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 Br ttany (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 106 bacteria cm−2) and a maximum in summer (6×7 bacteria cm-2). Large and highly pigmented fragments presented a less marked density reduction in winter. Bacterial densities from the frond tips and decaying fragments remained high (6×7 bacteria cm-2 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. © 1990, Walter de Gruyter. All rights reserved.