Global mapping of cost‐effective microalgal biofuel production areas with minimal environmental impact

Sustainable alternatives to fossil fuels are urgently needed to avoid severe climate impacts and further environmental degradation. Microalgae are one of the most productive crops globally and do not need to compete for arable land or freshwater resources. Hence, they may become a promising, more sustainable cultivation alternative for the large‐scale production of biofuels provided that substantial reductions are achieved in their production costs. In this study, we identify the most suitable areas globally for siting microalgal farms for biodiesel production that maximize profitability and minimize direct competition with food production and direct impacts on biodiversity, based on a spatially explicit multiple‐criteria decision analysis. We further explore the relationships between microalgal production, agricultural value, and biodiversity, and propose several solutions for siting microalgal production farms, based on current and future targets in energy production using integer linear programming. If using seawater for microalgal cultivation, biodiesel production could reach 5.85 × 1011 L/year based on top suitable lands (i.e., between 13% and 16% of total transport energy demands in 2030) without directly competing with food production and areas of high biodiversity value. These areas are particularly abundant in the dry coasts of North and East Africa, the Middle East, and western South America. This is the first global analysis that incorporates economic and environmental feasibility for microalgal production sites. Our results can guide the selection of best locations for biofuel production using microalgae while minimizing conflicts with food production and biodiversity conservation.

pre-industrial levels, a commitment that has been ratified by 185 parties following the 21st Conference of the Parties to the United Nations Framework Convention on Climate Change (IPCC, 2015). Through the transformation of biomass into bioenergy (McKendry, 2002), biofuel systems can provide an alternative to fossil fuels in the transport sector, especially for the shipping and aviation industries, which in the midterm cannot be fully powered by electricity (Fulton, Lynd, Körner, Greene, & Tonachel, 2015).
With 16% of transport energy demands potentially fulfilled by biofuels in 2040 (IEA, 2017b), microalgal biofuel production systems could become an important alternative for offsetting fossil fuels in the transport sector, provided that significant reductions in their production costs are achieved (Acién, Molina, & Fernández-Sevilla, 2018;Chia et al., 2018;Norsker, Barbosa, Vermue, & Wijffels, 2011;Slade & Bauen, 2013). Costs reductions can derive from the development of biorefinery systems that produce high-value coproducts (e.g., food and animal feed) along with biofuels (Chia et al., 2018;Ruiz et al., 2016); the identification, development, and cultivation of fast-growing microalgal strains (Ajjawi et al., 2017;Mata et al., 2010); the colocation of microalgal production systems with free nutrient and CO 2 sources (e.g., from wastewater operations and industries) (Beal, Archibald, Huntley, Greene, & Johnson, 2018;Mu et al., 2014;Orfield, Keoleian, & Love, 2014;Roostaei & Zhang, 2017); the production of biogas and the recycling of nutrients (i.e., by anaerobic digestion) (González-González et al., 2018;Uggetti et al., 2014); and the implementation of governmental incentives based on the relative environmental benefits of biofuel production alternatives (Correa et al., 2019). However, considerable land areas will still be required to offset fossil fuels by microalgal cultivation, although lower compared to firstgeneration biofuels (Chisti, 2008;Correa et al., 2017). Here, we evaluate global opportunities for large-scale microalgal biodiesel production while minimizing direct competition with agricultural lands and biodiverse areas, taking into account attributes that maximize the profitability in microalgal biodiesel production: water availability, lipid productivity, flat land availability, proximity to main transport networks, gross national income (GNI) per capita (as a substitute for labor costs), and proximity to known industrial CO 2 sources. Based on four scenarios for microalgal cultivation, which combine two main approaches to decrease microalgal production costs and freshwater use (i.e., availability of free CO 2 and availability of seawater)-Scenario 1 (i.e., use of fresh, brackish, or saltwater), Scenario 2 (i.e., use of fresh, brackish, or saltwater adjacent to known industrial CO 2 sources), Scenario 3 (i.e., use of seawater), and Scenario 4 (i.e., use of seawater adjacent to known industrial CO 2 sources)we: (a) Identify the most suitable areas globally for siting microalgal farms for biodiesel production (i.e., microalgal cultivation systems along with the associated infrastructure), while avoiding direct competition with food production and direct impacts on biodiversity, which are considered the two main impacts of first-generation biofuels (Correa et al., 2017;Immerzeel et al., 2014), (b) Explore the relationships between microalgal production, agricultural value, and biodiversity, and (c) Explore solutions for siting microalgal production farms based on current and future targets in energy production. Because we aim at finding areas globally for siting microalgal production farms, we assume free trade for microalgal biofuel commercialization in the context of a globalized economy.
For minimizing competition with food production, the selected attribute corresponded to the agricultural value of lands (i.e., potential annual gross economic rents from agricultural lands) (Naidoo & Iwamura, 2007). For minimizing impacts on biodiversity, the selected attribute corresponded to the biodiversity value. Biodiversity value was based on the number of vertebrate species and the number of threatened vertebrate species (i.e., considering amphibians, birds, and mammals) (Jenkins, Pimm, & Joppa, 2013), the presence of islands (which harbor higher proportions of endemic and threatened species compared to the mainland) (McCreless et al., 2016;Tershy, Shen, Newton, Holmes, & Croll, 2015), and the presence of areas with low human pressures (which is related to the integrity of ecosystems) based on the Global Human Footprint (Venter et al., 2016). Water bodies (Lehner & Döll, 2004), protected areas (UNEP-WCMC, 2016), Key Biodiversity Areas (BirdLife International, 2016), and urban areas (i.e., based on built areas) (Schneider, Friedl, & Potere, 2009) were assumed to be unsuitable for microalgal cultivation and excluded from final suitability maps (i.e., assigning No Data to water bodies and zero to the other layers).
We developed four scenarios for microalgal cultivation, based on the type of available water and the inclusion of known industrial CO 2 sources (Scenarios 1-4): Scenarios 1 and 2 included the use of fresh, brackish, or saltwater, while Scenarios 3 and 4 included the use of seawater, which is abundant and does not compete with scarce freshwater sources (Gleeson, Wada, Bierkens, & Beek, 2012;Vorosmarty et al., 2010). Scenarios 2 and 4 included the proximity to known industrial CO 2 sources (but not to anaerobic digesters as this information is currently not available), which is a cost-effective way to increase microalgal biomass productivities (Borowitzka et al., 2012;Klise et al., 2011;Lundquist et al., 2010;Orfield et al., 2014;Quinn et al., 2013;Slade & Bauen, 2013;Venteris et al., 2014;Wigmosta et al., 2011), while Scenarios 1 and 3 did not include the proximity to known industrial CO 2 sources ( Figures S2-S5).
Land covers potentially replaced by microalgal production farms were identified for top suitable microalgal production lands (i.e., suitability values ≥0.7), using the MODIS-derived global mosaic for 2012 at a resolution of 5 arcminutes (Channan, Collins, & Emanuel, 2014). The proportion of top suitable lands within politically unstable countries, which could challenge potential large-scale microalgal biofuel production, was calculated based on the Fragile States Index in 2016 (FFP, 2017) for Scenarios 2-4, considering countries with total values ≥80 as politically unstable. This index is based on social, economic, and political risk indicators that lead to higher values in politically unstable countries. Potential biodiesel production was estimated for top suitable lands, along with the percentage of transport energy demands that could be fulfilled based on the most plausible production scenarios (i.e., Scenarios 2-4) in 2016, 2030, and 2040 (see Supporting Information for details). Future transport energy demands were based on the Current Policies, New Policies, and the Sustainable Development Scenarios for 2030 and 2040 (IEA, 2017b). The Current Policies Scenario takes into account policies that have been enacted in mid-2017 for reducing greenhouse emissions, while the New Policies Scenario additionally includes announced policy intentions for reducing global warming, and the Sustainable Development Scenario aims at limiting global warming consistent with the Paris Agreement and the United Nations 2030 Agenda for Sustainable Development.

| Development of sensitivity analysis on slope and lipid productivities
In order to determine how changes in model parameters influence the siting of microalgal production farms and potential biodiesel production, a sensitivity analysis was developed based on slope and lipid productivities. For this, the slope was increased from a membership midpoint of 5° to 10°, and 15°; and lipid productivity was both increased and decreased in 20% and 40% from a midpoint of 13,000 L ha −1 year −1 (see Supporting Information for details on fuzzy memberships and midpoints). We used the one-at-a-time method, in which the changes in values for each factor were evaluated in turn (Malczewski & Rinner, 2015).

| Siting of microalgal production farms based on targets in transport energy demands
In order to find locations for siting microalgal production farms based on targets in transport energy demands, an integer linear optimization model (Beyer, Dujardin, Watts, & Possingham, 2016) was developed using the software R and Gurobi Optimizer (see Supporting Information for calculation details). The model aimed at maximizing profitability while minimizing direct competition with agricultural lands and biodiverse areas through the following formula: where i corresponds to each pixel, P corresponds to microalgal profitability (ranging from 0 to 1), x corresponds to the decision variable given by the software (ranging from 0 to 0.8 and representing the available area for placing microalgal ponds), "maximum" corresponds to the maximum value among agricultural value A (ranging from 0 to 1) and biodiversity value B (ranging from 0 to 1), D corresponds to productivity values in units of energy (GJ pixel −1 year −1 ), and T represents the targets in energy demands globally in 2016, 2030, and 2040 (GJ/year) based on the IEA (2017b) energy production estimates (i.e., Current Policies Scenario, New Policies Scenario, and Sustainable Development Scenario). Using the square of the profitability as the numerator and the maximum value between A and B as the denominator ensures that pixels with low or average profitabilities, and with high agricultural or high biodiversity value, are excluded in the final solutions.
We investigated alternative solutions in which the agricultural and biodiversity values were not taken into account (see Supporting Information), and in which targets in microalgal biodiesel production increased from 10% to 40% based on 2016's transport energy demands. Finally, the amount of cultivation land needed to meet 10%, 20%, 30%, and 40% of total transport energy demands in 2016, 2030, and 2040 was determined based on the several IEA (2017b) energy production scenarios (Current Policies Scenario, New Policies Scenario, and Sustainable Development Scenario) and current estimated microalgal lipid productivities (Moody et al., 2014).

| RESULTS
The most suitable areas for microalgal biodiesel production, while avoiding direct competition with agricultural and biodiverse lands, were located in human-transformed dry tropical and subtropical mainlands (Figures 1 and 2). For Scenario 1 (i.e., cultivation based on fresh, brackish, or saltwater), top suitable microalgal production lands (suitability values ≥0.7) could reach around 1,422.8 thousand square kilometers, concentrated in dry areas in North and East Africa, the Middle East, South and Central Asia, and South America, mainly overlapping with barren and sparsely vegetated lands (60%), open shrublands (22%), and grasslands (9%) ( Table 1). Significant competition with scarce freshwater resources would occur in dry areas, where low-density microalgal production farms could be established (Figure 3). Scenario 2, which is restricted to known industrial CO 2 sources, could reach around 464.2 thousand F I G U R E 1 Global suitability map for microalgal biodiesel production based on the maximization of microalgal productivity, minimization of competition with food production, and minimization of direct impacts on biodiversity for (a) Scenario 1 (use of fresh, brackish, or saltwater without considering known industrial CO 2 sources) and (b) Scenario 2 (use of fresh, brackish, or saltwater adjacent to known industrial CO 2 sources) square kilometers mainly over barren and sparsely vegetated lands (57%), open shrublands (17%), and grasslands (8%). The cultivation Scenarios 3 and 4 (i.e., cultivation based on seawater) could reach around 305.3 thousand square kilometers and 132.9 thousand square kilometers, respectively, mainly over barren or sparsely vegetated lands and open shrublands.
For Scenarios 2-4, which are the most feasible options for widespread microalgal biodiesel production in terms of reduced competition with scarce freshwater, 61%, 45%, and 34% of top suitable lands (suitability values ≥0.7), respectively, fell within several politically unstable countries in Africa, the Middle East, and South F I G U R E 2 Global suitability map for microalgal biodiesel production based on the maximization of microalgal productivity, minimization of competition with food production, and minimization of direct impacts on biodiversity for (a) Scenario 3 (use of seawater without considering known industrial CO 2 sources) and (b) Scenario 4 (use of seawater adjacent to known industrial CO 2 sources)  Irrigation dams Rivers Suitability Scenario 1 (HD) Suitability Scenario 1 (LD) Brackish water Major groundwater basins Average groundwater recharge (km³ /year) 0-6.9 6.9-32.6 32.6-106.5 106.5-300.5 300.5-856.4 Asia (i.e., Afghanistan, Egypt, Iran, Iraq, Lebanon, Libya, Mauritania, Niger, Pakistan, Somalia, Sudan, Syria, Turkey, and Yemen). Based on these scenarios, potential total microalgal biodiesel production ranged between 5.85 × 10 11 and 1.81 × 10 11 L/year, representing between 17% and 6% of total transport energy demands in 2016, respectively (Table 2). Among these scenarios, maximum levels of biodiesel production would be achieved in Scenario 2, followed by Scenarios 3 and 4, which are restricted to the use of seawater. Less than 0.5%, 5.8%, 3.5%, and 5.1% of threatened amphibians, birds, mammals, and reptiles, respectively, would overlap top suitable microalgal production lands (i.e., suitability values ≥0.7) for Scenarios 2-4. Around 25% and less than 2.5%, 2.8%, and 3.6% of this set of threatened amphibians, birds, mammals, and reptiles, respectively, would face competition with microalgal production in more than 20% of their distribution ranges (Figure 4). This competition would be highest for Scenario 3 compared to Scenarios 2 and 4 (Table S3).
At a global scale, potential conflicts could arise among microalgal production and areas of high agricultural and biodiversity value, mainly in Central America, tropical and subtropical South America, Africa, India, and Southeast Asia ( Figure 5). If agricultural and biodiversity values are not considered, microalgal cultivation for one of the most feasible cultivation scenarios (i.e., Scenario 3, which is based on seawater) would include larger tracts of humid lands in the tropics (e.g., in Southeast Asian islands and Madagascar when just avoiding areas of high agricultural value; and in Central and South America, Southeast Africa, India, and Southeast Asian mainland when just avoiding areas of high biodiversity value) ( Figure 6). Locations for microalgal cultivation would change as a function of targets in microalgal biofuel production (Figure 7). Potential conflicts with areas of higher agricultural and biodiversity value (e.g., in Central and South America, Africa, South and Southeast Asia, and China) would increase if fulfilling higher targets in microalgal biofuel demands (i.e., from 10% to 40% of total transport energy demands in 2016). Finally, more lands would be needed to fulfill higher targets in microalgal biofuel demands based on current and future energy consumption scenarios (IEA, 2017b; Figure 8).

| DISCUSSION
We provide the first global analyses on cost-effective areas for microalgal biodiesel production that minimize direct competition with food production and biodiversity, considering variables that increase the profitability in microalgal biofuel production. Our analyses are based on four scenarios for microalgal cultivation (i.e., use of fresh, brackish, or saltwater; use of fresh, brackish, or saltwater adjacent to known industrial CO 2 sources; use of seawater; use of seawater adjacent to known industrial CO 2 sources). Furthermore, we explore how microalgal production, agricultural value, and biodiversity are related, and how changes in current and future targets T A B L E 2 Estimates of microalgal biodiesel production for Scenarios 2-4 in top suitable microalgal production lands (suitability values ≥0.7) (see Supporting Information for calculation details). The percentages of transport energy consumption fulfilled by each cultivation scenario are shown for 2016, 2030, and 2040. Scenarios of transport energy consumption (Current Policies Scenario, New Policies Scenario, and Sustainable Development Scenario) are based on the IEA (2017b) energy production estimates in energy demands alter the siting of microalgal production farms. These results can help in decision making toward the selection of best areas for microalgal biodiesel production at lower conflicts with food production and biodiversity. Based on a MCDA, our results show that dry tropical and subtropical mainlands in areas subject to high human pressures on the environment (i.e., human-transformed dry tropical and subtropical mainlands) are the most suitable areas for large-scale microalgal biodiesel production. While avoiding direct competition with agricultural and biodiverse lands (i.e., based on the richness of vertebrates, presence of threatened vertebrates, presence of islands, and presence of areas with low human pressures), these areas still provide access to water and flat lands for microalgal cultivation, access to transport networks that ensure supply of inputs and distribution of biodiesel, and low labor costs (here measured as GNI per capita) that reduce production costs. As expected, microalgal suitability increases where high solar irradiance and temperature facilitate larger microalgal biomass and lipid yields (Lundquist et al., 2010;Moody et al., 2014;Quinn et al., 2012;Venteris, McBride, et al., 2014a;Wigmosta et al., 2011), which occurs in tropical and subtropical regions of the world.
The use of drylands for microalgal production, which in general are less suitable for cropping (Alexandratos & Bruinsma, 2012) and hold lower biodiversity values compared to more humid regions (Gaston, 2000), would decrease direct competition with high-value agricultural and biodiverse lands. In contrast, several studies developed in the United States show that humid regions are the most feasible locations for large-scale microalgal production Venteris, McBride, et al., 2014a;Venteris et al., 2013;Wigmosta et al., 2011). These studies indicate that the consumption of water per liter of microalgal oil and the costs associated with water pumping would be lower in the Southeastern United States (i.e., mainly around the Gulf and East Coasts) compared to the drier southwestern lands, where water demands and water pumping costs increase as a result of higher evaporation rates relative to precipitation. Notwithstanding, the use of humid areas for microalgal production would inevitably lead to direct competition with food production and biodiversity (although lower compared to first-generation biofuels because of their higher biofuel productivities per unit area) (Correa et al., 2017). Furthermore, targeting humid areas for carbon sequestration, where forests can grow (Saatchi et al., 2011), is an effective solution for climate change mitigation (Canadell & Raupach, 2008).
The establishment of low-density microalgal production farms would be a more sustainable option in regions where significant competition with freshwater resources is expected to occur, including dry areas around the Nile river in North Africa and the Tigris and Euphrates rivers in the Middle East, F I G U R E 5 Overlapping of microalgal biodiesel profitability for Scenario 1 (i.e., use of fresh, brackish, or saltwater without considering known industrial CO 2 sources) with (a) agricultural value and (b) biodiversity value. The agricultural value corresponds to the potential gross economic rents from agricultural lands in USD/ha (Naidoo & Iwamura, 2007). The biodiversity value (i.e., ranging from 0 to 1) is based on the number of vertebrate species (considering amphibians, birds, and mammals), the number of threatened vertebrate species (Jenkins et al., 2013), the presence of islands, and the presence of areas with low human pressures (Venter et al., 2016)  as well as along low-recharge aquifers in North America, South America, North and East Africa, Southern Europe, the Middle East, South and Central Asia, and China (Gleeson et al., 2012;Vorosmarty et al., 2010). Scenarios 3 and 4 (i.e., based on seawater use) would become a more feasible alternative for large-scale microalgal biodiesel production, in terms of reduced competition with scarce freshwater resources. However, top suitable lands (i.e., suitability values ≥0.7) would decrease from 1,422.8 thousand square kilometers for Scenario 1 to 305.3 thousand square kilometers and 132.9 thousand square kilometers for Scenarios 3 and 4, respectively. In these areas, the use of microalgal strains tolerant to a wide range of salinity conditions could prevent the use of freshwater and minimize the use of seawater that would maintain pond salinities as water evaporates (Borowitzka & Moheimani, 2013). Additionally, the recycling of harvest water could facilitate nutrient recovery while reducing water requirements (Venteris et al., 2013;Yang et al., 2011). Currently not considered in other studies, the political stability of countries could constitute an additional challenge for the widespread adoption of microalgal biofuel production systems (i.e., between 61% and 34% of top suitable lands for Scenarios 2-4 fell within politically unstable countries). However, several of these countries already have a well-developed infrastructure for oil production and processing (e.g., Egypt, Iran, Iraq, Libya, Sudan, and Turkey are among top oil producers globally), which would facilitate the transition toward a more sustainable future fuel production based on microalgae. Furthermore, microalgal biofuel production may represent an important development option to improve livelihoods and build sustainable economies in these countries.
Potential microalgal biodiesel production is a function of the cultivation scenarios and changes in membership midpoints applied to the different variables. Between 5.85 × 10 11 and 1.81 × 10 11 L/year could be produced in top suitable lands (suitability values ≥0.7) for the most feasible cultivation scenarios (i.e., Scenarios 2-4). For these scenarios, changing the midpoint in slope from 5° to 15° would increase potential microalgal biodiesel production by between 8% and 10%, while decreasing the midpoint in lipid productivity from 13,000 to 7,800 L ha −1 year −1 (i.e., in 40%) would increase potential microalgal biodiesel production by between 8% and 32%, and increasing the midpoint in lipid productivity from 13,000 to 18,200 L ha −1 year −1 (i.e., in 40%) would decrease potential microalgal biodiesel production by between 45% and 82% ( Figures S6 and S7). Biodiesel production estimates are expected to increase with the cultivation of fast-growing and high-lipid-producing microalgal strains (Ajjawi et al., 2017;Mata et al., 2010;Slade & Bauen, 2013), along with the adoption of more efficient cultivation, harvesting, and processing techniques that increase microalgal biomass and lipid productivities (González-González et al., 2018;Pierobon et al., 2017). Reducing the uncertainty in global microalgal potential biodiesel production would require the refinement of models based on resource availability (e.g., inclusion of nutrients from wastewater sources, inclusion of CO 2 sources from anaerobic digesters, and inclusion of freshwater restrictions) and economic feasibility (e.g., considering land costs, opportunity costs with other economic activities, and several microalgal production technologies).

| Relationships among microalgal production, agricultural value, and biodiversity value
Potential conflicts among microalgal production and areas of high agricultural and biodiversity value could arise within the tropical region ( Figure 5), which faces the highest deforestation rates globally as agricultural activities expand for meeting food and biofuel demands (Hansen et al., 2013;Laurance, 2015;Laurance, Sayer, & Cassman, 2014), in spite of harboring most of Earth's biodiversity (Dirzo & Raven, 2003;Kier et al., 2005). In fact, if agricultural and biodiversity values are not considered, microalgal production for Scenario 3 (i.e., use of seawater) would shift to areas of higher agricultural value and ecological importance (e.g., in Central and South America, Africa, India, and Southeast Asia), similarly to food F I G U R E 8 Potential microalgal cultivation area needed to meet 10%, 20%, 30%, and 40% of total transport energy demands in 2016, 2030, and 2040. Scenarios for transport energy consumption (Current Policies Scenario, New Policies Scenario, and Sustainable Development Scenario)   crops for biofuel production. This would intensify the pressures on food production and biodiversity in regions currently impacted by agriculture and biofuel expansion, including the Southeast Asian tropical forests (Fargione et al., 2010;Koh, 2007;Koh et al., 2011), as well as in regions with current little agricultural development such as the Amazon and Congo tropical forests (Laurance et al., 2001;Wich et al., 2014) and the South American and African savannas (Laurance, 2015;. Avoiding areas of high agricultural and biodiversity value for microalgal cultivation would help in decreasing direct competition with food production and biodiversity loss, which is unlikely by using food crops (Correa et al., 2017;Searchinger & Heimlich, 2015).
The consideration of the trade-offs among microalgal biodiesel profitability, cultivation water source (i.e., fresh, brackish, and saltwater) and its availability, along with the agricultural and biodiversity value of lands, could limit direct competition with food production and prevent further direct habitat loss in biodiverse regions (Correa et al., 2017). The use of human-transformed dry mainland coasts within the tropics and subtropics for microalgal production seems to be the most sustainable option in terms of reduced competition with freshwater resources, high-value agricultural lands, and biodiversity. However, dry coastal areas hold unique and threatened biodiversity (Brito et al., 2014;Durant et al., 2012;IUCN, 2016;Vale, Pimm, & Brito, 2015) and provide a wide range of important ecosystem services (e.g., coastal protection, maintenance of fisheries, and tourism) (Barbier et al., 2011). In fact, top suitable microalgal production lands for Scenarios 2-4 harbor as much as 3.1% of threatened vertebrates globally (i.e., mainly birds and reptiles), and some of them would face competition with microalgal production in more than 20% of their distribution ranges (Table S3). Several of these areas could be easily avoided without significantly impacting species and microalgal production (i.e., for the amphibian Eupsophus queulensis and the Lima leaf-toed gecko, with distribution ranges smaller than 393.5 km 2 ). For species with larger distribution ranges (i.e., the Syrian hamster, the Atacama toad, the Peruvian plantcutter, the four-toed jerboa, and the rufous flycatcher, with distribution ranges larger than 4,713.6 km 2 ), microalgal cultivation could avoid their habitat patches. Furthermore, functional connections, among dry terrestrial ecosystems and mangroves, mudflats, saltmarshes, and coral reefs (Martínez et al., 2007), should be preserved by avoiding pollution (e.g., through harvest water recycling).

| Locations for siting microalgal farms for biodiesel production based on energy targets
Locations for microalgal biodiesel production would not only change as a function of trade-offs among profitability, water availability, agricultural value, and biodiversity but also along with targets in microalgal biofuel production. As expected, more lands would be needed to fulfill higher targets in microalgal biofuel demands. Furthermore, as targets in microalgal biofuels increase, regions of higher agricultural and biodiversity value would be considered suitable for microalgal cultivation. In fact, increasing microalgal production from fulfilling 10% to 40% transport energy demands in 2016 would lead to the inclusion of regions with higher agricultural and ecological importance within Central and South America, Africa, South and Southeast Asia, and China, potentially compromising food production and biodiversity in these areas.
Future assessments based on global and national targets in energy and food production, economic development (e.g., urbanization, mining, tourism), biodiversity conservation, and provision of ecosystem services (e.g., carbon sequestration and coastal protection), in the context of climate change, can improve the understanding of the socioeconomic and environmental role of microalgal biofuels. Spatially explicit comparisons with biofuel production alternatives (e.g., firstand second-generation biofuels) can guide the identification and adoption of more sustainable biofuel production systems (Correa et al., 2019). These comparisons can help to assess the impacts of microalgal biofuels in dry regions, in contrast to systems that rely on agricultural lands and more biodiverse areas for crop production (e.g., oil palm and sugarcane) (Jaiswal et al., 2017;Ocampo-Peñuela, Garcia-Ulloa, Ghazoul, & Etter, 2018).
Although we propose that avoiding the cultivation of microalgae within agricultural and biodiverse areas would be the best option for reducing direct competition with food production and biodiversity, further assessments on the overall environmental impacts of microalgal production in humid areas are needed. These assessments could consider the replacement of areas of currently established biofuel crops by microalgal biofuel systems-which offer higher biofuel yields per unit area (Chisti, 2008;Correa et al., 2017)-along with the colocation of microalgal systems with free nutrient sources (e.g., from wastewater and agricultural residues) (Chiu & Wu, 2013;Fortier & Sturm, 2012;Mu et al., 2014;Orfield et al., 2014;Roostaei & Zhang, 2017) and free CO 2 sources (e.g., from industrial operations, including anaerobic digesters and biorefineries with fermenters) (Lundquist et al., 2010;Orfield et al., 2014;Wigmosta et al., 2011).

| CONCLUDING REMARK S
We propose best locations for siting microalgal farms for biodiesel production that meet substantial biofuel production levels while avoiding direct land-use competition with agricultural lands and biodiverse areas, through a GISbased multiple-criteria decision analysis and integer linear programming. We conclude that potential conflicts with food production and biodiversity conservation, as well as with freshwater consumption, can be reduced if cultivation is restricted to human-transformed dry mainland coasts in tropical and subtropical regions of the world, in contrast to first-generation biofuels, which need agricultural lands and freshwater (Correa et al., 2017). However, even in these areas, the prevention of environmental impacts associated with microalgal production would be required. This includes halting direct habit loss for threatened species, by avoiding microalgal production within habitat patches while preserving functional connections among ecosystems (e.g., terrestrial dry ecosystems, mangroves, mudflats, saltmarshes, and coral reefs). Potential total biofuel production decreases with the accumulative number of constraints (i.e., from 5.85 × 10 11 to 1.81 × 10 11 L/year for Scenarios 2 and 4, respectively, based on top suitable microalgal production lands). Locations for microalgal biodiesel production would not only change as a function of trade-offs between profitability, water availability, agricultural value, and biodiversity but also along with targets in microalgal biofuel production. Higher targets in microalgal biofuels would inevitably lead to competition with areas of higher agricultural and biodiversity value, mainly within the tropics and subtropics. Future assessments that include optimized cultivation technologies, cultivation of more productive microalgal strains, availability of nutrients (e.g., from wastewater sources and agricultural residues), availability of CO 2 (e.g., from anaerobic digesters), restrictions on freshwater use, regional changes in land costs, and trade-offs among ecosystem services (e.g., carbon storage and coastal protection) can further refine the assessment of opportunities for microalgal biofuel production at a global scale. Microalgal production could become an important economic alternative in areas with little potential for agricultural development and relatively low biodiversity value (i.e., human-transformed dry tropical and subtropical mainlands), thereby helping in poverty alleviation while reaching substantial energy and environmental targets.