essential carbohydrates, while the fungus offers protection from desiccation, UV light, and provides water and minerals. This association is a form of mutualism, not parasitism, where the algal partner is protected and enhanced, not harmed or destroyed by the host.