Bacterial diversity in relation to secondary production and succession on surfaces of the kelp Laminaria hyperborea

Abstract

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.

Figure 1

Bacterial production and bacterial diversity on kelp surfaces. (a) The rarefaction curves show the observed 97% OTU richness with increasing sequencing depth of the kelp biofilm sampled at the three different sites in July 2009 for bacterial production measurements. The dotted line shows at what sequencing depth rarefied OTU richness was calculated. (b) There is no clear relationship between 97% OTU richness and bacterial production as measured by incorporation of ³H-Thymidine. (c) 97% OTU evenness is positively correlated with bacterial production.

Figure 2

Successional changes in bacterial diversity. (a) The rarefaction curves show the observed 97% OTU richness with increasing sequencing depth of the kelp biofilm sampled across a gradient of kelp surface age (March 2007–November 2007, see Supplementary Figure S1). Sample id is indicated by abbreviated month names and ‘L' and ‘M' for the kelp lamina and meristem, respectively. Numbers in parentheses indicate the approximate age of the kelp surface in months. The dotted line shows at what sequencing depth rarefied OTU richness was calculated. (b) Rarefied 97% OTU richness increases with kelp surface age. (c) 97% OTU evenness, measured by Pielou's evenness index, increases with kelp surface age.

Figure 3

Bacterial community composition in relation to bacterial production and succession. (a) The nMDS ordination illustrates similarity of OTU composition between samples in the production experiment. The diameter of the bubbles is proportional to the bacterial production measured in each sample. The ellipses represent 95% confidence intervals of sample ordination grouped by sampling site. (b) The nMDS ordination illustrates similarity of OTU composition between samples of different kelp surface age. The contours show the gradient of kelp surface age (see Supplementary Figure S1), fitted to the ordination plot using function ordisurf in R.

Figure 4

General characteristics of bacterial communities on kelp surfaces. (a) Every dot represents a bacterial 97% OTU that is plotted by its frequency of detection (percentage of samples it is present in) against its relative abundance in the entire data set (n=1108). OTUs are color coded according to which of the major phylogenetic groups they belong to. Individual OTUs mentioned in the text are indicated by their number. (b) The rarefaction curves show the number of 97% OTUs that belong to the five most represented bacterial groups. Numbers in parentheses give the percentage of sequences represented by that group, and the percentage of OTUs, respectively. Most of the sequences belong to Planctomycetes, but Alphaproteobacteria and Bacteroidetes are more diverse (have a higher OTU richness).