ALBERT

Description: Diseases increasingly threaten aquaculture of kelps and other seaweeds. At the same time, protection concepts that are based upon application of biocides are usually not applicable, as such compounds would be rapidly diluted in the sea, causing ecological damage. An alternative concept could be the application of immune stimulants to prevent and control diseases in farmed seaweeds. We here present a pilot study that investigated the effects of oligoalginate elicitation on juvenile and adult sporophytes of Saccharina japonica cultivated in China and on adult sporophytes of Saccharina latissima cultivated in Germany. In two consecutive years, treatment with oligoalginate clearly reduced the detachment of S. japonica juveniles from their substrate curtains during the nursery stage in greenhouse ponds. Oligoalginate elicitation also decreased the density of endobionts and the number of bacterial cells on sporophytes of S. latissima that were cultivated on sea-based rafts. However, the treatment increased the susceptibility of kelp adults to settlement of epibionts (barnacles in Germany and filamentous algal epiphytes in China). In addition, oligoalginate elicitation accelerated the aging of S. japonica adults. Based upon these findings, oligoalginate elicitation could be a feasible way to provide “environmentally friendly” protection of kelp juveniles in nurseries. The same treatment causes not only beneficial, but also unwanted effects in adult kelp sporophytes. Therefore, it is not recommended as a treatment after the juvenile stage is completed. Future tests with other elicitors and other cultivated seaweed species may allow for the development of more feasible applications of targeted defense elicitation in seaweed aquaculture.

Description: Saccharina japonica is a commercially farmed seaweed of global importance. However, disease occurrence during different stages of cultivation can result in substantial economic losses. Identification of the causative agents of disease remains a significant bottleneck to the large scale cultivation of S. japonica. In this study, an aerobic heterotrophic, flagellated, rod-shaped Gram-negative bacterial strain X-8 was isolated from the bleaching diseased S. japonica sporelings. Pathogenecity of strain X-8 was tested by re-infection assay. The ultrastructural changes of infected S. japonica cells by strain X-8 indicated that chloroplasts were the first organelle responding to X-8 infection with deformed structure and later followed by fragmented nucleus. However, the ultra-structure of mitochondria and cell wall remained intact during the re-infection. Based on 16S rRNA gene sequence, morphological and biochemical characteristics, strain X-8 was designated as Pseudoalteromonas piscicida X-8. The pathogenicity of P. piscicida X-8 was identified by Koch's Postulate under laboratory conditions. Our results will not only help to establish a stable experimental model between the pathogenic bacteria and the host S. japonica to further elucidate the virulence mechanisms, but will also provide information for disease management to effectively prevent and mitigate the occurrence of bleaching disease of S. japonica at nursery stage.