Abstract
The intertidal zone often has varying levels of environmental stresses (desiccation, temperature, light) that result in highly stress-tolerant macrobiota occupying the upper zone while less tolerant species occupy the lower zone, but little comparative information is available for intertidal bacteria. Here we describe natural (unmanipulated) bacterial communities of three Fucus congeners (F. spiralis, high zone; F. vesiculosus, mid zone; F. distichus, low zone) as well as those of F. vesiculosus transplanted to the high zone (Dry and Watered treatments) and to the mid zone (Procedural Control) during summer in Maine (United States). We predicted that bacterial communities would be different among the differently zoned natural congeners, and that higher levels of desiccation stress in the high zone would cause bacterial communities of Dry transplants to become similar to F. spiralis, whereas relieving desiccation stress on Watered transplants would maintain the mid-zone F. vesiculosus bacterial community. Bacteria were identified as amplicon sequence variants (ASVs) after sequencing the V4 hypervariable region of the 16S rRNA gene. Microbiome composition and structure were significantly different between the differently zoned congeners at each tissue type (holdfasts, receptacles, vegetative tips). ASVs significantly associated with the mid-zone congener were frequently also present on the high-zone or low-zone congener, whereas overlap in ASVs between the high-zone and low-zone congeners was rare. Only 7 of 6,320 total ASVs were shared among tissues over all congeners and transplant treatments. Holdfast bacterial community composition of Dry transplants was not significantly different from that of F. spiralis, but Watered holdfast communities were significantly different from those of F. spiralis and not significantly different from those of procedural controls. Additional stressor(s) appeared important, because bacterial communities of Dry and Watered transplants were only marginally different from each other (p = 0.059). The relative abundance of Rhodobacteraceae associated with holdfasts generally correlated with environmental stress with highest abundance associated with F. spiralis and the two high-zone transplant treatments. These findings suggest that the abiotic stressors that shape distributional patterns of host species also affect their bacterial communities.
Highlights
Intertidal zones are dynamic ecosystems where multiple abiotic stresses and biotic interactions occur with differing intensities across a few vertical meters
Pairwise tests of the interaction revealed that bacterial composition and structure were significantly different (p = 0.001, Supplementary Figure S1) between species at each tissue type
Despite the unpredicted lack of differentiation of receptacle and vegetative bacteria within some species (Figure 3 and Supplementary Figure S1), we found strong differentiation of bacterial communities in the differently zoned congeners across all tissues, with holdfast communities being distinct from vegetative and receptacle tissues both within and across congeners (Figure 3)
Summary
Intertidal zones are dynamic ecosystems where multiple abiotic stresses (e.g., temperature, desiccation, light, nutrient) and biotic interactions (e.g., competition, facilitation, parasitism, predation) occur with differing intensities across a few vertical meters. Abiotic stresses determine the upper distributional limit for macrobiota (e.g., Baker, 1909, 1910; Doty, 1946; Schonbeck and Norton, 1978, 1979; Dethier et al, 2005), whereas biotic interactions determine the lower limit (e.g., Connell, 1961a,b; Paine, 1974; Lubchenco, 1980; Hartnoll and Hawkins, 1985). This is not an absolute rule, (e.g., Underwood, 1980; Hartnoll and Hawkins, 1985). Bacteria can be opportunistic pathogens, possibly due to abiotic or biotic stresses decoupling mutualistic relationships (Egan et al, 2014; Kumar et al, 2016)
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