**2. Carbohydrate importance and composition**

*Microalgae - From Physiology to Application*

piece for the production of certain products.

Microalgae are considered to have the potential to produce third-generation biodiesel (due to its capability of fixing carbon dioxide (CO2), which is eventually converted to biomass and other products), which can also be referred as thirdgeneration feedstock, providing mainly lipids, proteins, and carbohydrates. The utilization of these molecules in a sequential way allows the treatment of biomass in biorefineries, including its use in fermentative production of a range of platform biochemical [5]. Through this process, carbohydrates are used as a fundamental

Besides the cell wall and reserves of photosynthetic reaction, the carbohydrates can be excreted by the cell. The exopolysaccharides (EPS) are complex carbohydrates produced from some microalgae, which are long chains composed of sugar derivative structures, mucilaginous and with reactive functional groups, such as sulfate, hydroxyl, or carboxylic [6]. The major components of EPS include mainly the polysaccharides and others as proteins, nucleic acid, and lipids [7]. Addition of these molecules is considered to be of extreme importance for enriching the nutritional value of food items [8]. Although these characteristics are beneficial, the extraction of these compounds from microalgae becomes a real challenge. In this context, several treatments can be performed for the disruption of microalgal cells, including chemical modifications and mechanical, thermal, or ultra-sonication processes [9]. Although promising, it is still hard to manage the cost and work that developing new technologies have for investors (in the industry for applied approaches and academia for basic development and standardization), which presents some limitations for the advance of research in this area. On the long run, an implementation of the use of microalgae as a substitute for many of the other crop options still used will have an important impact on the economics, environment, and more sustain-

Due to the vast diversity of species of microalgae, we present an analysis of the current trends and importance and potential of the use of carbohydrates present in

*Diagram of the main topics involved in the harnessing of microalgae carbohydrates.*

**158**

**Figure 1.**

able practices.

Carbohydrates from microalgae are considered to have a great application in industry, which has led to the development of new techniques and studies but that has exposed new challenges to industry [11, 12]. The diversity of microalgae, the composition, and cell organization are some of the few trials that many scientists are currently facing.

Carbohydrates are poly- or oligosaccharides that can be present in vacuoles and cell walls or that could also be excreted as exopolysaccharides (EPS) [13]. Microalgae come to be an interesting key organism to study due to the high content

**Figure 2.** *Glucose polymers found in microalgae.*

of carbohydrates that some of them have. Some examples are *Desmodesmus spp*. (41%) from wastewater and landfill leachate treatment [14], *Chlorella stigmatophora* (~55%) [5], or *Chlorella vulgaris* (60% [15] or >52% [16]) cultivated under conditions of nitrogen depletion.

Similarly, indigenous microalgae species have been bioprospected in nitrogendepleted environments, such as *Desmodesmus sp*. (57%), an unidentified one with more carbohydrate content (70%) [17], and *Arthrospira platensis* that accumulated up to 74% of carbohydrates [18].

Despite their vast potential, using different species imposes a few challenges when trying to establish a consistent methodology for extraction and use. Based on the differences in metabolism that each species presents, a possible and feasible approach is to consider that they all have reserves made out of polymers of glucose (glucans), such as chrysolaminarin (1:11), laminarin (3:1) (β-1,3 and β-1,6 branches), paramylon(β-1,3), glycogen-type, cyanophycean, floridean (semiamylopectin: α-1,4 and α-1,6 branches), and amylose-type starch (α-1, 4) or both (**Figure 2**). The external covering of cell with polysaccharides could be peptidoglycan matrices, cellulosic wall, and galactose polymer matrix, and others [18–20].

The potential of using microalgae for the development of various products, using carbohydrates as the main source, exposes some of the challenges that using these organisms can have. The main of it is the process of extraction of the diverse saccharides present in each species, but in comparison with plant-derived products, it is actually a much easier path, since they do not present lignification of the cell wall [11, 21]. In the same way, having various species together as a main feedstock source may require adaptations to a proper and more efficient method of extraction.
