Seasonal variations of the cell density and bacterial community composition in biofilms growing on the surface of the kelp Laminaria hyperborea from 2 sites on the southwestern coast of Norway were investigated using total cell enumeration and denaturing gradient gel electrophoresis (DGGE) fingerprinting. The major taxonomical groups of bacteria inhabiting the biofilms were identified by DGGE band sequence classification. The microbial cell density of the biofilm appeared to be highly affected by the seasonal growth cycle of the kelp and was found to be lowest on growing kelp in March (minimum 8.3 × 10 2 cells cm -2), while on non-growing kelp in July to February, it was around 1.0 × 10 7 cells cm -2 with large fluctuations. The composition of the bacterial community of the biofilm followed a continuous seasonal succession that may be explained by the influence of both biotic factors such as seasonal changes in the kelp substrate and abiotic factors such as seawater temperature. Planctomycetes and Alphaproteobacteria were frequently detected throughout the year, while Verrucomicrobia, Cyanobacteria, Gammaproteobacteria, Betaproteobacteria, and Bacteroidetes were more sporadically detected. The bacterial communities of the biofilm on kelp showed little overlap with that of the surrounding seawater.
Kelp forests worldwide are known as hotspots for macroscopic biodiversity and primary production, yet very little is known about the biodiversity and roles of microorganisms in these ecosystems. Secondary production by heterotrophic bacteria associated to kelp is important in the food web as a link between kelp primary production and kelp forest consumers. The aim of this study was to investigate the relationship between bacterial diversity and two important processes in this ecosystem; bacterial secondary production and primary succession on kelp surfaces. To address this, kelp, Laminaria hyperborea, from southwestern Norway was sampled at different geographical locations and during an annual cycle. Pyrosequencing (454-sequencing) of amplicons of the 16S rRNA gene of bacteria was used to study bacterial diversity. Incorporation of tritiated thymidine was used as a measure of bacterial production. Our data show that bacterial diversity (richness and evenness) increases with the age of the kelp surface, which corresponds to the primary succession of its bacterial communities. Higher evenness of bacterial operational taxonomical units (OTUs) is linked to higher bacterial production. Owing to the dominance of a few abundant OTUs, kelp surface biofilm communities may be characterized as low-diversity habitats. This is the first detailed study of kelp-associated bacterial communities using high-throughput sequencing and it extends current knowledge on microbial community assembly and dynamics on living surfaces.
In this study of Laminaria saccharina the length-to-width ratio of newly grown lamina tissue was observed to vary throughout the year. The ratio was at its lowest during the period of slow growth and at its highest during the period of rapid growth. An increase in both length and width growth took place at mid-winter simultaneously with a decrease in carbon content of the lamina, indicating consumption of stored carbohydrates. Long laminae were more prone to distal erosion than short ones during the autumn. The distribution of more growth in the width direction during summer and autumn may therefore maximize the lamina area during autumn and winter and thus increase the amount of stored carbon available for the plants at this time. It may be interpreted as a morphological adaptation to a period of slow growth. A period of rapid lamina growth during late winter and spring was observed. Both length and width growth declined early in the summer, probably due to nitrate limitation. At the end of the investigation the plants were harvested and their age determined. The lamina elongation was found to be generally lower in third-year plants than in second-year ones, while width growth was similar in the two groups. This means a decreased length-to-width ratio of the new tissue produced with increasing age. Depopulation of the tagged plants was at its highest during the period of slow growth. , Liming 1986, Liming 1988. Ecological aspects Seasonal growth in European Laminaria saccharina related to fitness have been given less attention in (L.) Lamour. has been investigated in Great Britain studies of seasonal growth patterns. For example, (Parke 1948, Conolly andDrew 1985) and in Helgo-growth in the various parts of the thallus in kelp land (Liming 1979). A general growth pattern of species has been little studied. Laminaria spp. have maximum growth during late winter and spring and thalli which are divided into holdfast, stipe and lamminimum growth during the summer and autumn has ina. Lamina may have genetically fixed features such been demonstrated. However, in some Laminaria spe-as splits or bullations. The morphology of Laminaria cies geographical variations in the seasonal growth species may also show considerable phenotypical varpattern due to environmental factors such as tempera-iation. Several species of Laminaria develop narrower ture (Sundene 1964) or nitrate availability (Gagne et lamina at wave-exposed localities than at sheltered al. 1982) have been demonstrated. Information on ones. Gerard (1987) demonstrated that mechanical seasonal growth in L. saccharina from northern Eu-stress caused L. saccharina to grow more in the length rope is therefore of general interest. and less in the width direction. In addition, lamina width of L. saccharina varies during the year (Parke Much work has been done in order to find the factors 1948). The ecological implications of this morphoaffecting seasonal growth in Laminaria sjpp. (see for logical plasticity in Laminaria spp. are not well example Chapman and Craigie 1977,...
The surfaces of kelp are covered with bacteria that may utilize kelp-produced carbon and thereby contribute significantly to the carbon flux in kelp forest ecosystems. There is scant knowledge about the identity of these bacteria and about which kelp-derived carbon sources they utilize. An enrichment approach, using kelp constituent carbon sources for bacterial cultivation, was used to identify bacterial populations associated with the kelp Laminaria hyperborea that degrade kelp components. In order to assess whether the cultured bacteria are significant under natural conditions, partial 16 rRNA gene sequences from the cultured bacteria were compared to sequences obtained from the indigenous bacterial communities inhabiting natural kelp surface biofilms. The results identify different members of the Roseobacter clade of Alphaproteobacteria in addition to members of Gammaproteobacteria that are involved in kelp constituent degradation. These bacteria are observed sporadically on natural kelp surfaces and may represent opportunistic bacteria important in degradation of dead kelp material. Many of the cultured bacteria appear to be generalists that are able to utilize different kelp carbon sources. This study is the first to link culturable kelpassociated bacteria with their occurrence and possible roles in the natural environment.
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