3.1. Basil Subsystem

Table 1 shows a summary of sweet basil varieties and cultivars reported in 88 articles in addition to the number of times they were reported. Interestingly, 58 out of 88 peer-reviewed publications did not report the variety name or cultivar type. Of the publications that used sweet basil, Genovese was the most commonly used variety (21 articles), followed by Italian large leaf (six articles) and purple ruffles (four articles) varieties. Three peer-reviewed articles reported the following species; holy basil (Ocimum tenuiflorum), lime basil (Ocimum Americanum), and lemon basil (Ocimum africanum). Nine publications reported only “basil” identifying neither the species name nor the variety/cultivar type. Among the reviewed publications, only four articles compared different basil varieties/cultivars. Ferrarezi and Bailey [10] assessed five diverse cultivars of basil in the UVI outdoor commercial aquaponics system in the summer and seven different cultivars in the fall. They concluded that basil yield was higher in summer (May–August) than in the fall (September–November). The authors stated that they visually observed the bolting occurrence in some cultivars throughout the harvesting period of 4 weeks. They explained that the more reoccurring trimming to avoid bolting possibly resulted in stress for the plant and consequently reduced their yield. Morphological characteristics including branching change when plant shoots are harvested multiple times in the cycle may cause variations in their productivity [28]. These authors noticed that basil productivity reduced in the last month of the summer, and they explained that this reduction could be because of the plant senescence due to multiple harvests. Hence, they recommended harvesting basil after 3 months in summer and restarting another cycle. They also reported that between all cultivars, Genovese and spicy globe produced the highest yields in the two seasons (summer and fall) within the geographical and environmental conditions of that study. Further research comparing the different basil varieties or cultivars of the same species and/or different species in aquaponic systems would be a valuable consideration.

Figure 1 illustrates the different hydroponic component types used to cultivate basil in aquaponic systems. From the 100 reviewed publications, 91 articles provided the type of hydroponic component used in their aquaponic system. Most of these papers (72 articles) used one hydroponic subsystem type to grow basil, and 19 articles compared two or more hydroponic components (20.8% of the total number of publications). Between the reviewed publications, 38.7% used deep water culture (DWC), 31.1% used media bed (MB), and 17.9% used nutrient film technique (NFT) to grow basil. Only 12.3% of the reviewed publications used different types of hydroponic systems such as dynamic root floating (DRF), aeroponics (AERO), A-frame ebb and flow, tray ebb and flow, perlite slabs, and floating in the fish system. Aslanidou et al. [29] reported that the most frequently used plant grow beds have been DWC, MB, and NFT, which is in agreement with the current review. Those authors pointed out that the substrate method used in the hydroponic unit of aquaponic systems is a matter that requires further investigation.

Multiple published papers that cultivated basil in aquaponic systems did a comparison between some of the above-mentioned hydroponic components; nevertheless, the results commonly differed from article to article. When comparing basil yield from the different hydroponic components of the different articles, differences in environmental conditions, and aquaponic and/or filtration systems, and different experiences in running an aquaponic system could explain the basil yield variability [30]. Knaus et al. [1] cultivated sweet basil in northern Germany and compared the yield from three diverse hydroponic components; an ebb-and-flood gravel substrate (MB), raft (DWC), and grow pipes (NFT). They found that after 41 culture days, there was no significant difference in the dry and wet weights between the subsystems. However, basil was significantly longer in the MB subsystem (101.8 ± 8.3 cm), next where the grow pipes (96.7 ± 7.0 cm), and lastly DWC (94.8 ± 8.6 cm). When compared with other plant crops, basil requires a higher amount of water for optimal growth in addition to a suitable drainage period [31]. The largest plant biomass occurred with a flooding interval of every 4 min using the ebb-and-flood gravel subsystem. These authors explained that the increased air contact during the dewatering phase could have provided additional nitrification through the beneficial bacteria activity (e.g., Nitrosomonas spp., Nitrobacter spp.) in the media. The substrate could have performed as a biofilter with supplemental nitrification and directly produced nitrate at the interface of the water-plant root [15]. Additional studies evaluating basil productivity in different types of hydroponic components and comparing different varieties/cultivars of basil are critical requirements.

3.2. Fish Subsystem

Figure 2 shows that tilapia was the most commonly cultured species with basil in aquaponic systems (44%), followed by catfish (14%) and carp (9%). Liang and Chien [32] stated that the foremost fish species produced in an aquaponics system was Nile tilapia. DWC and MB plant grow beds were the most common hydroponic subsystem used for cultivating basil with tilapia and carp, whereas NFT and DWC were the most commonly used with catfish.

Criteria used for the selection of aquatic organisms cultured in aquaponic systems include the availability of juveniles, the value of the species of interest, ease of culture, and tolerance of suboptimal water quality factors [33]. The number of aquatic organisms cultured with basil was divided into three categories: human consumption, ornamental, or conservation (total peer-reviewed publications number 93, Figure 3). Of those 93 papers, 86% of the cultured organisms were for human consumption, 13% were ornamental organisms, and 1% were for conservation purposes. Although the price of ornamental species is relatively high compared to fish for human consumption and has increased significantly throughout the last decade, Maucieri et al. [34] stated that research using ornamental species in aquaponic systems is lacking. In 2021, the market of ornamental fish was valued at $5.4 billion; however, a sudden reduction in exports of ornamental fish by 13.1%, from $390 to $339 million, occurred between 2021 and 2022. Currently, the ornamental fish trade represents about 0.0014% of total world trade, and their market is expected to increase at a compound annual growth rate (CAGR) of 8.5% between 2022 and 2030 [35]. The production of ornamental species in aquaponic systems has enormous potential for higher income and productivity with minimum requirements of water, land, labor, and additional inputs.

In contrast to food-grade fishes, a limited number of ornamental species have been used in aquaponic systems, which mostly include koi, Cyprinus rubrofuscus [33], koi, C. carpio [36, 37], and goldfish, C. auratus [11, 36, 38]. Among these species, goldfish and koi (C. rubrofuscus var. “koi”) from the family Cyprinidae are the most popular and draw aquaculturists, hobbyists, and researchers’ attention all over the world [33], which agrees with the current review findings. This could be explained by their resilient nature, which makes them an excellent laboratory species besides being an aquarium species. It has been reported that goldfish and koi are resistant to different water quality levels, along with their fast growth ability and high reproduction ability, have made them the perfect ornamental candidates for the culture with plants in aquaponic systems [33, 39]. Further research on the optimal productivity of ornamental species in aquaponic systems with basil and other crops is needed. In addition, intensification of ornamental fish production is required to meet the rising demand from hobbyists and ornamental fish markets.

3.3. Multiple Crop Evaluations

Of the reviewed publications, 93 articles provided the basil species, variety, and/or cultivars grown in aquaponic systems (Figure 4). Among those publications, 55.9% cultured one aquatic organism in conjunction with basil, followed by 41.9% that cultured one aquatic species while growing additional crops with basil. The percent of articles that evaluated more than one species in combination with basil was 8.6%, and 5.4% cultured more than one species while producing basil with one or more crops. The choice of fish species and cultivated plants could impact the stability of an aquaponic system, especially coupled systems. Nevertheless, research on different fish or crustacean species’ influence on plants is scarce [40].

Palm, Bissa, and Knaus [41] cultured two fish species (O. niloticus and C. gariepinus) with basil and other crops including lettuce, cucumber (Cucumis sativus), and tomato (Solanum lycopersicum) in four plant boxes (2 m² each) and reported higher growth of plants in conjunction with tilapia. After a 92-day cycle, basil yielded 159.00 ± 96.91 g when cultured with Nile tilapia, as opposed to 117.94 g ± 92.30 g when cultured with African catfish. It appears the choice of fish or crustacean species has an effect on plant yield as well as aquaponic system stability.

Different basil cultivars can be grown for diverse markets, such as a culinary herb for dry or fresh spices, pharmaceutical companies for the extraction of their essential oils, or ornamental plants [10, 28]. In the current review, only 3.2% of the publications compared more than one basil variety or cultivars of the same species. A comparison between different basil cultivars in aquaponic systems with different fish or crustacean species would be a valuable consideration.

3.4. Feed Amount as a Function of Grow Bed Area

Figure 5 depicts the feed amount (in grams) per square meter of plant grow bed area per day in the aquaponic system (total number of papers 29). A few articles used multiple fish and/or hydroponic component types. Two out-of-range values were not included in Figure 5 (336 and 366 g/m² grow space/day; [42, 43]). For tilapia, the amount of feed per square meter of grow space per day ranged between 20.3 and 81.6 g. Amount of feed per square meter grow space per day values ranged between 29.2–68.9 g feed/m²/day when culturing Pangasius, 20–25 g feed/m²/day when culturing catfish, and 4.4–16.9 g feed/m²/day when culturing carp in an aquaponic system with basil. A minor number of publications were found studying eel, barramundi, and axolotls in aquaponic systems. The number of papers using different types of hydroponic components with the quantity of feed per square meter of grow bed area per day regardless of the aquatic species in the aquaponics system is presented in Figure 6. Values ranged between 11.8 and 71.5 g feed/m² for DWC, 18.3–112.6 g feed/m² of grow area per day when using MB, and 2.6–67.5 g feed/m² of grow area per day when using NFT. In a recent review of tilapia and lettuce in aquaponic systems, the amount of feed offered to tilapia varied with the type of hydroponic component [44]. Research has shown that the hydroponic system type can influence plant growth and nutrient uptake efficiency. A review by Somerville et al. [45] on aquaponics discusses how different system designs impact plant nutrient availability and overall system performance. However, it remains unclear if the difference in feed input values in the published papers signifies the plant subsystem requirements or is dependent upon the fish density and feed consumption.

Alarcón-Silvas et al. [46] argued that the feeding rate ratio (FRR; g feed/m²/day) data for diverse aquaponic systems are usually not comparable due to variations in the experimental conditions, including size and growth phase of aquatic organism, their densities, nutritional requirements and dietary nutrient concentrations, and crop species. Delaide et al. [47] proposed a FRR of 42 g feed/m²/day for a tilapia-lettuce aquaponic system, which is lower than the amount recommended by UVI (57 g feed/m²/day; [26]). Mariscal-Lagarda et al. [48] used a FRR of 24 g feed/m²/day for a shrimp–tomato aquaponic system. This variation suggests the need for establishing a recommendation for a species-specific optimal feed input that supplies sufficient nutrients for specific crops, which can serve as a basis for future research. Furthermore, future studies on feed input, nutrient budgets, and system modeling under controlled conditions are necessary to distinguish between plant subsystem requirements, fish needs, and density effects.

3.5. Abiotic/Environmental Factors

Temperature is one of the major factors that affect the growth and development of plants and aquatic organisms in a controlled environment [45, 49]. The majority of vegetables have a wide range of suitable temperatures from 18 to 30°C. However, it has been recommended that the optimal water temperature for basil growth is 20.0–25.0°C [9, 10, 28], and the maximum recommended temperature is 30.0°C [45]. Data in Table 2 indicates basil evaluations have been conducted within a large range of water temperatures (17.4–31.6°C). Some of the recorded variation may be attributed to meeting the thermal optima of the aquatic organisms. Nevertheless, about 90% of the reviewed publications reported experimental temperatures between 23.6 and 26.6°C, which indicates that the majority of publications were conducted within the recommended range for cultivating basil.

Somerville et al. [45] stated that warm-water species such as tilapia, catfish, and common carp, as well as the nitrifying bacterial community in the system, grow well in elevated water temperatures of 22−29°C, along with many of the common vegetables like basil. Hendrickson et al. [72] compared the growth of 17 cultivars of basil (Italian large leaf, Spicy bush, Amethyst, Sweet Thai, Red rubin, Prospera, Cardinal, Nufar, Cinnamon, Karpoor tulsi, Elide lemon, Dark opal, Rutgers devotion, Lime, and Genovese) from six main types at three different temperatures 23, 27.5, and 31°C to determine the optimal temperature for growing different basil types hydroponically. They concluded that the yield of all basil cultivars except Italian large leaf and purple basil grown in hydroponic systems was higher when cultivated at a water temperature of 27.5°C. The highest yield for Italian large leaf and purple basil was found to be variable between temperatures of 23 and 31°C. Hence, basil is considered a heat-tolerant crop [28]; however, it is considered sensitive to temperatures below 10°C, which has been shown to damage the developmental and growth processes [73]. Basil has been cultured with both warm-water species (tilapia, catfish, carp, seabass, and shrimp) and cool—and cold-water species (trout, sturgeon, and perch). Considering this wide range of cultured aquatic organisms with basil, the optimum temperature determination for species-specific culture with different varieties of basil is a critical necessity to enhance the production of both species.

In coupled aquaponic systems, water flow rate is one of the factors that affect the physiological requirements for optimal fish and crop production, energy consumption, and nutrient loading. The water flow rate in the aquaponic systems ranged between 0.05 and 400 L/min in the reviewed literature. However, about 90% of the reviewed publications reported a flow rate between 6.3 and 71 L/min (Table 2). Most of the articles that used MB applied intermittent flow using the flood and drain techniques except two articles that used a pump inside the grow bed. Water flow rate directly influences the hydraulic loading rates (HLRs) for optimum production of different fish and plant species in an aquaponic system [74, 75]. Endut et al. [74] stated that the HLR (m/day) of an aquaponic system can be calculated by “dividing the flow rate of water, Q (m³/day) through the system by the surface area of the grow bed, A (m²). Angkha et al. [76] compared four HLR (0.72, 1.44, 2.16, and 2.88 m/day) in an aquaponic system and reported the effects on the performance of tilapia and basil. They found that tilapia and basil production were the highest at an HLR of 1.44 m/d compared with the other treatments. They also found that the highest nitrate (42.25 ± 1.56%), potassium (40.44 ± 1.270%), and phosphate (64.80 ± 2.011%) removal percentages were observed at HLR of 1.44 m/d. Based on those findings, these authors recommended an HLR of 1.44 m/d for the culture of Nile tilapia and holy basil in aquaponic systems. It has been reported that the optimal flow rate in aquaponics depends on the specific requirements of the fish, plants, and the system design. It balances the delivery of oxygen, nutrient supply, and removal of waste [3, 15, 45]. Further investigations are required to identify the optimal water flow rate and HLR for the cultivation of basil with different aquatic organisms in aquaponic systems. Additionally, studies on how the flow rate influences nutrient uptake and fish health under varying environmental conditions (e.g., temperature, pH) are a crucial requirement.

3.6. Basil’s Economic Value

In an international survey by Love et al. [77], 70%, 69%, and 64% of the survey respondents cultivated basil, tomatoes, and salad greens, respectively, in their aquaponic systems. In contrast, the most frequently grown crops in aquaponics systems recently recorded by industry-wide survey respondents were lettuce (83%), and leafy greens (81%), followed by basil (73%; [78]). Engle and Beem [79] compared the production and selling prices of basil and lettuce from UVI publications and other literature and reported that the production price was $6.15–$12.40/case of lettuce (24 heads of lettuce weighing 20–26 pounds) and $0.75/lb for basil cultivated in an aquaponic system. The market price is $20/case of lettuce (~$0.9/lb) and $10.20/lb of basil, suggesting that basil is a more profitable crop. Lobillo-Eguíbar et al. [80] stated that basil and lettuce represented more than 10% of the total market value of both vegetable/fish aquaponic production (the former because of its high price and the latter because of its high production). However, aquaponic producers still grow lettuce and other leafy greens more frequently than basil despite its higher profitability due to market demand and consumer preference, growth cycle considerations, and economic factors. In terms of market demand and consumer preference, lettuce is versatile and has a more established global market due to its popularity in salads and ready-to-eat meals such as sandwiches and wraps [81], while basil is more niche, primarily used as a culinary herb or for specific recipes [82]. Additionally, lettuce has a relatively shorter growth cycle, typically reaching market size in 30–45 days in aquaponic systems [83], compared to basil, which takes longer to mature (typically 50–70 days), which means fewer harvests per year [6]. Lastly, while basil can be more profitable due to its higher market value, the overall economic risk is higher. Lettuce, being less expensive per unit but much more reliable in terms of production volume, hence providing a steadier cash flow for producers. Given the shorter growth cycle and fewer risks associated with growing lettuce, it often represents a more stable financial investment.