Agar is an important gelling agent used for many applications in various industries, including the pharmaceutical, food, cosmetics and sciences (El Gamal, 2010; Kammler et al., 2024). The only sources for agars are a limited number of red macroalgae (seaweeds which include seaweeds from the Gelidium, Pterocladiella and Gracilaria genera (Armisen, 1995; Pangestuti and Kim, 2015; Santos and Melo, 2018; Lomartire and Gonçalves, 2023). Currently, a considerable portion of the worldwide supply of these seaweeds comes from collection of wild seaweed stocks. This causes agar supply—for the ever-growing global demand—to fluctuate together with the availability of these wild seaweeds. Water inlet jets were formed by eds (Santos and Melo, 2018). For establishing a constant supply chain of seaweeds for the agar industries, a shift towards mariculture of agarophytic red seaweeds is required. Of all the agar producing seaweeds, the genus Gracilaria has been demonstrated to be the most successful in terms of mariculture (Friedlander et al., 1987; Levy et al., 1990; Friedlander, 2001; Pereira and Yarish, 2008). Different Gracilaria species can grow in almost every ocean or sea in the world, as some species are more adapted to cold temperatures, while others are more tropical. Gracilaria can grow vegetatively and there is no need to establish sophisticated hatchery/nursery facilities for propagation through a sexual life cycle (Bast, 2014). Small cuttings from the tips of the Gracilaria thallus can be treated for epiphyte removal and growth enhancing stimulants and grown in small volumes until planting in the field conditions (Bhagiya et al., 2025). Around 4 million tons of Gracilaria are cultured yearly worldwide (FAO, 2020). Most of the growth is conducted in offshore facilities with the seaweeds tied manually to submerges long lines, baskets, or other floating devices (Bast, 2014; Veeragurunathan et al., 2019; Shah et al., 2021). Growth in offshore conditions leave the seaweeds exposed to constant fluctuations in its abiotic and biotic surroundings. Abiotic changes include diurnal and seasonal changes in ambient light and temperature regimes, as well as other, random events such as water flow rates and turbulence, nutrient availability and water pollution outbreaks. Biotic fluctuations can include the presence of herbivore organisms, pathogenic microorganisms and viruses, and competition with epiphytic or allelopathic algae.

Light regimes and water motion are important factors influencing seaweed growth. Light is of course important as the energy source for photosynthesis (Falkowski and Raven, 2007)Water motion controls the thickness of boundary layers forms around the seaweeds so that in general, faster relative water movement against the seaweed body will result in thinner boundary layers and higher diffusion rates of nutrients and gases into and from the seaweeds (Gonen et al., 1993; Friedlander and Levy, 1995; Hurd, 2000). High nutrient exchange rates are beneficial for rapid seaweed growth.

Offshore commercial growth of seaweed relies solely on ambient sunlight regimes. Daylight length varies between seasons and the light intensity is very influenced by clouds. In-land tank cultivation of seaweeds allows for better control over the light regimes, either through covering seaweed growth tanks or pools with shading nets, or through addition of light sources. However, illumination comes at a financial cost both for the equipment purchase and for the operational cost of electricity. It is thus necessary to investigate the right balance of illumination addition to growth and agar yields.

Land-based systems also allow for control over the water motion around the seaweeds (Gonen et al., 1993; Friedlander and Levy, 1995). Currently water turbulence is created in such systems either by the use of paddle wheels or through rigorous air bubbling (Friedlander and Levy, 1995). Paddle wheels are considered efficient at water displacement, but their mechanical nature makes them prone to cause physical damage to the seaweeds (Friedlander and Levy, 1995). Air bubbling is efficient yet has considerable infrastructure and operational costs (Ben-Ari et al., 2014).

In this work we built a novel land-based tank system for seaweed cultivation. Using this system, we studied the effects of artificial light addition and of a water inlet spray system on the growth and agar yields of G. cornea towards designing a cultivation protocol with increased seaweed yields, decreased operational costs, and thus added commercial value.

Improving cultivation techniques for agar producing seaweed species is of great importance for a sustainable global production of agar (Santos and Melo, 2018). Cultivation of Gracilaria sp. in land-based systems can allow for better control over ambient parameters such as light regimes and water turbulence, as compared to growth in offshore conditions where the seaweeds are exposed to fluctuating natural conditions.

Since seaweeds are photosynthetic organisms and rely on light energy for their growth it is important to measure the effect of added light on their performance and define its parameters. In our setup it was apparent that adding only 4 extra hours of illumination at medium intensities (500 μmol photons m^-2 s^-2) to the naturally occurring 10-hour daylight regime was not enough to significantly increase the growth rates of G. cornea. On the other hand, the addition of light throughout the entire daytime coupled with extra 4 hours of illumination did increase growth rates in a significant matter. This implies that although G. cornea under our ambient condition was exposed to irradiance of around 900 μmol photons m^-2 s^-2 during daytime, it was not yet saturated with light, and thus the addition of another 500 μmol photons m^-2 s^-2 of illumination increased its productivity. This is of special interest since current literature suggests that Gracilaria sp., including G. cornea, is already usually light-saturated at much lower intensities around ~365 μmol photons m^-2 s^-1 (Reviewed in Salomon, 2025). This can perhaps be attributed to the different experimental photosynthetic rates measurements setups performed in the different reviewed reports and needs to be studied in more detail. It is also interesting that exposure to full sunlight was harmful to the seaweeds, and thus a shading net was required, whereas adding the artificial illumination was helpful. This means that to increase productivity we had to partially shade the sunlight while simultaneously adding more light. Several reasons for this paradox may be made. First, it is possible that the harm for the seaweeds results from the UV spectrum of the light, which is not found in the LED illumination systems. UV exposure may create cellular damage, especially in long exposures. In addition, the direct impact of the sun light on the growth tanks may cause them to heat. In our system this does not seem to be the problem since water temperatures remained stable throughout the day due to sufficient water replacement rates. Finally, it is perhaps possible that the maximal photosynthetic rates of G. cornea in our cultivation systems occur at light intensities around 1,400 μmol photons m^-2 s^-2. Thus, illumination with only 900 μmol photons m^-2 s^-2 (under the shading net) or with direct sunlight (~2000 μmol photons m^-2 s^-2may result in decreased photosynthetic rates and limited growth.

No effect on agar yields (percentage of the seaweed weight) was observed even in the 14 hour light addition protocol as compared to the ambient conditions, and some increase was observed in the water spray system compared to the bubbling treatment ( Figure 3 ). There is a notable difference in the agar yields between the G. cornea seaweeds grown for the light addition experiment and the water movement experiment, this is perhaps because of the different seasons in which the seaweeds were cultivated, but the exact reason for this difference is unknown to us. Seasonal variance of agar yield was previously reported for G. cornea grown in the Yucatan cost, Mexico, where it averaged 31.6% during the cold season and 21.4% during the rainy season (Freile-Pelegrín and Robledo, 1997). These values are higher than compared to our findings, however these can be explained perhaps by the many differences in growth habitat between the Bay of Mexico and in our open land-based system.

Although the increase in seaweed growth as a result of 14 hours of light addition is encouraging, it is up to each seaweed farmer to closely examine the cost versus benefit of this approach. In our systems we calculated that the operational cost of the extra 14 hours of 100W illumination (not including the price of the equipment) was around 0.24 $ US per day. The additional biomass was estimated to be around 30 g day^-1 m^-2. This implies that illumination costs for 1 Kg of extra seaweed (fresh weight) will be roughly 8 $ US. Using a conservative estimation of 10% dry mass (from the wet weight of the seaweed) and maximum 20% agar yields (on a dry weight basis) we can calculate that the 1 Kg fresh Gracilaria cornea will yield 20 grams of agar. So, for 1 Kg of agar and investment of around 400 $ US in electricity is required. This is currently not a financially viable option since the retail price of 1 Kg of agar can reach 580 $ US per Kg and up, depending on grade (taken online from Merck website ¹ ). Nevertheless, in cases where the costs of electricity are lower, or the electricity is produced locally (by solar panels for example), adding artificial illumination should be considered, even when there is apparently sufficient ambient light.

Water motion is especially important for seaweed growth. Rapid relative water movement helps decrease the thickness of the naturally occurring water boundary layer surrounding the seaweed surfaces. This allows for faster diffusion and better nutrient and gas exchange (Gonen et al., 1993; Friedlander and Levy, 1995; Hurd, 2000). Currently, literature regarding land-based seaweed growth systems describes the use of air bubbling or paddle wheels as the means for maintaining the water and the seaweeds in motion. However, a previous report calculated that the costs of air pumping can account for a substantial percentage of the operational costs of seaweed tanks (Ben-Ari et al., 2014). Here we suggest the use of water jets as an alternative. Our results indicate that there were no negative effects on growth rates or agar yields on Gracilaria cornea seaweeds grown using the water spray method as compared to the traditional bubbling method. This eliminates the need for capital investment in the air pumps and tubing setup as well as the operational costs involved in air bubbling. We suggest that the water flow rates provided by the inlet water jets were enough to allow for sufficient diffusion and nutrient uptake rates for the seaweeds growth, at least compared to the air bubbling mode. We believe that there are some other disadvantages of the bubbling system which are not found in the water spray method. In the air bubbling system, it must be considered that in many cases the bubbles also induce the movement of the seaweed together with the water, thus reducing the relative water flow rates against the seaweed body, air mixing increases the risks of oxidative reactions which may harm the seaweed (Friedlander and Levy, 1995). Enriching the water with air may also cause high oxygen levels which may interfere with the photosynthetic reaction.

Several factors still need to be determined, especially regarding upscaling the water spray system. We suggest the use of long shallow tanks, up to 40 cm high, like in our system, where the water inflow pipe can be easily located in its bottom, while providing sufficient water turbulence. This should especially be feasible in U or V shaped tanks where the water spray inlet tube is located in the middle. The long tanks can be fed by a single, long, water pipe ranging the entire length of the tank, or by a series of shorter pipes, this will depend on the inlet water pressure and needs to be determined in situ. In such a system, if for some reason air bubbling is required, the water spray inlet pipe can serve as an air tube when connected to an air compressor, while water inlet can be made by an alternative pipe, from the top of the tank. The location of the water jet head inside the tank is of potential importance due to the resistance of the tank water column against the water jets, which may require additional water pressure in the inlet flow tubing. In addition, the water spray tubes may entangle or interfere with the seaweeds, although this was not observed in our setup, which serves as another reason to keep them the water spray tubes as close to the bottom of the tank as possible.

In conclusion, we demonstrated that addition of artificial LED illumination, even during daytime, for 14 hours enhanced G. cornea daily growth rates. Importantly, the water spray method is introduced as a low cost yet efficient alternative for maintaining good water flow and seaweed growth rates and agar yields in land-based cultivation systems. The combination of higher growth rates coupled with decreased expenses on electricity demand for air compressors for air bubbling should result in more increased net values for the cultivated Gracilaria seaweeds and their agar. This may drive the global agar industry toward increased mariculture practices and less reliance on depleting natural seaweed stocks, and ensuring a constant, sustainable supply of agar and other seaweed extracts.