Salinity and Time Can Alter Epibacterial Communities of an Invasive Seaweed

Abstract

The establishment of epibacterial communities is fundamental to seaweed health and fitness, in modulating ecological interactions and may also facilitate adaptation to new environments. Abiotic factors like salinity can determine bacterial abundance, growth and community composition. However, influence of salinity as a driver of epibacterial community composition (until species level) has not been investigated for seaweeds and especially under long time scales. We also do not know how abiotic stressors may influence the 'core' bacterial species of seaweeds. Following an initial (immediately after field collection) sampling of epibacterial community of an invasive red seaweed Agarophyton vermicullophylum, we conducted a long term mesocosm experiment for 5 months, to examine the influence of three different salinities (low, medium and high) at two different time points (3 months after start of experiment and 5 months, i.e., at the end of experiment) on the epibacterial community richness and composition of Agarophyton. Metagenomic sequencing showed that epibacterial communities changed significantly according to salinity and time points sampled. Epibacterial richness was significantly different between low and high salinities at both time points. Epibacterial richness also varied significantly between 3 months (after start of experiment) and 5 months (end of experiment) within low, medium and high salinity level. Irrespective of salinity levels and time points sampled 727 taxa consistently appeared in all Agarophyton samples hinting at the presence of core bacterial species on the surface of the alga. Our results indicate that both salinity and time can be major driving forces in structuring epibacterial communities of seaweeds with respect to richness and β-diversity. We highlight the necessity of conducting long term experiments allowing us to detect and understand epibacterial succession over time on seaweeds.

Keywords: community; epibacteria; invasive; metagenome; salinity; seaweed; time.

FIGURE 1

Epibacterial abundance (A) and species richness (B) at time points T0, T1, and T2 for each salinity level. Epibacterial abundance on algal surface is based on DAPI counts and expressed as cells/cm2 of alga (n = 5). Species richness is the number of distinct taxa at the species level observed at time points T0, T1, and T2 for each salinity level [n = 5, except T0 (n = 3) and T2 high salinity (n = 2), and T1 low salinity (n = 4)]. low = green, medium = blue, pink = high, the median is marked by the line that divides the boxes, the top and bottom of the box are the 75th and 25th percentiles respectively, and the whiskers show the minimum and maximum values.

FIGURE 2

Epibacterial composition (NMDS based on Jaccard dissimilarity). Time point is indicated by shape (T0 = circles, T1 = triangles, T2 = squares), Salinity is indicated by color (low = green, medium = blue, pink = high). Tank walls are indicated by crosses (+).

FIGURE 3

Mean relative abundance of top 200 dominant bacterial taxa at the class level [n = 5, excluding T0 (n = 3), and T1 low salinity (n = 4)].

FIGURE 4

The relative abundance of the core epibacterial taxa that were shared between all salinity levels (top 25 most dominant shown, corresponding to 50% of the total abundance). The plot is faceted by Class and the color denotes the species. Shared taxa is defined as taxa present in all replicates for all treatments.