2.1.4. Microalgae Culture

Microalgae are single-cell photosynthetic micro-organisms mainly found (but not only) in aquatic environments. These organisms are able to synthetise important amounts of lipids, proteins, carbohydrates as well as other compounds with biological activity in a very short timeframe from three basic ingredients: solar radiation, carbon dioxide (CO2) and fertilizers/nutrient-rich water. Microalgae have been reported over a long time to be very important in the biofixation and storage of CO2, as a way to neutralize the huge production and release of greenhouse gases (GHG) worldwide, derived from the enormous use of fossil fuels at various scales, including the industrial sector. The several benefits of microalgae biomass as a feedstock for biofuel production compared to traditional biomass resources are listed as follows:


Different microalgae were previously studied concerning their biofuel production potential. The conversion route and technology depend on different parameters such as the biomass composition, selected biofuel product, operating conditions, process time and production cost, in order to assure either economically viability or environmental sustainability.

Tables 1 and 2 resumes the main characteristics of each type of selected and studied biomass (including microalgae).

*Energies*

**2020**, *13*, 937


**Table 2.** Optimal conditions for the cultivation of the selected and studied biomasses (including microalgae).

*Energies* **2020**, *13*, 937

The use of energy crops and microalgae for bioenergy presents some opportunities for rural areas, such as employment generation, land recuperation from an abandonment state, rural development especially of isolated regions, increased income for farmers and economic benefits for companies or entities with an interest in developing or using these species for the production of bioenergy. Yet, implementation of these biomasses can also lead to some constraints. For this reason, some steps have to be taken before implementation of such projects:


To study the possibility of the implementation of crops in an intercalated form of species with different ages or of several other typologies, given the rotation of these, and avoid extensive use and massive soil damage.

### *2.2. Low Indirect Land-Use Change (ILUC) Risk Soils*

Low indirect land-use change (ILUC) risk soils refers to areas that can be considered for the implementation of dedicated energy crops [7], once the soils present a low quality for food and feed species, caused by a multitude of natural or human factors. There is a wide diversity of soils considered to be at low ILUC risk such as contaminated soils, wasted land, devastated land, moderate and highly degraded soils, abandoned land such as pasture or arable soils, marginal and fallow lands [37], all of which are suitable for the implementation of energy crops or microalgae for bioenergy production. Wastes from farmland species and from food and feed species can also be harnessed for energy production. Fertility and productivity are different for each soil type, as shown in Figure 2.

**Figure 2.** Low indirect land-use change (ILUC) risk soils for biomass and biofuels production. X axis refers to increasing biomass (bioenergy) productivity potential. The red vertical dash line separates bad (low) and fairly good (moderately high) quality lands (Adapted from [37]).

Based on the literature, the more studied soils that represent low ILUC risk are the degraded, marginal and contaminated soils, which according to their properties meet the sustainability requirements. With the objective of implementing the energy crops previously mentioned in this type of soils for to reduce the risk of land use conflicts due to competition for food and feed, that they can bring additional revenue to land owners, thus contributing positively to economic growth. These three types of soils were then evaluated and selected in this study, being described below:

• Degraded land: areas that su ffer from a continuous deterioration process that can be caused naturally where lands with high carbon-laden are converted to dry land, causing changes in the physical, chemical and biological characteristics of the soil, reducing soil quality and causing severe erosion, namely, nutrient loss, soil infiltration problems [7] and wind erosion. Degradation can also occur by human action that generates progressive and continuous soil depletion causing biological and economic loss by decreasing the value of the land [38].

Considering the aforementioned definition, it can be confirmed that desertification is a state of the soil that is associated with degraded land and is characterized by very dry areas (dry sub-humid, arid and semi-arid) that reach this state for environmental or human reasons [39], being a factor that depending on the zone, is gradually increasing. Desertification causes alteration and destruction of ecosystems and increases the presence of invasive species, with loss of suitable areas for agriculture and decreased groundwater [40].

In this study, soil a ffected by natural factors that cause desertification [41] of these areas will be considered degraded;

• Contaminated soil: land with high concentrations of pollutants such as asbestos, gold, tin and tungsten, polymetallic, coal, base metals, iron and manganese, radioactive, among others [42], caused by human action, namely, in industrialized areas, intensively applied agriculture [7] and in areas where mining has occurred for a certain period of time.

Currently, in Portugal, two types of situations are identified in contaminated areas. In the first case the area remains contaminated without any possible use, becoming an abandoned area and in the second case, the recovery and valorization of this area is being accomplished, by companies, such as EDM, that have to monitor and control the recovery process along and after the application of gradual remediation methods [8]. The recovery process may take years to complete land reclamation, either for agriculture, recreational areas or residential areas.

Contaminated soils are also considered degraded soils [7]. However, in this study, degraded (desertified) and contaminated areas will be assessed separately. The areas massively exploited by human activity as mining areas characterized by the presence of heavy metals [42] were considered as contaminated soils;

• Marginal areas: although there is no clear and accurate concept of this type of soil [8], the APEC (Asia-Pacific Economic) Energy Working Group report presents a very broad definition of these areas which are characterized by poor weather conditions (low rainfall and high temperatures) and very poor soil physical-chemical characteristics (low quality and with physical constraints such as mountainous, extremely dry areas, saline, drenched, glacial and rocky areas) [43].

Based on data found in the literature, the saline soils are considered marginal, therefore, areas with moderate and high concentrations of saline elements are inadequate for the implementation of food crops [7]. The term salinization refers to areas with low precipitation and high evapotranspiration that causes salt accumulation making it impossible to wash on the soil surface. These areas can be found in the coastal part of the territory [8]. Much of the marginal land could be used for agriculture due to the quality and type of soil, however, many of them, are found in high zones, with high slopes, hard-to-reach areas or abandoned land, that are no longer used for this purpose [8] and now are considered suitable for other purposes such as the implementation of energy crops. For these reasons, in this study, we consider as marginal lands the pasture areas such as natural herbaceous vegetation, areas with dense, light dense undergrowth, dense and dense sclerophyte vegetation, other woody formations and, lastly, areas related to uncovered spaces or with sparse vegetation [44].

Based on a report by the 2014 Joint Research Center that presents an analysis of the extent of degraded areas in European Union (EU) countries, it can be stated that in the case of Portugal, the contaminated land has the largest area of the three types of soils presented above. The total extent of these areas is 2318 kha of which 12.3% represents highly saline soils (marginal land); 33.8% are areas with severe erosion (degraded areas) and the remaining, 53.9%, are contaminated soils. This document also identifies an area of 93 kha that will be available in Portugal by 2050 for the implementation of energy crops, but this implies the conversion of 19% arable and 44% forested areas in these lands [7]. According to the European Court of Auditors of 2018, 8% of EU territory which includes countries such as Bulgaria, Cyprus, Greece, Italy, Spain and Portugal, have areas with very high values highly sensitive to desertification (degraded soils), causing a decrease in land use for the agricultural sector [45].
