QPCR-based relative quantification of the brown algal endophyte Laminarionema elsbetiae in Saccharina latissima: variation and dynamics of host\u2014endophyte interactions

Miriam Bernard,Akira F Peters,Bertrand Jacquemin,Marion Ballenghien,Sylvie Rousvoal,Catherine Leblanc

doi:10.1007/s10811-017-1367-0

Abstract

Morphological changes—such as dark spots, twisted stipes and deformed blades—have been observed in wild and cultivated Saccharina latissima. The putative cause for the disease symptoms is the filamentous endophytic brown alga Laminarionema elsbetiae, which is known to invade stipes and fronds of its hosts. Little is known about this interaction and its occurrence in the field, although former studies indicated high endophyte prevalence in kelp populations. Previous epidemiological studies on kelp endophytes were mainly based on the examination of microscopic sections, followed by time-consuming isolation and cultivation steps in order to identify the endophyte and a reliable method to quantify endophyte infections was missing. As a novel approach, we established and validated a qPCR assay for relative quantification of the endophyte L. elsbetiae within its host S. latissima, which allows to examine both, the prevalence of endophytic algae and the severity of infections. The assay was shown to be highly specific and suitable to reliably detect small amounts of endophyte DNA in the host. Using this method, we detected very high endophyte prevalence in the investigated kelp populations, up to 100% in young S. latissima sporophytes in Brittany during spring. Furthermore, our results suggest that Saccharina sporophytes are infected early in their life and that seasonality and environmental factors have a significant impact on infection rates. In the future, this approach could also be applied to study other host-endophyte pairs using specific primers.

Highlights

The sugar kelp Saccharina latissima (Laminariales, Phaeophyceae) is an important primary producer in temperate to cold northern hemisphere coastal ecosystems (Bartsch et al 2008) and an economically relevant seaweed with high industrial potential (Østgaard et al 1993; Adams et al 2009)
The onset of endophyte infections in the field was explored by obtaining infection rates of young S. latissima sporophytes with different thallus lengths collected in March 2017 in Northern Brittany (Perharidy; 48.73° N, 4.00° W, N = 10) that were grouped according to the host length: 3–5 cm (N = 6), 6–10 cm (N = 7), 11–15 cm (N = 8), 16–26 cm (N = 4), and > 26 cm (N = 30)
Artificial mixtures with the same amount of host DNA and different amounts of endophyte DNA were used to test if varying amounts of L. elsbetiae could be detected reliably, even in low concentrations (Online Resource 2A + B)

Summary

Introduction

The sugar kelp Saccharina latissima (Laminariales, Phaeophyceae) is an important primary producer in temperate to cold northern hemisphere coastal ecosystems (Bartsch et al 2008) and an economically relevant seaweed with high industrial potential (Østgaard et al 1993; Adams et al 2009). Growing or being cultivated in the sea S. latissima is exposed to a high number of potentially harmful organisms such as fungi, bacteria or endophytic algae (Andrews 1977; Wu et al 1983; Apt 1988a; Potin et al 2002) Previous studies on the latter reported a high prevalence of filamentous endophytic algae in kelp populations, with up to 100% of infected individuals (Andrews 1977; Lein et al 1991; Peters and Schaffelke 1996; Schaffelke et al 1996; Ellertsdóttir and Peters 1997). There is no reliable technique to quantify endophyte infections, which is crucial to investigate the dynamics of this phenomenon

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