Kalkalgen

Kalkalgen, or calcareous algae, are a group of algae that form hard, lime-depositing structures by precipitating calcium carbonate within their tissues or cell walls. The majority belong to the red algae (Rhodophyta), with coralline algae (family Corallinaceae) being especially well known for encrusting rocks, shells, and reef substrates. Some green algae, such as Halimeda, also calcify, producing segmented, calcareous thalli that can accumulate as sand or sediment.

Ecology and function: Kalkalgen contribute to the structure and stability of many marine habitats. Coralline algae cover reef surfaces and contribute to framework formation, while calcified Halimeda can trap sediments and alter local sediment dynamics. They provide microhabitats for small invertebrates and serve as food sources for certain grazers. Because of their calcium carbonate content, calcareous algae are often used as indicators of seawater chemistry, including pH and carbonate ion availability.

Habitat and distribution: Kalkalgen are found in marine environments worldwide, particularly in shallow, sunlit waters where light and nutrients support photosynthesis. They are common on rocky shores, coral reefs, tidal pools, and subtidal substrates. Coralline algae tend to favor stable, moderately saline conditions and can tolerate a range of temperatures, from tropical to temperate zones.

In cultivation and aquaculture: In reef aquaria and other managed systems, kalkalgen contribute to alkalinity and pH buffering by consuming carbon dioxide during photosynthesis and depositing calcium carbonate. Maintaining adequate calcium and alkalinity, along with appropriate lighting and water quality, is important for their growth. They can be outcompeted or their calcification impeded by poor water chemistry, high nutrients, or low light.

Threats and conservation: Ocean acidification, increasing sea temperatures, and nutrient pollution threaten calcification rates and the overall health of kalkalgen populations. They are not typically targeted for conservation as a taxon, but changes in their abundance can reflect reef health and broader ecosystem status.