**Abstract**

The genus *Galdieria* refers to red algae and includes microscopic inhabitants of highly acidic (pH 1–2), often volcanic habitats. They are thermophilic or thermo-tolerant organisms, some of them surviving temperatures up to 56°C. As other extremophilic microorganisms, they exhibit unique features derived from their modified metabolisms. In this chapter, we will review the special abilities of *Galdieria* species such as metabolic flexibility to grow photoautotrophically, heterotrophically or mixotrophically, ability to utilize a whole range of unusual carbon sources, capability of surviving extreme environments or their extremely high resistance to metals. We will discuss the potential of *Galdieria* for applications in biotechnology, for example, phycocyanin production, nutrient removal from urban wastewaters, bio-mining, treatment of acidic mine drainage, selective metal precipitation, bioremediation of acidic metal-contaminated areas or recovery of critical and scarce metals from secondary sources.

**Keywords:** *Galdieria*, red algae, extremophiles, metals, biotechnology, recovery

## **1. Introduction**

The alga *Galdieria* belongs to the class *Cyanidiophyceae*, which are unicellular red algae classified into three genera *Cyanidium*, *Cyanidioschyzon*, and *Galdieria*. They have a relatively simple morphology consisting of spherical thick-walled cells containing one chloroplast, 1–3 mitochondria, a nucleus, a vacuole, and storage products. They inhabit diverse environments including hot sulfur springs, streams, mud, rock walls, endolithic habitats, etc. *Galdieria* is an extremophilic organism surviving in acidic geothermal environments with very low pH and high temperatures. It exhibits a unique metabolism enabling auto-, mixo-, and heterotrophic growth. Versatility in the utilization of carbon sources for heterotrophic growth is enormous including a large range of sugars and alcohols. Besides low pH and high temperature, the *Galdieria* have several unique mechanisms for metal tolerance, enabling them to grow in environments rich in toxic metals [1, 2], where other organisms cannot grow at all or with difficulties.

For its unique extremophilic properties, the alga *Galdieria* is an unrivaled organism for biotechnological applications in surroundings with high temperatures, low pH, and high concentrations of heavy metals and organic compounds, particularly sugars. It can be cultivated under conditions where other microalgae often become contaminated with microorganisms, which is a major problem for their

application in biotechnology [3]. The autotrophic cultivation of *Galdieria* follows predominant research trends in microalgal pigments, ß carotene, astaxanthin, and phycocyanin used in feed, in foods, in health applications or biofuels production. The composition of storage glycogen and lipids for biofuels can be largely changed depending on the growth conditions. *Galdieria* biomass has potential for use as food ingredients, both for protein-rich or insoluble dietary fiber-rich diets and for its low concentration of lipids.
