What is algae

Definition of the Term Algae

Algae represent a highly diverse consortium of ancient plants comprising different evolutionary lineages of mostly photoautotrophic organisms. The different groups of algae are of polyphyletic origin and epitomize the majority of existing divisions of plants. Algae are thallophytic; their vegetative body is not organized in roots and leafy stems like that of the kormophytes. Many algae are living in solitary cells, colonies, filaments, or primitive vegetation bodies and do not have a vascular system. In contrast to the phanerogams (plants producing seeds), the algae are cryptogams that propagate by concealed or hidden reproductive strategies. Following a conception of subdivision of living organisms into five kingdoms (Monera, Protoctista, Fungi, Animalia, and Plantae), the prokaryotic algae (blue-green algae, Cyanobacteria, Cyanoprokaryota) are placed in the Monera (Eubacteria) and the eukaryotic algae in the Protoctista. Hence, the algae do not belong to the kingdom of Plantae. Nevertheless, it is widely accepted (because of the photosynthesis as the mutual characteristic) to interpret algae as lower plants in distinction to the vascular higher plants.

The eukaryotic algae possess membrane-bound organelles such as nuclei, mitochondria, and plastids. The prokaryotic Cyanobacteria do not exhibit such organelles; their DNA and photosynthetic thylakoids lie free in the cytoplasm. The combination of both eukaryotic algal groups and prokaryotic Cyanobacteria to the algae demonstrates the heterogeneous and artificial character of this biological term. On the other side, the inclusion of Cyanobacteria in the group of algae is justified by several ecophysiological and evolutionary biological arguments. Like the eukaryotic algae, Cyanobacteria are able to photosynthetic oxygen production, and the theory of endosymbiosis has given evidence for evolution of chloroplasts by incorporation of Cyanobacteria into eukaryotic host cells. Endosymbiosis is discovered as a major force in the evolution of diverse algal lineages. Algae exhibit a fascinating diversity of life-forms and strategies in response to the environment. Most algae possess chlorophyll a. As primary producers, they use the sunlight energy to convert inorganic substances into simple organic compounds, and, provide the principal basis of food webs on the Earth. Furthermore, they produce oxygen that is essential for heterotrophic organisms. Algae populate a wide range of habitats from soil to water, whether it is cold or warm, alkaline or acidic, hyper- or hyposaline and we could cite a range of extremes. In this chapter, we focus on the algae of all types of inland waters.

Abstract

Eukaryotic algae and cyanobacteria as microscopic photosynthetic organisms represent a major group of primary producers in nature. Their role as a foundation of the ecosystems productivity is even more important on tepui summits than in other biotopes, considering that a large part of tepui tops and walls consists of exposed rock surfaces, which are covered with algal or cyanobacterial biofilms rather than macroscopic vegetation.

Cyanobacteria compose the majority of biomass in almost all types of habitats here, followed by desmids and diatoms. Rhodophytes and green filamentous algae play an important role in streams; coccal green algae, Euglenophyta, Synurophyceae, Cryptophyta, and Dinophyta do occur in the various habitats, but they are rare. As almost any group of organisms in Pantepui, algae and cyanobacteria seem to exhibit a significant level of endemism: 10 new species of diatoms, four cyanobacteria, one green filamentous alga, and two desmids have been described so far, but more are likely to be added as further studies explore the microscopic biodiversity on tepuis.

I. INTRODUCTION

Algae play many important and beneficial roles in freshwater environments. They produce oxygen and consume carbon dioxide, act as the base for the aquatic food chain, remove nutrients and pollutants from water, and stabilize sediments. Excessive algal growths, however, can cause detrimental effects on aquatic systems, endangering the organisms that live in or depend on these systems and hampering or preventing human uses of the infested waterways.

When we refer to the kinds of problems that algae cause, it is helpful to divide algae into three groups according to their growth habits: microscopic algae (primarily phytoplanktonic), filamentous mat-forming algae, and the Chara/Nitella group. Each group poses its own unique problems to aquatic systems. This chapter describes the problems caused by each of these three groups and then covers the control methods that typically are used for these algae.

Algal Uptake

Algae while growing in the ponds take up nutrients from waste water for their cell synthesis. The major nutrient extracted by algae for synthesis of cell mass is ammoniacal nitrogen. The growth of algae and cell division mainly depend on the photosynthetic activity and also the availability of the nutrients. Therefore, this carbon requirement of algae must be supplied by carbon dioxide. The source of carbon dioxide may be obtained by diffusion of dilute carbon dioxide gas into the culture media or using some biogradable organic matter e.g. sewage effluent. For optimum growth of algae, the culture pond should be exposed to sunlight so that light penetrates without obstruction into the algae culture pond for optimum photosynthesis. The process is similar to the oxidation pond system with the exception that the growth rate of algae is higher with consequent high removal of ammoniacal nitrogen from the culture media, i.e., effluent.

In actual process, oxidation pond-like shallow ponds are used for the treatment of the effluent. In most cases, this treatment is adopted as a secondary or tertiary treatment. Usually a collection and equalization pond is used at the front end of the shallow algae culture pond. The water flows to the pond having a suitable detention time for removal of ammoniacal nitrogen by algae. Under suitable conditions of depth of the pond, concentration of algae and ammoniacal nitrogen in the pond, good sunlight, adequate carbon dioxide supply etc. the uptake of ammoniacal nitrogen is quite appreciable. The algae thus produced is harvested and used as cattle feed or organic manure. Oxidized nitrogen and urea nitrogen bearing effluents may also be treated adopting this process. It is also possible to develop a fish pond at the rear end of the algae culture pond where the algal mass can be used as fish food. Figure810 illustrates the removal system of ammoniacal/urea/nitrate nitrogen by algae culture.

Figure810. Removal system of ammoniacal/urea/nitrate nitrogen by algae culture.

4.1 Harvesting/Dewatering Technologies

Algae after cultivation in open raceway ponds or closed PBRs exist as dilute solution of algae (0.1-10 g/L). Recovering algae biomass from such dilute solutions poses many challenges, especially for open pond cultivation systems. Therefore, development of efficient processes to recover algae is critical for economic viability of algal biodiesel.

Most of the proposed strategies to recover algae from the growth media such as centrifuges, screens, and bio/chemical flocculation are expensive or unreliable in a continuous large-scale operation. Although some of these processes are used in commercial production of Spiriluna, the economics of operations are different economics as it is sold as human food. Therefore, to produce biodiesel from algae, it is critical to use simple, reliable, and low-cost algae recovery processes. Suitability of a particular harvesting technology is critically dependent on the strain of the algae. Therefore, pilot-scale tests must be conducted before any decision regarding the optimum harvesting technology can be made.

2.3.3.1 Cell Structure

Algae can be unicellular, colonial (occurring as cell aggregates) or filamentous, resulting in great diversity in overall cell morphology. Algal cell walls surround cytoplasmic membranes and are thin and rigid but vary in their composition. They generally contain cellulose with a variety of other polysaccharides including pectin, xylans and alginic acid. Some walls are calcareous containing calcium carbonate deposits. Chitin (a polymer of N-acetylglucosamine) may also be present in some algae. The euglenoids, however, differ from other algae by lacking cell walls. For diatoms, the cell wall is composed of silica giving rise to fossils. Other cell wall-associated structures include gelatinous capsules outside the cell wall for adhesion and protection, and flagella arranged in different patterns on the cell for motility.

All algae also contain membrane-bound chloroplasts containing chlorophyll a and other chlorophylls, such as chlorophyll b, c or d. Some contain differently colored pigments called xanthophylls, which can give rise to differently colored algae. Many algae also contain pyrenoids, which serve as sites for storage and synthesis of starch. Starch is one of the many types of carbohydrate storage that algae use to support respiration in the absence of photosynthesis. Other types of storage molecules include paramylon (β-1,2-glucan), lipids and lammarin (β-1,3-glucan).

3.4.2.2.2 Algae

Algae can be used to treat both municipal and industrial wastewater. Algae play a key role in the aerobic treatment of waste in the secondary treatment process. Algae-based municipal wastewater treatment systems are mainly used for nutrient removal (removal of nitrogen and phosphorus). Algae have the ability to accumulate the heavy metals and thereby remove toxic compounds from the wastewater. In some cases, algae also play a role in the removal of pathogens in the tertiary treatment process. Many different mechanisms play a role in disinfection in high rate ponds. These include predation, sunlight, temperature, dissolved oxygen (DO), pH, sedimentation, and starvation (Fallowfield et al., 1996). Algal photosynthesis causes an increase in the pH because of the simultaneous removal of CO2 and H+ ions (Fallowfield et al., 1996) and uptake of bicarbonate when the algae are carbon limited (Craggs et al., 1997). According to Rose et al. (2002), a pH of 9.2 for 24h will provide a 100% kill of Escherichia coli and most pathogenic bacteria and viruses. Parhad and Rao (1962) also found that E. coli could not grow in wastewater with a pH higher than 9.2.

2.02.4.1 Algae

Algae are chlorophyll-bearing and photosynthetic non-vascular organisms, which unlike plants do not differentiate into roots, stems or leaves. They include a diverse group of microorganisms found in a wide range of aquatic systems. Most algae are eukaryotic and autotrophic organisms, although there are some heterotrophic systems too. Recently, with the advent of large-scale biodiesel production, they have captured a lot of attention due to their capacity to accumulate lipids as a survival mechanism under unfavourable growth conditions. These lipids can then be extracted and transformed into biodiesel, and in some algae species, the lipids can account for more than 60% of the dry cell weight. Another energy-storage compound found in algae are carbohydrates in the form of starch or laminarins. The nutrient composition of the growth medium (nitrogen and phosphorous concentrations) can affect the metabolism and drive the accumulation of one or the other compound. Additionally, as a result of rising environmental concerns, the utilization of algae for CO2 sequestration has furthered interest in these organisms. The ability of algae to use CO2 during photosynthesis, their fast growth rates and higher fixing rates than terrestrial plants have made them an interesting alternative for CO2 biofixation.

Culturing of algae mainly occurs in two types of system: closed and open ponds. In the former, also known as photobioreactors, environmental conditions such as light intensity and nutrient concentration can be controlled. Open ponds have the advantage of utilizing natural light; however, they are more prone to the invasion of other organisms and the control of cultivation conditions becomes more difficult.

Applications of algae in biotechnology go far beyond biodiesel. Just in the energy field, different fuels can also be obtained: ethanol, biobutanol, methane and fuel-grade alcohols. In the food industry, proteins, lipids, vitamins and minerals can all be derived from algae biomass. Also protein-rich animal feed can be produced. Algae from the genus Chlorella are cultivated in northern climates for the production of Chlorella growth factor and agricultural feed products. High-value products such as plastics, resins and lubricants are already been produced commercially. Antibiotics and applications in medicine and agriculture have been exploited and continue to develop as algae physiology and genetics are better understood.