Mn

Manganese (Mn) is an elemental metal used in trace amounts by photosynthetic organisms bacteria, algae, and plants, and is essential in the oxygen-evolving complex used in photosynthesis.

Manganese is considered a plant micronutrient, and is often included in aquarium plant fertilizers, enriched substrates, and can be found in fish food such as Spirulina.

Links

 * Wikipedia