Evaluating optimal cultivation sites for microalgae as a sustainable biofuel energy resource

Microalgae are promising candidates for the production of sustainable biofuels due to their high productivity, ability to grow in diverse environments, and carbon capture potential. However, the selection of suitable cultivation sites is crucial to achieve efficient and sustainable microalgae cultivation for biofuel production. This study discusses the evaluation of optimal cultivation sites for microalgae as a sustainable biofuel energy resource, with a focus on the naturally favorable conditions for microalgae cultivation in district Gwadar, Pakistan. The global demand for renewable energy sources has increased due to the rising energy crisis, and microalgae has emerged as an efficient source of biofuel on a large scale. The study utilizes geographic information systems to highlight suitable microalgae cultivation areas in district Gwadar. The results indicate that district Gwadar has the potential to overcome the energy crisis by cultivating microalgae as a biofuel energy source. The research provides a promising idea for designing a master plan to install a microalgae cultivation system in the highlighted sites for the concerned authorities.


Introduction
The global energy crisis has led to a surge of interest in cultivating algae for the production of biodiesel and other biofuels, particularly in unfavorable land.As an alternative source of raw material for biofuel production, microalgae, especially liquid fuels produced from microalgae lipids (oils), can provide an important component of the future biofuel's mixture (Borowitzka et al 2012, Rawat et al 2013, Boruff et al 2015).In addition to generating electricity, environmentally friendly energy sources can be used to produce a variety of products.Because of the presence of energy-rich contents, algal biofuels can be potentially used as a sustainable and green alternates of fossil-based gas and oil (Darzins et al 2010, Scott et al 2010), and it is estimated that one acre of an algal bloom can generate over 2,000 gallons of fuel each year (Carriquiry et al 2011).
The cultivation of microalgae typically involves two main types: Raceway Pond system (commonly referred to as open cultivation system) that include open ponds, tanks, and raceway ponds, and controlled closed cultivation systems which use different types of bioreactors (Narala et al 2016).Open raceway ponds were used in the first attempt to scale up microalgae cultivation (Johnson et al 1988).Open cultivation has advantages including lower startup and operating expenses as well as less energy needed for blending the cultures.On the other hand, closed cultivation systems are referred to as closed photobioreactors (PBRs), which can be tailored to specific strains and occupy less space.Thus, they are more efficient and precise as optical conditions such as light intensity can be maintained and contamination can be minimized.PBRs do, however, have several disadvantages, such as bio-fouling, overheating, cleaning difficulties, and an excessive buildup of dissolved oxygen, which might limit growth.PBRs also have substantial construction and maintenance expenses (Molina Grima et al 1999, Chisti 2008).There are specific cultivation system designs and principles required for different algal strains (Schenk et al 2008) such as open ponds established in wastewater treatment plants can take circular or gravity-driven forms.Fundamental tubular designs of photo bioreactors (PBRs) have been improved over a period of time to cater a diverse range of products from pharmaceuticals to high nutritional substances.This was done by enhancing light exposure and culture mixing.Continuous operation increases the efficiency of PBRs (Otero andFábregas 1997, Mata et al 2010).While continuous closed systems are capable of producing more biomass, they are not suited for inducing lipid buildup, which is necessary for the synthesis of biodiesel.Despite substantial study on open and closed cultivation methods, two-stage hybrid systems have received little attention.These systems were put up as useful methods to separate lipid accumulation from biomass development (Olaizola 2000, Schenk et al 2008, Su et al 2011).According to a recent life cycle analysis, hybrid farming has a significantly smaller environmental impact than various open and closed systems (Adesanya et al 2014).In contrast to long-term open pond cultures, a different study on large-scale hybrid cultivation showed the economic viability of PBRs for giving consistent inoculum to short-term batch open pond cultures and reducing system failures (Huntley et al 2015).
Open and closed cultivation systems on a large scale are widely used (Borowitzka 2013).Different species of microalgae have been used in these systems in various countries, including China, Australia, Japan, the United States, Thailand, and India (De Boer et al 2012).Since 1970s, Brazil and the United States have been producing biofuels using corn to develop bio-ethanol, with outstanding results.According to the US Department of Energy's Pacific Northwest National Laboratory, renewable fuels made from algae alone may replace 17% of the world's oil imports, making prospective sources of renewable biofuels incredibly interesting (Roesijadi et al 2008).Many countries around the world, including Europe, Asia, and the Americas, have their own researchbased microalgae initiatives for bioenergy to drive eco-friendly power production (Wang 2013).
Microalgae have the unique ability to survive in extremely harsh conditions, particularly in agricultural locations where seasonal weather variations are unfavorable.As a result, using land as a growth medium is not problematic since wastewater can be used instead of freshwater (Li et al 2008, Raja et al 2008).Unlike other biodiesel raw materials like soybean, rapeseed, sunflower, and vegetable oil, different microalgae species can be kept alive under various environmental conditions (Chisti 2007).Microalgae with a high growth rate have a high output compared to crops, conventional forestry or other aquatic plants.Additionally, less surface area is required than biodiesel feedstocks, which is 49 or 132 times less than soybean or rapeseed, to produce algae biomass with a 30 percent (weight per unit weight) oil content (Chisti 2007).Consequently, competition between cultivated land and other crops, particularly for human consumption, has decreased.Biodiesel, methane, hydrogen, ethanol, and other sustainable fuels can be produced from microalgae (Delucchi 2003), and the use of biomass rather than petroleum products not only overcomes the energy crisis cost-effectively but is also an environmentally friendly process that harnesses water, air, and soil for bioenergy production during the photosynthetic process (Wang 2013).
Numerous algal species with a size range from tiny, single-celled microalgae to multicellular macroalgae, can be found in a number of habitats, including moist areas and bodies of fresh or salt water with.Algae appear to be an appealing energy feedstock among renewable resources, for the quick production of carbohydrates and lipids, by utilizing their high photosynthetic potential.Large-scale cultivation of algae offers the advantage of mitigating greenhouse gas emissions, as it involves the consumption of CO 2 for photosynthesis during the growth process (Ben et al 2019).Additional advantages could include algae utilization as a source of byproducts with economic value or, albeit in a limited capacity, as human food (Roostaei et al 2018).In 2023, Hasnain et al reported that with respect to specie efficiency of biofuel production is different and required different cultural conditions such as climatic parameters, water content and nutrients.Table 1 shows the interactive effect of temperature on the growth of algal species while in our study the temperature conditions are shownso the relevant algal species can be used for cultivation.Like-wise, further research required for a comprehensive understanding to optimize the cultural conditions of different species.The inception of commercial microalgae cultivation can be traced back to the 1950s, starting with the production of Chlorella as a health food in Japan and Taiwan.This was followed by the cultivation of Spirulina (Arthrospira) in the 1970s in Mexico and the USA, Dunaliella salina for β-carotene production in the 1970s in Australia and USA, Haematococcus pluvialis for astaxanthin production in the 1980s in USA, and Crypthecodinium cohnii for the production of polyunsaturated long-chain fatty acid docosahexaenoic acid (DHA) in USA.The two largest commercial algae production facilities in the world located in Australia, in Whyalla, South Australia, and Hutt Lagoon, and Western Australia (Borowitzka et al 2012).
Number of studies have attempted to identify the ideal locations for developing open pond algal biofuel production facilities shown in table 2. Maxwell et al (1985) made the first attempt, using a quasi-GIS method to identify manufacturing sites in the southwest of the US.This approach focuses on physical characteristics like slope and land usage, as well as climatic elements like insolation, temperature, precipitation, and evaporation.The ideal locations were determined by combining weighted input criteria.Lundquist et al (2010) adopted a more complex strategy in California.They used cost-based weighting in an additive model, like Maxwell et al (1985), taking into account physical characteristics and climatic conditions as well as access to the production inputs including saline water, CO 2 , and fertilizers.Location wise algal biofuel production potential was estimated through GIS-based methods by Wigmosta et al (2011), Klise et al (2011), Quinn et al (2012).Notably, Wigmosta and colleagues from the Pacific Northwest National Laboratory (PNNL) (2011) focused on environmental aspects for site selection before modelling production potential based on climate across USA.Although water and nutrient sources were absent from their original evaluation, a more sophisticated web-based GIS tool called the Biomass evaluation Tool (BAT) is being developed to incorporate these elements depending on user-defined parameters (Wigmosta et al2011).In 2014 Boruff and co-workers extend previous research, considering the unique context of Western Australia.Their study incorporated environmental factors, practical aspects like soil workability, and logistical considerations such as transportation and employment opportunities.Similarly, Lozano-Garcia et al in 2019 analyzed potential land for microalgae cultivation in Mexico, while Correa et al 2019 developed criteria for sustainable biofuel production alternatives.2 summarize the GIS based approaches of different studies for targeting suitable sites of microalgae cultivation.
In the context of biofuel production, diligent efforts have been made by scientists.Pakistan's extensive agricultural land, which constitutes 30% of its total area (80 million hectares), provides a promising prospect for biofuel crops, including microalgae.Considering the favorable parameters identified in previous studies (table 2), which significantly impact microalgae growth, this study aims to utilize geographic information system (GIS) to highlight suitable microalgae cultivation sites as a renewable energy solution to address energy challenges in Gwadar district of Pakistan with the estimation of the biofuel production in suitable sites.

Study area
This study focuses on the district Gwadar (figure 1), which is approximately 72 km from the Iranian border and 320 km from Cape al-Hadd in Oman.Of particular significance is Gwadar's proximity to the Persian Gulf, located near the mouth of this crucial body of water and around 400 km from the Strait of Hormuz, through which nearly 40% of the world's oil tankers pass.The whole district Gwadar is divided in five tehsils or subdistricts such as Gwadar, Jiwani, Ormara, Pasni and Sunster (Mengal et al 2021).This district is situated approximately 460 km west of Karachi, near the entrance to the Gulf of Oman and the Persian Gulf (Anwar 2010).While Pakistan boasts a coastline stretching around 700 km and hosting two major ports: Karachi Port and Gwadar Port.Both ports, Karachi and Gwadar, are essential to Pakistan's commercial and economic operations and each makes a distinct contribution to the growth of the country.It's important to realize that every port has unique advantages of its own.Analysing Gwadar and Karachi ports side by side demonstrates their unique benefits and strategic importance.Gwadar Port's location in the southwestern part of Pakistan lends it particular significance.Its proximity to key energy and trade routes positions it as a potential nexus linking South Asia, Central Asia, the Middle East, and beyond.Anchored along the Arabian Sea, Gwadar Port stands nearer to the Strait of Hormuz, a critical conduit for global energy supplies (Gholizadeh et al 2020).This strategic positioning holds the promise of establishing more streamlined trade corridors, consequently reducing transportation distances.An integral component of the China-Pakistan Economic Corridor (CPEC), Gwadar Port stands central to connecting the western region of China with Gwadar through a comprehensive network of transportation and energy infrastructure (Gholizadeh et al 2020).Our project will benefit from the improvement of the road networks that make up Gwadar port's infrastructure, which will result in economic growth.

Material and methods
The primary objective of this study is to pinpoint the appropriate sites for cultivating microalgae, a renewable energy resource, in district Gwadar, Pakistan.To achieve this goal, the methodology employed in this research involves cultivating microalgae sites approach in a manner similar to other relevant studies (table 1) to identify suitable locations.The study aims to determine the ideal orientation of the site with the approximate calculation of the biofuel production from microalgae, which is crucial for optimal growth conditions of microalgae, using Geographic Information System (GIS) techniques.By utilizing GIS, the study intends to identify areas with the most favorable environmental factors, such as sunlight, temperature, and humidity, that are conducive to microalgae cultivation.The material and methods (figure 2) applied in this research will help to identify the most suitable locations for microalgae cultivation in the Gwadar district of Pakistan, which could contribute to sustainable energy development in the region.

Data and its sources
As Geographic Information System (GIS) plays a pivotal role in gathering, managing, analyzing, and visualizing spatial data (Kenneth et al 2015).In this study, we collected raw climatic data from the Global Weather Data for Soil and Water Assessment Tool (SWAT) provided by the National Centers for Environmental Prediction (NCEP) in 2016.The daily climatic data was averaged and the parameters such as temperature, precipitation, solar radiation, relative humidity, wind speed, and elevation were used to identify the ideal climatic conditions required for the growth of microalgae.
In addition to climatic data, water bodies and metropolitan waterways were also considered since water is a crucial factor in the growth of microalgae.Figure 3 displays the location of targeted water resources.To identify suitable sites for microalgae cultivation, we employed various GIS techniques.These techniques included image classification (supervised classification), Inverse Distance Weighting (IDW) and weighted overlay analysis for suitability mapping.By employing these techniques, we were able to identify the most suitable locations and assess the potential of different sites in Gwadar district for microalgae cultivation.

Image classification
In this study, the Landsat 8 image was used, obtained from Earth Explorer to monitor the land surface reality.The Landsat 8 satellite image has a resolution of 30 meters, was chosen as it is a valuable source of information in locating the optimal production sites for microalgae based on accessible land.The supervised classification technique was employed in ArcGIS to categorize the image into four distinct classes including water, vegetation, populated/built-up land, and non-built-up land.It should be noted that the eastern side of the Gwadar district is densely populated, while the southernmost part has a coastal line.However, the maximum land surface of the study area is still undeveloped, as reported in previous studies (Gholizadeh et al 2020).Figure 3 depicts the supervised classification of the study area, which is a crucial component in identifying the suitable sites for the cultivation of microalgae.
Furthermore, GIS data was also used to extract the information of the study area's land use and land cover.The classification of the Landsat 8 image provides essential information regarding the distribution of various land covers and assists in locating suitable areas for the cultivation of microalgae.The study area's land cover information was analyzed using the supervised classification technique, and the results were verified using ground truth data collected from the study area.This technique efficiently classify land use and land cover information, providing crucial input data for identifying the potential sites of microalgae cultivation (Nnam et al 2013).

Inverse distance weighting
In order to identify suitable locations for the cultivation of microalgae in the Gwadar district, this study utilized the Inverse Distance Weighting (IDW) technique.The IDW technique is a commonly used spatial interpolation method in GIS that is used to estimate values for points based on the surrounding measured values.The IDW technique was applied to the various data parameters such as temperature, precipitation, solar radiation, relative humidity, wind speed, and elevation.
The IDW technique produced layers of data parameters that were visually represented using color-coded maps.These maps provided a visual representation of the spatial distribution of the various data parameters across the study area.By combining the various IDW layers, the study was able to identify the most suitable locations for microalgae cultivation based on the ideal climate conditions required for its growth.Overall, according to Liu et al 2021 the IDW technique was an effective method for producing the necessary layers of data parameters and visually representing them in a way that allowed the study to identify the most suitable locations.

Temperature
Microalgae cultivation is influenced by various environmental factors, and temperature is one of the most important parameters affecting algal growth and productivity.In both open and closed systems, temperature plays a crucial role in regulating the metabolism and photosynthetic activity of microalgae.Although extremely high temperatures can cause damage to the cultivation, microalgae can tolerate temperatures that are up to 15 °C below the ideal range (Mata et al 2010).In closed systems, overheating can be a significant issue, particularly on hot days, where the reactor's internal temperature can rise up to 55 °C.To mitigate this problem, an evaporative water-cooling system can lower the temperature by approximately 20 °C-26 °C (Moheimani 2005).
The ideal temperature range for algae strains used in large-scale culture is between 20 °C and 35 °C (Lundquist et al 2010).Furthermore, specific strains may have different temperature requirements for optimal growth and biomass production.In this regard, it is crucial to identify and select the appropriate strains for cultivation under specific temperature conditions.
Recent research has focused on creating a raster layer of temperature to understand the temperature variations within a region.For instance, the data values obtained in a research region indicated that the temperature ranges from 22.3418 °C to 27.2025 °C (figure 4).These temperature conditions are within the ideal range for the growth of microalgae and can be leveraged to maximize algal productivity and biomass yields (Wen and Johnson 2009).

Precipitation
Environmental conditions in which microalgae are grown can significantly impact their growth and productivity.Closed photobioreactors provide more controlled environmental conditions than open ponds, making them less sensitive to weather variations.In contrast, open ponds are more susceptible to changes in weather conditions such as the amount of daylight available and temperature, which can have direct seasonal and yearly impacts on algae productivity.Additionally, precipitation, evaporation, and other weather parameters can affect water supply and water quality, further impacting microalgae cultivation (USDOE 2010).
Precipitation is a critical weather parameter that can impact microalgae growth, particularly under different climatic conditions.Researchers have utilized precipitation data to create GIS layers to understand the spatial and temporal variations in precipitation across a given area.For instance, the precipitation conditions of the study area are ideal for microalgae growth, with an annual precipitation range of 71.6202 mm to 104.553 mm (figure 4) (Mata et al 2010).
Furthermore, the water source used for microalgae cultivation is crucial, and the quality and quantity of the water must be considered.In this regard, the use of wastewater as a nutrient-rich water source for microalgae cultivation has gained attention due to its potential for sustainable and cost-effective algal production (Liu et al 2021).
Briefly, environmental conditions in which microalgae are grown significantly impact their productivity and growth.Use of closed photobioreactors provides more controlled environmental conditions, while weather conditions such as precipitation, temperature, and daylight availability impact open pond productivity.
Precipitation data can be leveraged to understand the spatial and temporal variations in precipitation across a given area, which can aid in identifying ideal locations for microalgae cultivation.

Relative humidity
The relative humidity of the air surrounding microalgae is an essential factor that can impact their growth and productivity.According to Chisti (2008) and Lundquist et al (2010), relative humidity is a crucial factor in the cultivation of microalgae.It has been found that a high relative humidity can cause condensation on the algal cells, which can negatively impact their growth and ultimately reduce productivity (Chisti 2008).Therefore, controlling the relative humidity during microalgae cultivation is critical for optimal growth and biomass production.
The study region's relative humidity was determined using GIS to create a raster layer, which ranged from 43.1041% to 66.1065% (figure 4).This information can be useful in identifying suitable locations for microalgae cultivation, as it is essential to ensure that the relative humidity remains within a range that promotes optimal growth and productivity (Lundquist et al 2010).
In addition to relative humidity, other environmental factors such as temperature, light intensity, and nutrient availability also play crucial role in microalgae cultivation.By understanding how these factors interact and impact microalgae growth, researchers can optimize the conditions for biomass production and improve the efficiency of microalgae-based bioprocesses.

Solar radiation
Solar radiation is a crucial factor that directly affects the growth and productivity of microalgae.It plays a significant role in microalgae cultivation systems by providing energy for photosynthesis and influencing metabolic processes.According to Borowitzka et al (2012) and Moheimani (2005), solar radiation is one of the most important environmental factor in microalgae cultivation, and its intensity and duration can have a direct impact on microalgae production.
In this study, the solar radiation parameter was created as a raster layer using IDW interpolation.The values of solar radiation in the study region ranged from 22.4896 MJ m −2 to 22.9926 MJ m −2 (figure 5).These values are considered appropriate for the development of microalgae and can be used to identify suitable locations for microalgae cultivation (Borowitzka et al 2012).
Furthermore, it is important to note that the intensity and duration of solar radiation can vary based on various factors, such as location, time of day, and weather conditions.Therefore, it is necessary to monitor and control the amount of solar radiation during microalgae cultivation to optimize the growth and productivity of the microalgae (Lundquist et al 2010, Sarker 2022).
In brief, solar radiation is a critical environmental factor that directly impacts microalgae cultivation and productivity.By understanding how solar radiation influences microalgae growth, researchers can optimize cultivation conditions to improve the efficiency of microalgae-based bioprocesses.

Wind speed
Wind speed is an important environmental factor that affects the growth and productivity of microalgae.According to Wigmosta et al (2011) andQuinn et al (2012), wind speed is an essential parameter to consider in microalgae cultivation because it influences the mixing of the culture medium, gas exchange, and temperature control.To identify suitable locations for renewable generating sites in the Gwadar district, a GIS-based wind layer was created.The study area's wind speed data ranged from 3.36742 m s −1 to 5.1077 m s −1 (figure 5).
Extremely high wind speeds, such as those experienced during powerful wind storms, can have negative impacts on microalgae farming.Literature reported that high wind speeds can cause the loss of microalgae biomass and disrupt the mixing of the culture medium, which can lead to reduced growth rates and productivity.However, the wind speed data in the study area is appropriate and suitable for microalgae cultivation, as it falls within the acceptable range for microalgae growth.
In short, wind speed is an important environmental factor that affects microalgae cultivation, and its influence on the mixing of the culture medium, gas exchange, and temperature control must be considered.The GIS-based wind layer created in this study provides valuable information for identifying suitable locations for renewable generating sites in the Gwadar district.The wind speed data in the study area is within the acceptable range for microalgae growth and has no negative effects on microalgae farming.

Elevation
The elevation of a site is an important factor to consider when selecting a location for microalgae cultivation.According to several studies (Li et al 2008, Wang et al 2013), elevation can have a significant impact on the microclimate of a location, including temperature, precipitation, and wind patterns.In this study, we utilized the IDW technique to generate a raster layer of elevation data for suitable microalgae cultivation sites in the study area.The elevation in our study area ranges from −9323.44 m to 744.825 m (figure 5).However, literature suggests that lower elevations may be more preferable for microalgae cultivation as they are associated with higher temperatures and greater access to water (Chisti 2007, Li et al 2008).Therefore, we recommend selecting sites with lower elevations within the study area for optimal microalgae cultivation.

Weighted overlay analysis
In this study, the weighted overlay analysis technique was employed to identify the most suitable locations for microalgae cultivation based on favorable growing conditions.The weighted overlay approach is a widely used method for overlay studies as it generates an integrated analysis for various input criteria by applying common scale values.To determine the suitable locations, all the relevant data parameters, including elevation, relative humidity, precipitation, temperature, solar radiation, wind speed, waterways, and water bodies, were processed and prioritized according to their suitable ranges.
Additionally, the supervised classification of the Landsat 8 image of the study area was incorporated into the analysis to enhance the accuracy of the results.The integrated analysis was then classified into four categories, namely most suitable, moderate suitable, less suitable, and non-favorable sites, based on the target requirements.The resulting map from the weighted overlay analysis highlights the most favorable locations for microalgae cultivation in the Gwadar district of Pakistan.This information can be useful for decision-makers and stakeholders involved in planning and implementing renewable energy projects in the region.

Results and discussion
Our study demonstrated that GIS techniques are a valuable tool for evaluating optimal cultivation sites for microalgae as a sustainable biofuel energy resource.Geospatial analysis incorporated various environmental and geographical factors to identify the potential sites for microalgae cultivation (Borowitzka et al 2012).Our study is consistent with previous researches as documented in table 2 on microalgae cultivation that highlights the importance of environmental factors.Findings of our study and criteria used in the analysis aligned with earlier studies such as Boruff et al 2015, Correa et al 2019, Wigmosta et al 2011, Quinn et al 2012 by using environmental and geographical factors including climatic parameters, water bodies, water ways, elevation and land surface analysis to identify the suitable cultivated sites and their biofuel potential from microalgae in Gwadar district.
Findings of this investigation demonstrated that the highest capacity for microalgae cultivation resided in the western sector of the study area.This specific region exhibited advantageous environmental factors including abundant solar radiation, moderate temperatures, and readily available water resources.Furthermore, analysis highlighted that the Gwadar district boasted an expansive coastline area with exceptionally conducive climatic conditions, rendering it an optimal site for microalgae cultivation.Comparable observations were made by Borowitzka et al in 2012 and Boruff et al in 2015, who identified suitable microalgae cultivation sites along the coastal regions of Western Australia.
Our assessment categorized microalgae cultivation suitability into four classes, ranging from most suitable to less favorable (figure 6).The Gwadar district with subdivisions Gwadar, Jiwani, Ormara, Pasni and Sunster (Mengal et al 2021).Sites that are most suitable for the cultivation of microalgae, referring to figure 6 and table 3. Areas including in the most suitable sites located in the west of Gwadar district namely; Gwadar, Sunstar, and Pasni, comprising an area of 6403.42 km 2 that would produce 1.28 × 1011 g km −2 yield.This whole region favors microalgal growth substantially.The largest portion of the Gwadar district is classified as having moderate suitability, Omara, Pasni and Jiwani which take over an area of 12032.04km 2 are also moderately suitable for microalgal growth i.e., 2.41×10 11 g km −2 biofuel production per day.This area comprises maximum area of Gwadar district, making it an optimal site for growth and cultivation of microalgae.It is noted that less suitable sites are extremely rare and are located mainly in the extreme east of the study area.Table 3 comprehensively describe the biofuel production within these categorized locations also highlighted in figure 6.But the principal constraint for microalgae cultivation is the presence of populated or developed regions, which are classified as non-favorable.
Variables influencing the total production cost of microalgae encompass factors such as the choice of algal strain, the specific photobioreactor (PBR) type, and the technology employed for biomass generation.If the installation is substantial, the cost of construction will be significantly high (Ma et al 2017).One viable strategy to substantially decrease the production costs is to utilize wastewater instead of adding minerals or nutrients through growth media (Roostaei et al 2018).This approach can lead to the production of over 150 tons per hectare per year of algal biomass (Norsker et al 2011).Moreover, using flat panel tubular PBRs rather than open ponds is another approach to reduce costs, although this will only lower costs by up to €1.28/kg.While using this technology, the production cost can be effectively reduced to €0.70 or €0.68 per kilogramme (Negi et al 2020, Oostlander et al 2020).Arable land and high-quality irrigation are not recommended, the utilization of saltaffected or marginal land, including desert regions, along with saline or wastewater, remains a viable option and it could be used if suitable strains of algae are available (Amiri and Ahmadi 2020).
Briefly, our study could be useful for policymakers, investors, and researchers to identify the best locations for the development of sustainable and cost-effective biofuel production systems.Future research can build on these findings by incorporating additional factors such as economic feasibility, social acceptability, roads network representation for the transportation of biofuel from the cultivated sites, labor availability and inclusion of carbon dioxide and nutrients for inducing the growth of microalgae to further refine the suitability analysis for microalgae cultivation.Finally, introducing open and closed pond cultivation systems to the designated locations represents a substantial undertaking for future research endeavors.

Conclusion
In conclusion, this research utilized GIS methodologies to assess the appropriateness of locations for large-scale microalgae cultivation as a sustainable biofuel energy source within the Gwadar district of Pakistan.The analysis highlighted suitable sites based on climatic variables, the presence of water bodies, waterways, and land surface characteristics, utilizing GIS techniques.Our findings revealed that the western region of the study area exhibited substantial potential for microalgae cultivation, while areas with high population density were deemed less suitable.Furthermore, the estimated biofuel production levels at the most suitable and moderately suitable sites were notably high.This holds significant importance in addressing energy shortages, bolstering economic stability, and generating employment opportunities in Pakistan.As Gwadar district has economic significance within the context of the CPEC, the production of biofuel from these identified sites also holds promise for international exportation of biofuel.This study not only underscores the immense potential of microalgae as a renewable energy source but also equips planning and development departments with a valuable tool for devising projects that optimize resource utilization.
(GIS) techniques designed to manage spatial data, have diverse applications in planning, telecommunications, engineering, and more (Kenneth et al 2015, Maliene et al 2011).On a large scale, open and closed pond microalgae cultivation system through GIS based suitability analysis was conducted by Klise et al 2011, Quinn et al 2012, Wigmosta et al 2011.Table

Figure 3 .
Figure 3. Water ways, water bodies and supervised classification of study area (source: author).

Figure 4 .
Figure 4. Temperature, precipitation and relative humidity of study area (source: author).

Figure 5 .
Figure 5. Solar radiation, wind speed and Elevation of Study area (source: author).

Figure 6 .
Figure 6.Suitable sites of study area for microalgae cultivation (source: author).

Table 1 .
Interactive effect of temperature on the growth of algae strains (Source: Abideen et al 2023).

Table 2 .
Showing the approaches of different researches.