Green Spot Algae

Clinical Identification

GSA manifests as intensely green, perfectly circular spots ranging from 1 to 5 millimeters in diameter.

• Substrate Preference: Ubiquitously found on the upper strata of aquarium glass and the adaxial surfaces of slow-growing leaves directly exposed to intense photosynthetically active radiation (PAR).
• Coalescence: Under severe nutrient imbalances, individual thalli will amalgamate, forming extensive, impenetrable green crusts that completely obscure the underlying substrate.
• Plant Pathophysiology: Heavy colonization on plant leaves drastically reduces the host's photosynthetic efficiency by blocking light transmittance and impeding gas exchange across the stomatal and epidermal layers.

Primary Etiology

1. Phosphate (PO4) Deficiency: The defining trigger for GSA sporulation is a critical deficit in aqueous phosphate, typically falling below 0.5 ppm. In oligotrophic conditions, higher plants undergo metabolic stasis, whereas GSA utilizes highly efficient phosphatase enzymes to scavenge the minimal available phosphorus, granting it a supreme competitive advantage.
2. Excessive Photoperiod and Intensity: High photon flux density (intense lighting) combined with prolonged photoperiods provides the massive energetic input required for GSA's calcification and rapid lateral expansion.
3. Ionic Imbalance: A high calcium-to-magnesium ratio combined with low orthophosphates creates an optimal geochemical environment for Coleochaete biomineralization.

Remediation Protocols

• Phosphate Titration: The immediate pharmacological intervention requires aggressive dosing of an inorganic phosphate source. Products such as Flourish Phosphorus or ProScape P Macroelements should be utilized to elevate PO4 levels to a sustained 2.0 - 3.0 ppm. This systemic shift actively suppresses GSA metabolic pathways and reactivates host plant growth.
• Mechanical Excision: Due to calcification, chemical treatments like glutaraldehyde are largely ineffective. Eradication necessitates physical removal using sharp metallic or rigid plastic scrapers on glass surfaces.
• Biological Grazing: Only specialized algivores possessing radula-like dentition, specifically Clithon sp. (Horned Nerite Snails) and Neritina sp. (Zebra Nerite Snails), are capable of rasping the calcified GSA crusts off surfaces.

Long-Term Prophylaxis

1. Macronutrient Homeostasis: Implement a rigorous and balanced fertilization regime. The continuous and non-limiting provision of N-P-K, such as through the APT Estimative Index (APT e), prevents the biological voids that GSA exploits.
2. Photoperiod Modulation: Strictly regulate lighting intensity and limit the photoperiod to 6-8 hours daily, effectively capping the energy available for algal calcification.
3. Strategic Planting: Position slow-growing, broad-leaf epiphytes in shaded or low-PAR regions of the aquascape to reduce their vulnerability to phototropic colonization by GSA.