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
For its unique extremophilic properties, the alga
2. Taxonomy and biodiversity
The order Cyanidiales from the class Cyanidiophyceae are a group of asexual, unicellular organisms that diverged from ancestral red algae around 1.3 billion years ago [4, 5]. These unicellular red algae were classified into three genera,
Different Cyanidiophycean species, including
Research based on environmental PCR studies revealed an unexpected level of genetic diversity among Cyanidiales. It was demonstrated that the Cyanidiales comprise a species-rich branch of red algae [9]. The high divergence rates in the Cyanidiales could be possibly explained by an elevated mutation rate in these taxa, resulting potentially from DNA damage in their extreme environments. The analyses also reject the putative mesophilic origin of Cyanidiales and suggest ancestral thermo-acidotolerancy of this lineage [9].
Sequencing of the
Based on molecular phylogenetics, three well supported
Consequently, the main lineages were identified:
Generally,
Phylogenetic analyses of the
3. Morphology and extremophile properties
Morphology of the unicellular taxa Cyanidiales is relatively simple. Thick-walled cells are of a spherical shape and usually contain one chloroplast, 1–3 mitochondria, a nucleus, a vacuole, and energy reserve products [8, 12, 15, 23, 24].
Representatives of the order Cyanidiales are unparalleled among phototrophic microorganisms (eukaryotes) in their ability to thrive in acidic (pH 0.5–3.5) and high temperature (38–56°C) geothermal environments. Soils, sediments, and endolithic habitats around hot springs, boiling mud pools, and steaming fumaroles are typical for these extremophiles, which are dominant in local microbial communities [5, 9, 10, 14, 17, 18, 19, 20, 25, 26, 27, 28, 29, 30]. They are the principal photosynthetic organisms found in hot acidic waters [31], where even photosynthetic prokaryotes, such as the cyanobacteria, are completely absent [32, 33, 34].
Morphological similarities between
Strain DB01 of
4. Genomes
Genetic information for the red algae
The mitochondrial genome is extremely small in size with a very low genetic content. It is characterized by the highest guanine-cytosine content among all red algae.
The plastid genome contains a large number of intergenic stem-loop structures but is otherwise rather typical in size, structure, and content in comparison with other red algae. It is assumed that the unique genomic characteristic resulted from both the harsh conditions in which
5. Biotechnological applications
5.1 Wastewaters
Recycling of valuable components and nutrients from wastewaters using algae has recently been studied extensively. But only limited types of wastewaters can be treated because wastewaters are generally acidic and most algal species grow with difficulty at low pH, and absorption rates that can be achieved by bioaccumulation decrease substantially [46, 47, 48, 49]. The acidophilic alga
5.2 Rare earth elements
Lanthanides [Rare Earth Elements (REEs)] have unique magnetic and catalytic properties and are, up to now, irreplaceable materials in numerous technologies, for example, wind turbines, solar panels, batteries, fluorescent lamps, computer and mobile monitors, TV screens etc. They are also used as fertilizers in agriculture, in aquaculture, or as animal growth enhancers.
Methods for extraction of lanthanides from ores, including pyro-metallurgy and hydro-metallurgy, have severe negative environmental impacts, as well as being expensive. Currently, industrial extraction of lanthanides from monazite involves either a basic process that uses concentrated sodium hydroxide or an acidic process that uses concentrated sulfuric acid. These processes generate large amounts of hazardous waste containing thorium and uranium [55]. Moreover, requirements for REEs are continually increasing, becoming critical due to risks of reduced availability of resources and their possible exhaustion.
One way to solve the problem would be efficient, sustainable, and cheap recycling of REE-containing wastewaters and others industrial wastes.
Considerable research efforts have been directed toward the development of efficient biological methods for recovering small amounts of these materials from wastewater systems [48, 49]. Research has recently focused on environmentally friendly technologies of metal recovery, including REEs, from secondary resources [56, 57] including bio-sorption by algae or cyanobacteria [52, 58]; for review, see [59, 60].
However, if REEs were present in an aquatic environment, together with other metals, most algae could not accumulate high concentrations of REEs [61, 62] due to metal-inhibited growth. The extraction of REEs or other metals have now been simplified by the use of
Similar to its relative
Recycling from different mineral ores and electronic wastes (luminophores) often meets difficulties in that REEs are not suitable for bio-sorption because they are present in solid forms and are almost insoluble in nutrient solutions for algal cultivation. The material can be readily dissolved in aqueous acid, but the efficiency of metal bio-sorption for most algae is usually decreased under acidic conditions or the algae cannot grow at a low pH. Application of extremophilic red alga
Another rich source of lanthanides is bauxite residue, called red mud, which is a by-product of the production of alumina (aluminum oxide) from bauxite. However, less than 2% of the residue produced annually is currently being reused [65], due to difficulties related to high pH, salinity, low solid content, size of fine particles, and the leaching of metals [66]. The ability to grow in the presence of red mud and accumulate REEs was successfully tested with
To conclude, the alga
5.3 Phycocyanin
Phycocyanin is an important compound that can be obtained from microalgal and cyanobacterial cultures. It is used as a fluorescent marker in diagnostic histochemistry [68, 69] and as a dye in foods and cosmetics [70] or as a therapeutic agent [71, 72]. Production of phycocyanin as a photosynthetic pigment in most microalgae grown heterotrophically is low and not suitable for biotechnological applications [73]. In contrast, phycocyanin as a major pigment of
For example,
The feasibility of utilizing crude glycerol (a major waste by-product of biofuel production from oilseed rape) as a carbon source for heterotrophic growth of green microalgae [54] was confirmed for
The high tolerance of
The ability of
Since a number of positive health effects have been associated with phycocyanin [79], and phycocyanin from
In addition to phycocyanin,
5.4 Biofuels
The world-wide and continuous increase in fossil fuel consumption, leading probably in the relatively near future to an exhaustion of resources, has led to increased research for alternative energy sources. Production of biofuels by algae might provide a viable alternative to fossil fuels; however, this technology must overcome a number of serious obstacles before it could compete in the fuel market and be broadly deployed. Application of remarkably extremophilic
Microalgae often become contaminated with other microorganisms in large-scale outdoor cultivations, which is a major problem that inhibits algal growth and decreases the quality of biofuel and high-value products. A lack of resistance to these factors could be catastrophic for future algae farmers. The red alga
5.5 Glycogen
The extremophilic red algae, similarly to other Rhodophyta, produce glycogen as energy and carbon reserves, instead of starch, which is characteristic of other microalgae and higher plants [81]. Glycogen, in contrast to starch, is readily soluble in cold water and more accessible by enzymes. In red algae, glycogen accumulates in a lower molecular weight form than glycogen from other microalgae and is a highly branched (higher than any other glycogen) glucose polymer [81]. Amylopectin, as a highly branched glucose polymer in starch, is used in various products such as peritoneal dialysis solutions and sports drinks. However, it is costly to prepare because of its insoluble, granular nature. The application of glycogen offers a cheaper alternative.
The alga
Apart from
Large amounts of glycogen were obtained in mixotrophic cultures, [53] where the maximum glycogen content per mL of culture was almost 10- and 2-fold greater than those of autotrophic and heterotrophic cultures, respectively. The accumulation of glycogen was enhanced by the addition of glucose, and the amount and composition of glycogen were determined by growth conditions. It is assumed that in addition to glycogen, other forms of carbon may be stored, although pathways are, as yet, unknown [75, 86].
5.6 Nutritional applications
Because of their high content of protein, algal biomass, in general, and green algae particularly, have been used in many foods, mostly in the form of dried biomass.
Difficulties in introducing microalgal-based ingredients into foods are technological and include sensorial obstacles such as its unattractive green-brownish color and unpleasant fishy smell increasing after longer storing [87]. Another problem is bacterial contamination, which decreases the commercial quality of algal biomass. Such disadvantages of green algae are not encountered using
6. Conclusions
The unicellular red alga
Acknowledgments
We thank prof. J. D. Brooker for critical reading and language corrections of the manuscript. We are grateful to Dr. Jens Hartmann, Donau University, Krems, Austria, for the electron-microscopy photograph.
This work was supported by the National Program of Sustainability I, ID: LO1416 and by the Program Interreg V-A Austria-Czech Republic, project ATCZ172 REEgain.
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