Seaweed Kappaphycus alvarezii Cultivation for Seagrass Ecosystem Conservation

Abstract

The physical characteristics of the seagrass ecosystem indicate that the shallow sea waters are ideal for seaweed cultivation. The rapid development of Kappaphycus alvarezii seaweed cultivation in coastal areas of Indonesia should not cause damage to the seagrass ecosystem. The study on the cultivation method of K. alvarezii seaweed in three seagrass ecosystems in Indonesia showed high growth rates, biomass production and carrageenan content due to high nutrient concentrations and high water clarity as well as the current optimal conditions in the cultivation environment. K.alvarezii cultivated in seagrass ecosystems prevents the damaging effects of UV-B radiation on those ecosystems. The right cultivation method applied is the off-bottom method.

1. Introduction

As many as 25,742 hectares of seagrass spread out in the coastal areas of Indonesia. The seagrass ecosystem is one of the most productive (high organic productivity) marine ecosystems and with high biodiversity. It is supporting fishery production, namely as feeding ground, spawning ground, nursery ground, as well as shelter from various predators and from the hot sun for various fish species.

Seagrass ecosystem are flowering plant (Angiospermae) vegetation that are formed by one or more species with high or rare density that are able to fully adapt in high salinity waters, which cover a coastal shallow marine zone. These physical characteristics of the seagrass ecosystem seems to be the ideal location for seaweed cultivation.

Seaweed cultivation and business in Indonesia are experiencing rapid development and plays an important role in improving people’s welfare. However, seaweed cultivation in coastal areas should be carried out in an environmentally friendly manner, without damaging the seagrass ecosystem. K. alvarezii is a species of red seaweed (Rhodophyceae) which changed its name from Eucheuma cottonii to Kappaphycus alvarezii on the basis of the kappa-carrageenan it contains [1], is one of economically important red (macro) algae species that has a high demand on the world market.

Based on the above considerations, a series of studies were conducted in the period 2011–2020 to find the right K. alvarezii cultivation method in the seagrass ecosystem that shows high growth rates, biomass production, and carrageenan content, and evaluates its impact on the condition of the seagrass ecoosystem. The research was conducted in three seagrass ecosystems in coastal areas of Indonesia, namely in the coastal waters of Tanakeke, Mappakasunggu District, and the coastal waters of Laikang Bay, Mangarabombang District, both are administrative areas of Takalar Regency, South Sulawesi Province. The water depths of both waters of the research location were 500 cm and 250 m, respectively, during high tide. Both have the same current pattern with current velocities ranging from 4.0–8.5 m/second outside the seagrass ecosystem. Another location is the grass ecosystem in Karampuang Island, Mamuju Regency, West Sulawesi Province with a depth of 400 cm during high tide. These three seagrass ecosystems are in one waters, namely the Makassar Strait.

The cultivation method tested in the seagrass area in Takalar Regency was a longline system with three methods, namely the floating surface longline method, the off bottom method, and the bottom method. The floating method was placed 20 cm below the sea level, the off-bottom method was placed 50 cm above the bottom sediment of the seagrass ecosystem, and the bottom method was placed right on the surface of the bottom sediments. While the method used in Mamuju Regency was only the floating surface longline method. Seaweed seeds of K. alvarezii as much as 100 g/bundle were tied to nylon ropes with a distance of 50 cm between ties and between ropes. Cultivation time was 42–45 days.

Seaweed growth was expressed as daily growth rate and calculated according to the formula suggested by Fortes [2]; Dawes et al. [3]; Mtolera et al. [4]; Hurtado et al. [5]; Munoz et al. [6]; Hayashi et al. [7]. Some water quality parameters were measured at the research site, including: light intensity falling on the sea surface using a Lux-meter, salinity using a handrefractometer, temperature using a thermometer, pH using a pH meter, current velocity with a current meter, and free CO2 with titration method [8]. Other chemical parameters were analyzed at the Water Quality Laboratory, Department of Fisheries, Faculty of Marine and Fishery Sciences, Hasanuddin University, Makassar, Indonesia, consisting of nitrate using the sulfuric acid method [9], ammonium and orthophosphate measured using a spectrophotometer [8]. Carrageenan levels were analyzed by extraction of dried seaweed with 0.5% KOH solution at a temperature of 90–95°C.

2. Seaweed growth

Seagrasses absorb nutrients directly from the water column through their leaves and in sediments by their roots [10], their sediments store a lot of accumulated organic matter, 60–80% of which is found in the roots (rhizomes) of seagrass plants, the rest is in the form of stem and leaf fragments of the plants, tissues fragment of aquatic animals, and others. Therefore, the sediment of the seagrass ecosystem is a good habitat for decomposing bacteria. Through the bacteria and various benthic organisms activities, the organic matter are decomposed and releases carbon, nitrogen, and phosphorus and other mineral nutrients. The carbon production of seagrass ecosystems is quite high, ranging from 900 to 4650 gC/m²/year [11] which is a source of high concentration of dissolved free CO2.

As a submerged soil, ammonium (NH4⁺)is the main product of mineralization (decomposition) of nitrogen-containing organic compounds in sedimentary soils of seagrass ecosystems [12]. Ammonium is the dominant form of nitrogen in seagrass ecosystems [13, 14]. Some of the NH4⁺ is oxidized by Nitrosomonas bacteria to nitrite (NO2⁻) in the initial phase of the Nitrification process, and the next phase is the oxidation of NO2⁻ by Nitrobacter to NO3⁻. In addition, under aerobic conditions at the bottom of the seagrass ecosystem that occur periodically at low tide, some ammonium is oxidized to nitrate, so that nitrate is also a form of nitrogen that is also quite abundant in seagrass ecosystems, and is also absorbed by seaweed. Therefore, as a source of nutrients, the sediment continuously supplies carbon, ammonium and nitrate to the above water layer through the processes of turbulence and diffusion. Aerobic bacteria that works around the roots of seagrass plants convert phosphorus that is in a complex state (bound with Fe ions so that it cannot be absorbed by seaweed) into dissolved phosphorus which can be absorbed by seaweed [15].

In the seagrass ecosystem in Takalar Regency, the concentrations of free carbon dioxide, orthophosphate, nitrate, and ammonium ranged from 6.40–67.92 ppm, 0.064–0.599 ppm, 0.015–0.65 ppm and 0.047–0.704 ppm, respectively down water column (at all cultivation methods). This value is suitable for seaweed cultivation. The N and P content in seagrass sediments with fine soil particles is higher compared to environments with sandy sediments [16] outside the seagrass ecosystem [15]. These nutrients are stored in high concentrations in sedimentary clay particles of seagrass ecosystems [17], then released into the water column for further absorption by K. alvarezii [18].

Nitrogen is an important nutrient in the process of cell division, for seaweed growth, but is most often reported limiting the growth of seaweed [19, 20]. Ammonium (NH4), which is a form of nitrogen ion (N) with a higher concentration than the concentration of nitrate (NO3), allows high growth rates of seaweed that was cultivated in seagrass ecosystems. The absorption rate of ammonium by seaweed is higher than that of nitrate [21], because ammonium is a form of nitrogen that enters the metabolic process. Nitrate is reduced to ammonium before combining into organic compounds [22]. Several previous studies have shown that ammonium is a form of N that is directly utilized by plants in protein biosynthesis. In plant cells, nitrate is reduced by the nitrate reductase enzyme to ammonium, the form of N which then combines with organic compounds in the stimulates vegetative growth (thallus) [23].

As a constituent of protoplasm, phosphorus plays a role in reducing plant abortion (stopping organ growth), plays a role in the formation of meristem tissue (tissue consisting of actively dividing cells), stimulates cell division and repairs damaged tissue [24]. This chemical is found in high concentrations in the water column of seagrass ecosystems.

The growth rate of seaweed in the floating method was 1.13–1.53%/day, lower than the off-bottom method (1.34–1.72%/day) on seagrass in the coastal waters of Laikang Bay. The low growth of seaweed using the floating method is probably due to the high exposure to UV-B radiation in the surface water layer. In the surface water layer, photosynthetic pigments (chlorophyll a and carotenoids) are damaged by excessive light intensity and by the damaging effect of ultraviolet UV-B radiation, known as photoinhibition, photodamage (damage by light) and photooxidation (oxidation by light) [25]. Light intensity measured at the water surface in the two seagrass ecosystems ranged from 3700 to 4100 Lux, exceeding the light intensity of 600 Lux for the maximum growth rate of seaweed. The optimal light intensity for seaweed growth ranged from 333 to 1000 Lux [26]. This low level is also caused by the Warburg Effect phenomenon due to aerobic (oxic) conditions, which is a very high concentration of dissolved oxygen in water beyond the optimal concentration due to the diffusion of oxygen from the atmosphere to the water column in the surface layer combined with dissolved oxygen produced during the photosynthesis of the seaweed itself that was cultivated with floating method. The Warburg effect is the phenomenon of competition between carbon dioxide and oxygen on the reactive site of RUBP carboxylase (an enzyme that binds carbon dioxide gas in the dark reaction of photosynthesis), causing only a small amount of free carbon dioxide to be reduced to carbohydrates by the enzyme in the pentose phosphate pathway of the dark (reaction) phase of photosynthesis [24, 27].

With the same off-bottom method, K. alvarezii cultivated in the seagrass ecosystem of Laikang Bay showed a lower growth rate (1.34–1.72%/day) compared to 2.20–2.54%/day those cultivated in the Tanakeke seagrass ecosystem. With the same cultivation method, the quantity of sunlight (the same range of 3700–4100 Lux the light intensity falling on the water surface of both waters) which is dispersed, attenuated, and reflected as light travels through the water column at low depths (200 cm in Laikang Bay Seagrass) just a little so that the intensity of sunlight that penetrates the column and exposes the tallus of seaweed is still quite high. Seaweed growth at lower water depths of the seagrass ecosystem, Laikang Bay may be caused by light intensity that exceeds 1000 Lux [28]. While the light hitting the thallus which is positioned at a greater depth (in Tanakeke seagrass ecosystem) the intensity has been greatly reduced (become lower and at least approximately in optimal range) by dispersion, attenuation, and reflection.

The difference in current velocity in these two locations also causes differences in the growth rate of seaweed in these two locations. The current velocity at the bottom of the seagrass ecosystem in Tanakeke is relatively faster (54–56, cm/second) which has an impact on better nutrient absorption by seaweed compared to nutrient absorption of seaweed in the Laikang seagrass ecosystem with a slower current velocity (20–40 cm/sec). The relatively faster current speed causes nutrient uptake relatively more efficient due to the thinner boundary layer (the space or layer between the water carrying nutrients with the seaweed thallus). However, current velocities of 20–40 cm/sec and 54–56, cm/second are considered optimal for nutrient absorption. With this method, photosynthesis and absorption of nutrients (CO2, NH4, NO3 and PO4), both via passive ion absorption through osmosis and diffusion processes and active absorption take place effectively. In addition, seaweed is also physiologically protected from the Warburg Effect at 450 cm water depth.

The low growth rate (1.01–1.27%/day) of seaweed using the bottom method in Takalar Regency can be caused by bacterial activity that breaks down the seaweed thallus which is in direct contact with the bottom of the waters.

High growth of K.alvarezii (2.26–2.42%) was only obtained in Mamuju District at the age of 30 days after growth. Beyond the 30th to 45th day of cultivation, there was a decrease in growth which was only 0.28–0.56%/day on the 45th day due to frequent rains at the cultivation location which caused the condition (range) of water quality parameters no longer optimal. The decrease in transparency (high water turbidity) and salinity, as well as the presence of pollutants in the form of waste from residential areas carried by surface run-off by rainfall events are the main causes of the low growth of seaweed. In addition, the influence of river flow that empties into the coastal waters of the cultivation site is the main factor causing the low growth rate of K. alvarezii. These factors were also thought to cause some seaweed thallus to be susceptible to a seaweed disease known as ice-ice which causes seaweed death.

The main cause of ice-ice disease is the condition of extreme abiotic factors that exceed the tolerance limit of seaweed [29] such as exposure to very high light intensity, low water salinity, and nutritional deficiencies that cause seaweed to become susceptible to bacterial infections including Pseudomonas, viruses and fungi. The initial symptom of this pathogen is in the form of white spots on the thallus, then it changes color to pale, the texture is easily crushed, rots and finally the thallus falls out of the clump.

3. Biomass production

In the seagrass ecosystem in Takalar Regency, the highest seaweed biomass production (18.2–30.83 g/clump) was obtained with the off-bottom method, in line with the growth rate which was also the highest at that particular method, and low in the bottom method (12.4–18.52 g/clump). Nitrogen available in high concentrations in seagrass ecosystems causes high biomass production of K. alvarezii cultivated in seagrass ecosystems. This nutrient determines the productivity of algae [14], is very important in the synthesis of chlorophyll a [30] which plays an important role in the photosynthesis process that produces biomass.

4. Carrageenan content

In the process of decomposing organic matter at the bottom of the water, bacteria utilize O2 and release CO2 into the water column which is then absorbed by seaweed in the process of photosynthesis which produces carbohydrates, followed by a secondary product in the form of carrageenan. Carrageenan (phycocolloid) is a natural additive that is widely used in various industries, especially the food, pharmaceutical and cosmetic industries. K. alvarezii seaweed cultivated in seagrass ecosystems in Takalar Regency produced a fairly high content of carrageenan (39.9–44.8%) in the seagrass ecosystem of Laikang waters, and higher (40.73–50.16%) those grown in the Tanakeke seagrass ecosystem. The carrageenan content is in line with the growth rate because the factors that affect growth are also factors that affect the carrageenan content. Several previous studies [31, 32], also showed that the carrageenan content of K. alvarezii was positively correlated with its growth rate. Those that grown in Mamuju Regency also have high levels of carrageenan ranging from 44.95–49.15%, although with a lower growth rate when compared to those that grow in Takalar Regency. These levels of carrageenan exceeds the quality standard set by FAO for industrial raw materials, which is 40%. The carrageenan content of K. alvarezii was relatively higher compared to the carrageenan content of 40.7% recorded by Munoz et al., [6], 31.2–38.1% by Hurtado et al., [33], and 30.57–36.93% by Syahrul et al., [34] all of which were cultivated outside the seagrass ecosystem.

Variations in carrageenan content are influenced by several factors, including cultivation location and climate [35]. The high content of carrageenan K. alvarezii cultivated in seagrass ecosystems is caused by the concentration of dissolved ammonium in seagrass ecosystem waters [13, 14]. Luning [36] suggested that nutrients (including nitrogen) in sufficient quantities can increase the synthesis of polysaccharides (carrageenan).

Since ammonium instead of nitrate is a form of nitrogen that is directly utilized in protein (amino acid) biosynthesis in plant cells, the abundant availability of ammonium in the seagrass ecosystem limits the energy (NADPH) that will be used in reducing nitrate to ammonium in protein biosynthesis in algal cells and more energy is used to reduce carbon dioxide in the synthesis of organic compounds (including carrageenan) in the dark reactions of photosynthesis [37] K. alvarezii. According to Sahoo and Ohno [19], the concentration of ammonium absorbed by seaweed causes high levels of carrageenan in seaweed.

5. Water temperature, pH and salinity

In addition to nutrients in the form of nitrate, ammonium and orthophosphate, seaweed growth is also influenced by environmental conditions such as salinity, temperature, and sunlight [38]. Water temperature, pH and salinity are environmental factors that affect the metabolism, growth and production of seaweed. Temperature affects the metabolic rate (photosynthesis and respiration) of algae [36, 39]. The temperature of the seagrass ecosystem in Takalar Regency in all vertical water layers is around 29–30°C. Meanwhile in Mamuju Regency the temperature ranged from 29.7–32°C respectively. In general, the temperature required for seaweed growth ranges from 20 to 30°C [36]. K. alvarezii can grow at a temperature of 25–29°C [40]. In the tropics the growth rate and biomass production of K. alvarezii is high in the temperature range of 25–30°C [41].

pH is a chemical factor that determines the availability of nutrients to be absorbed by plants so that it affects the growth of the seaweed. The range of water pH measured in seagrass ecosystems in Laikang, Tanake, and Mamuju Bays was 7.5–7.8, 7.2–8.0, and 7.5–8.4. The pH range indicates that the three seagrass ecosystems are classified as waters with high productivity. This pH range is optimal for the growth and development of K. alvarezii based on Trono’s [42] statement that seaweed can live in a pH range of 7.5–8.4. In this pH range, the decomposition of organic matter that accumulates at the bottom of the seagrass ecosystem takes place more quickly, and immediately releases various nutrients needed by seaweed. HPO4³⁻ is the predominant P ion that is absorbed at pH over 8.

The salinity of the seagrass ecosystem in Takalar Regency in all vertical water layers is 30–31 ppt. Meanwhile in Mamuju Regency the salinity ranged from 29.7–32 ppt. This salinity range is suitable for the growth of K. alvarezii seaweed in the tropics. The salinity range of 18–35 ppt is suitable for seaweed growth [6], and very good in the range of 22–30 ppt [43]. Good salinity range for the growth of Eucheuma sp. is 30–35 ppt [44]. Doty [45] stated that the desired salinity of K. alvarezii ranged from 29 to 34 ppt. Sulu et al. [40] stated that 30 ppt salinity was best for the growth of K. alvarezii.

6. Kappaphcus alvarezii cultivation and sustainability of seaweed ecosystems

Based on the above facts (data from studies conducted in seagrass meadows in Takalar and Mamuju districts as well as supporting academic references), K. alvarezii seaweed can grow well side by side with seagrass vegetation, even with high carrageenan content, without having to cut down the seagrass vegetation. There is no competition in meeting nutrient needs between the two because seagrass plants absorb more nutrients from sediment, while seaweed only takes nutrients that are soluble in water. Even, seagrass vegetation supports the growth of seaweed through nutrient cycles from the sediment to the water layer where the seaweed grows [15]. These nutrients are released into the water column after the decomposition of the leaves and other parts of the seagrass.

With stem and leaf morphology resembling the stems and leaves of land grass and reed plants, as well as their root system that propagates in and on the surface of the bottom sediment, seagrass acts as a sediment trap and stabilizes the bottom sediment [46], so it is not easy to stirred by the movement of water (current and waves), dampening the waves and currents that cause the water mass to become calm and clear (high transparency) in this ecosystem. This ecosystem has the characteristics of low current velocity, as a storehouse of mineral nutrients which are recycled from the sediment to the water column above it. This condition is really needed by seaweed for its growth and development. Therefore, high daily growth rate, biomass production and carrageenan content of K. alvarezii were obtained using off-bottom cultivation method in seagrass ecosystems. With the availability of natural food in the form of leaves of seagrass plants, epiphytic algae and animals attached to the sediment substrate, leaves, stems and roots of seagrass, the attack of herbivorous fish on seaweed can be reduced.

Until now, there is no indication of any negative impact of seaweed cultivation on seagrass growth, especially for Enhalus acoroides seagrass. Most seagrass species can survive in low light intensity by means of physiological as well as morphological, or the and anatomical structure adaptations of the leaves [47]. Growth is mainly influenced by water quality parameters. During the cultivation of seaweed in these three seagrass ecosystems, the seaweed farmers activities did not cause physical damage to the seagrass plants. Seagrass degradation is most likely to occur if the plant is trampled by the cultivator. However, when doing seaweed cultivation activities in the seagrass ecosystem, this does not happen because the series of activities ranging from attaching ropes to harvesting are all carried out by boat without stepping on seagrass plants at the bottom of the waters.

The seagrass meadows has high biodiversity and is one of the marine ecosystems with high organic productivity, which supports food chains and food webs in the sea, both based on herbivorous and detrivorous chains. Through the decomposition process of detritus and other organic matter, these waters are a natural food habitat for benthic animals and filter feeder invertebrates that consume detritus particles. With soft bottom sediments, seagrass ecosystems become niches for various benthic animals (zoobenthos), including polychaeta, crustaceans (shrimp, prawns, crabs), sea urchins, mollusks, and sea cucumbers. Important economic fish species that are often found foraging in this ecosystem include rabbit fish (Siganus canaliculatus, S. fuscescens, S. guttatus), Mugil spp., grouper, Epinehelus spp., Neonniphon sammara, Scarrus sp., Lethrinus harak, L. .orantus, Lutjanus kasmira, Pomacantus semicirculatus, Calatus spinidens, and Parupenous berberinus. Some of these fish are either as a seasonal resident, visitors, or occasional residents in this ecosystem. Other marine organisms are sea horses (Hyppocampus spp), turtles, and dugongs.

As a shallow marine ecosystem, seagrass ecosystems are very sensitive to the adverse effects of ultraviolet radiation, especially UV-B (wavelength 280–320 nm) which exposes and penetrates down the water column of seagrass ecosystem. With thin leaf anatomy, seagrass plants are sensitive to UV radiation, so that in the future, this radiation will have a negative impact on the sustainability of this ecosystem [48].

K. alvarezii, a species of red algae (Rhodophyta) known as a group of algae with high red, orange and yellow pigments in its tissues. These pigments are carotenoids and their derivatives, such as lutein, loroxanthin and siphonaxanthin content. Carotenoids are generally found in cell membrane systems where one of the main functions of these compounds is related to photosynthesis. The carotenoid content of K. alvarezii that we recorded in our study on the cultivation of this species in two districts of Takalar Regency was 0.02–0.03% in the dry season when light intensity was high enough, and 0.06–0.17% in the rainy season where relatively lower light intensity. This explains that the carotenoid synthesis is high under conditions of low light intensity, and low under high light intensity. Carotenoids are accessory pigments that function to capture sunlight energy at certain wavelengths beyond the wavelengths that can be captured by chlorophyll and other photosynthetic pigments [49]. The formed carotenoids then act as protecting compounds against the destruction effect of ultraviolet UV-B radiation on the seagrass plants, known as photoinhibition, photooxydation and photodamage [50, 51, 52]. Through cultivation in an environment with high nutrient concentrations, the carotenoid content of K. alvarezii cultivated in seagrass ecosystems is also high.

As a macroalgae, K. alvarezii contains mycosporine-like amino acid compounds (MAA), the compound that gives seaweed species the ability to tolerate the effects of UV-B radiation. Under certain conditions, through the pigmentation mechanism in which the synthesis of carotenoid pigments increases, and with a thick thallus, with a multicellular layer, as well as morphological adaptations where the K. alvarezii clumps become denser when there is an increase in light intensity, cultivating this algae species in the seagrass ecosystem provides shade for seagrass from the UV-B radiation, and can physiologically reduce or inhibit the negative effects of the radiation on seagrass plant tissues. Thus, through the off-bottom method of seaweed in this seagrass ecosystem, it is possible to minimize the destructive impact of UV-B radiation before exposing seagrass plants, and recover the damage from the effects of UV-B radiation. Thus, this cultivation can maintain the sustainability of the seagrass ecosystem as a provider of ecological and socioeconomic services.

7. Conclusions

Based on the discussion, it can be concluded that K. alvarezii seaweed can grow well with high carrageenan content in the seagrass ecosystem, without having to clear seagrass plants. By cultivating seaweed in the seagrass ecosystem, the seagrass vegetation is protected by the seaweed clumps from the negative effects of ultraviolet radiation which can damage the seagrass vegetation in the long term. With the off-bottom cultivation method and 100 grams the initial weight of the seedling with a tie spacing of 50 cm, the cultivation of K. alvarezii in the seagrass ecosystem did not inhibit the growth and development of seagrass vegetation.