Density and Morphology of Epiphytic Bacteria on the Kelp Laminaria digitata

Samples were taken from three parts of the frond of Laminaria digitata, from algal particulate fragments and from ambient seawater, over a period of 13 months on the coasts of Brittany (France). Bacteria associated with algal surfaces and seawater were studied using SEM and culturing methods. Total bacterial counts of the meristematic and middle parts of the frond followed a seasonal pattern, with a minimum observed in winter (about 10 bacteria cm~) and a maximum in summer (6 χ ΙΟ bacteria cm"). Large and highly pigmented fragments presented a less marked density reduction in winter. Bacterial densities from the frond tips and decaying fragments remained high (6 χ ΙΟ bacteria cm" annual average) and relatively steady throughout the year. Numbers of culturable bacteria increased with tissue age, suggesting a change in the bacterial communities. This change appeared to be correlated with a change of morphological bacterial types associated with the different algal growth levels. Coccoid forms, corresponding to capsulated bacteria were dominant on the meristematic part of the frond, principally during the active growing phase of the kelp. The percentage of rod-shaped bacteria increased when tissues became older. Relationships with algal metabolism (production of tissue, production of antimicrobial compounds, algal cell lysis) are discussed. j , ti Manus 1969, Shiba and Taga 1980, Lakshmanaperumalsamy and Purushothaman 1982). Bacterial abunLaminaria digitata (Hudson) Lamouroux (Phaeophydance has been followed by culture methods on Lamceae, Laminariales) occurs in the upper part of the inaria longicruris Pylaie (Laycock 1974), Laminaria infralittoral zone of the eastern North Atlantic Ocean, pallida (Greville) ex J. Agardh and Ecklonia maxima This species is intensively exploited for algin extras Ostbeck (Mazure and Field 1980). Total microflora tion on the coast of Brittany (France). The biology has been determined essentially from stranded kelp and ecology of the species has been described by (Kqop et al 1982, Bouvy et al. 1986) or incubated Gayral and Cosson (1973), Cosson (1978) and its· algal material (Linley et al 1981, Stuart et al. 1981, physiology by Floc'h (1979,1982), Davison and StewReichardt and Dieckmann 1985). Only single data are art (1984), Davison et al (1984) and Davison and available from in situ growing kelp. Koopetal. (1982) Reed (1985), but little is known about its epiphytic noted 2.3 χ 10 bacteria cm" on freshly cut fronds bacterial communities. " of Ecklonia maxima in South Africa, while Bouvy et al (1986) found 5 χ ΙΟ bacteria g" of wet weight Several authors have shown that macroalgal surfaces on lacerated fronds of Macrocystis pyrifera (L.) C. constituted a favorable substratum for bacterial setAgardh in the Subantarctic region, however they were tlement and quantitative evaluations have been pubinterested specifically in the stranded kelp biodegralished (Conover and Sieburth 1964, Chan and Mac dation processes. Botanica Marina / Vol. 33 / 1990 / Fasc. 6 Copyright © 1990 Walter de Gruyter · Berlin · New York 516 Corre and Prieur: Density and morphology of epiphytic bacteria on Laminaria digitata The present study followed the annual development of both culturable and total microflora associated with Laminaria digitata and on naturally detached fragments as a function of the age of the algal tissues