Preliminary Studies On Characteristics And Infection Progress Of A Bacterial Pathogen Infecting Gametophytes Clones Of Laminaria Japonica Aresch(Phaeophyta)

Laminaria japonica Aresch, is a commercially cultured kelp species and it hasprecious nutrition and medical value. By now, kelp mariculture has developed into ahuge-scaled industry in China. However, like any other marine algae, L. japonica isplagued frequently by diseases caused by bacteria, fungi, protists, or viruses, and thatcause great losses to economy. As a key tool in germplasm conservation andutilization, gametophytes clones of L. japonica plays an important role in kelp seedingand breeding, as well as genetic and physiological research. As a result, studies ondiseases of gametophytes, are of great significance to kelp mariculture.In this study, a new disease infecting gametophytes clones of L. japonica isdiscovered. The pathogen was isolated from decayed gametophytes, and itsidentification as well as biological, physiological and biochemical characterizationswere studied. Meanwhile, the infection progress was investigated using both lightmicroscope and scanning electron microscope (SEM). In infection progress, theinfluence of bacterial inoculation on algicidal efficency was also studied. The resultswere showed as follows:The macroscopic symptoms of the diseased gametophytes were usuallyflocculation, accompanied by gradual bleaching, while the healthy ones were regularand dark brown. An alginolytic marine bacterium strain marked as A-1was isolatedfrom the diseased gametophytes which was identified as one strain of the genusAlteromonas based on the phylogenetic analysis of16S rDNA sequences as well asthe morphological, biochemical and physiological characteristics. Test of Koch’postulates further confirmed that the strain A-1was the pathogen of the newdisease of gametophytes clones of L. japonica. In a certain range (10℃-30℃), thebacteria grew faster with the increase of temperature. The optimal conditions ofgrowth and alginase production of strain A-1were NaCl3.5%, pH8and sodiumalginate0.6%.In the early stages of infection progress, a mass of bacteria that gathering aroundand attaching to the cell walls of the host could be found. Later, the bacteria began topenetrate the cell wall of host and the invaded points could be observed occasionally.In the late stages of infection, parasites propagated rapidly at expense of host cells.Meanwhile, the pigments and protoplasts of host cells were increasingly drained away.Finally, the host cells were filled with bacteria, and then groups of bacteria emergedout, leaving behind the collapsed host cells.The healthy gametophytes clones of L. japonica grew well when cultured at both10℃and15℃but badly at20℃, so the optimal culture temperature forgametophytes clones was10℃-15℃. Temperature was the most important factor thataffecting the infection progress. In a certain range, the higher of temperature, thefaster gametophytes clones got diseased. During the infection progress, with theincrease of cultured temperature, the content of total chlorophyll of laminariangametophytes was decreased, but the total protein concentration and SOD activityincreased at first and then decreased.