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INTRODUCTION
Kappaphycus alvarezii (Doty) L. M. Liao seaweed is a major economically important fishery commodity (Ask & Azanza, 2002; Bixler & Porse, 2010; Bindu & Levine, 2011; Zuniga-Jara & Marin-Riffo, 2015).The development of the K. alvarezii culture is very fast due to the demand for carrageenan, which is commonly used as the main raw material in many industrial activities (Munoz et al., 2004; Yunque et al., 2011; Barbosa et al., 2013; Hurtado et al., 2015; Porse & Rudolph, 2017).
Kappaphycus alvarezii is a seaweed commodity that has been developed in many countries, such as Indonesia, Malaysia, the Philippines, Vietnam and several other countries (Hung et al., 2009; Nadirah et al., 2012; Hurtado et al., 2015; Kasim & Mustafa, 2017). In the seaweed culture system, ice-ice disease is a major obstacle because it can harm cultivation efforts (Vairappan, 2006; Solis et al., 2010; Luhan et al., 2015). Ice-ice disease causes a decrease in seaweed farming production ranging from 70%-100% (Loureiro et al., 2010).
Outbreaks of ice-ice diseases attacking K. alvarezii cultivation development activities have been reported in various regions of Indonesia, including Karimunjawa Islands, East Java; Bungin Island and Muna Island, Southeast Sulawesi; and Seribu Islands, DKI Jakarta (Porse & Rudolph, 2017; Syafitri et al., 2017; Aeni et al., 2019).
Bacteria play a role in the development of ice-ice disease in seaweed cultivation. The bacteria that cause ice-ice disease in K. alvarezii are Vibrio, Aeromonas, Cytophaga, Flavobacterium, Pseudomonas and Bacilllus (Largo et al., 1995; Riyaz et al., 2019). Aquatic organisms infected with bacteria indirectly cause damage to tissue. Changes in tissue structure make aquatic organisms sick and even cause death (Korun & Timur, 2008; Oh et al., 2019).
Disease transmission in aquatic organisms can occur vertically or horizontally. Horizontal spread is the process of transferring pathogens from disease-infected organisms to healthy organisms (Manin & Ransangan, 2011; Karthikeyan & Sudhakaran, 2018; Ben-Asher et al., 2019). This study aims to determine the horizontal transmission of ice disease on growth, carrageenan content and changes in the tissue structure of K. alvarezii seaweed.
MATERIALS AND METHODS
Sample Collection
Kappaphycus alvarezii seaweed samples used were healthy seaweed with a wet weight of 35 g obtained from the K. alvarezii seaweed aquaculture in Kupang, East Nusa Tenggara, Indonesia, whereas the wet weight of seaweed that was attacked by ice-ice was 30 g (treatment A), 35 g (treatment B), 40 g (treatment C) and 45 g (treatment D) obtained from the waters of Panggang Island, DKI Jakarta, Indonesia (S 05o44’ 30,7’’ dan E 106036’04’’). In addition, there was also a negative control treatment as a comparison to treatment A, B, C and D. Negative control, was a treatment with thallus free of ice-ice disease.
Container Preparation
This test was carried out in vivo with an aquarium container of 45 cm x 30 cm x 30 cm, the volume of seawater media used was 80 liters. The values of each water quality parameter especially temperature, salinity, and dissolved oxygen are maintained in an ideal range. Temperature 27-29oC, salinity 30-35 ppt, and dissolved oxygen 4.8-6.2 mg / L.
Seaweed Acclimatization
Before testing the transmission of ice-ice disease, seaweed was first acclimatized for 3 days.This acclimatization was carried out to adapt seaweed to new environmental conditions. During acclimatization, the media were given liquid organic fertilizer (POC NASA) as much as 5 ml per aquarium volume of 80 liters. The test container was equipped with constant aeration.
Observation of Clinical Symptoms
The determination of the health status was based on clinical signs macroscopically, which is characterized by changes in the thallus to become rough, wilted and wrinkled followed by the appearance of white spots on the surface of the thallus seaweed. Thallus wet weight measurements were carried out every hour for 5 days.
Carrageenan Content Analysis
Determination of seaweed carrageenan concentration was expressed in the percentage of carrageenan dry weight of seaweed following the method of Hayashi et al.(2007). The analysis of carrageenan content was performed on the seaweed thallus in each treatment, including the negative control.
Histological examination
Histology observation is a microscopic observation to see changes in tissue structure due to the transmission of ice-ice disease from seaweed which is attacked by ice-ice disease to healthy seaweed at each treatment. Observation of the structure of the seaweed tissue was carried out for 5 consecutive days. Histological observation and the staining of Mayers Hematoxylin and Eosinfollowed the procedure by Jaiswal et al. (2019).
RESULTS
The spread of ice-ice disease in cultivated seaweed organisms is closely related to the process of transferring disease agents from thallus affected by ice-ice to healthy thallus. The symptoms of ice-ice disease are indicated by changes in the color of the thallus which leads to symptoms of the emergence of ice-ice disease.
Analysis of variance showed that there was no significant interaction between treatment and time with a value of 0.99 (p> 0.05), while the severity of the treatment of infected thallus to the transmission of ice-ice disease showed a significant effect with a value of 0.03 (p <0.05). Treatment with the transmission time level showed a significant influence on the appearance of ice-ice disease 0.01 (p <0.05).The results of the statistical analysis did not show the effect of each treatment, but based on the observation time they showed a significant effect in each treatment. Based on Duncan’s test results, it shows that the treatment on the fifth day has a lower value (28.0472) compared to the first (32.451), second (32.6769), third (32.2361), and fourth (30.2365) days. This shows that over time the transmission process of ice-ice pathogens increases in the thallus of healthy seaweed.
This can be further proven by the appearance of ice-ice disease symptoms from each treatment compared to negative controls that did not show ice-ice symptoms during the observation. The speed of transmission of ice-ice disease to healthy seaweed thallus shows different results. In treatment A, transmission of ice-ice disease infection occurred on the fourth day. Treatment B, the transmission occurred on the third day. Treatment C, the transmission occurred on the fourth day. While treatment D, the transmission occurred on the second day.
Transmission of ice-ice caused morphological changes in seaweed thallus showing the same symptoms in each treatment.The change begins with a change in the condition of the thallus that has lost mucilage on the surface of the thallus. These conditions cause the surface of the thallus to become rough and gradually change color.
Changes in the thallus surface conditions and changes in color and the appearance of bleaching at the tip of the thallus were followed by changes in the wet weight of the thallus seaweed (gram). The relationship is shown by the tendency to decrease the fresh weight of seaweed thallus after transmission test (Fig. 1). The change in color and the decrease in wet weight cause changes in the conditions of the thallus tissue. According to the results of the histological observations, the thallus tissue of the seaweed showed differences in the structure of the tissue (Fig. 2). The healthy condition of seaweed tissue shows that the components of the cells making up the tissue are intact such as cortical cells and modular cells are still compact, neatly arranged, while the seaweed tissue structure infected with ice-ice disease shows the tissue structure from the first day to the fifth day showing changes in cell degradation (cells making up damaged tissue or lysis).
Figure 2 Photomicrograph of seaweed thallus tissue Kappaphycus alvarezii that is attacked by ice-ice disease (A = Day 0, B = Day 2, C = Day 3, D = Day 4, E = Day 5) [Arrows in image B-E indicate the occurrence of cell death or lysis].
Changes in carrageen an content of seaweed are different in a healthy thallus condition with seaweed with ice-ice disease and can be seen in Table 1. According to the analysis of the carrageenan content of the seaweed thallus in the negative control, the values ranged between 28% and 35%. When compared with seaweed thallus content from the transmission test results, the range of carrageenan content values is lower with a range of 2.9% - 4%.
Table 1 Results of carrageenan content analysis on day of the experiment
| Treatment | Carrageenan content (%) |
| A | 4,0 % |
| B | 3,1 % |
| C | 3,1 % |
| D | 2,9 % |
| Negative control | 28-35 % |
The transmission test shows that there is a change and causes a tendency to significantly decrease carrageenan levels with each treatment. Based on the analysis of carrageenan content of seaweed after transmission test, show a tendency to decrease carrageenan levels significantly from each treatment. Based on the results of the analysis of variance (p<0.05), it shows an interaction of the level of severe treatment of infected thallus with the transmission test time to the response to decrease in carrageenan content. The interaction of each treatment level was determined by further statistical tests with the Duncan test. The results of the analysis show the differences of each interaction level of treatment before and after the ice-ice disease transmission test.
DISCUSSION
In fish, shrimp and seaweed cultivation activities, disease attacks are the main obstacles (Loch & Faisal, 2015; Mohamad et al., 2019; Dey et al., 2019; Ward et al., 2019). The results of this study indicate that the transmission of ice-ice disease to healthy seaweed thallus causes discoloration and weight loss.
The phenomenon observed in the present study of ice-ice disease has similarities with those reported by Ask et al. (2002), Yong et al. (2015) and Tsiresy et al. (2016), that the incidence of ice-ice disease in a seaweed cultivation region begins with changes in thallus that are experiencing stress, followed by bleaching and ultimately thallus damage. Stress characterized by changes in the color of the thallus is an early symptom of seaweed that has a disruption of ice-ice disease, along with the maintenance of the thallus tip turns white and finally porous (Sulu et al., 2003).
In addition, the transmission of ice-ice shows different tissue structures in seaweed that show changes in the degradation of cells making up the lysed tissue. Transmission of disease agents can cause changes in tissue structure (Magi et al., 2009; Senapin et al., 2018). This is due to the attachment of disease agents from the organism to the production of extracellular substances, that is highly correlated with time and weight loss of the new hosts due to the lysis of new host cells (Salyers & Whitt, 1994).
Ice-ice infection also causes a decrease in carrageenan content in K. alvarezii seaweed (Rantetondok & Latama, 2017; Ward et al., 2019). This is indicated by the loss of pigment due to lysis of the cells making up seaweed tissue. Another impact of cell lysis is the occurrence of significant degradation of organic substances from the thallus tissue of seaweed. The study of Mendoza et al. (2002), showed that the viscosity and carrageenan levels of K. alvarezii seaweed thallus decreased when ice-ice infection occurred.
This condition is in line with the results of research Hayashi et al. (2010) that, the condition of seaweed thallus which experienced pigment loss and showed symptoms of ice-ice disease had a low carrageenan content ranging from 0.5% - 25%. The quality of K. alvarezii seaweed is determined by carrageenan content which ranges from 32% - 75%, and decreases its quality when it is below that range (Munoz & Sahoo, 2004). The content of carrageenan and gel strength decreased significantly when the symptoms of ice-ice disease began to appear in the thallus, and decreased with time and the level of thallus damage. The decrease in carrageenan content is directly proportional to the time of bacterial infection (Riyaz et al., 2020).
CONCLUSION
The results of this study indicate that the transmission of ice-ice disease can occur horizontally from thallus affected by ice-ice to healthy thallus. This transmission causes discoloration accompanied by a decrease in thallus weight. Transmission also causes changes in tissue structure and decreases carrageenan content.
