Abstract
Seagrass roots host a diverse microbiome that is critical for plant growth and health. Composition of microbial communities can be regulated in part by root exudates, but the specifics of these interactions in seagrass rhizospheres are still largely unknown. As light availability controls primary productivity, reduced light may impact root exudation and consequently the composition of the root microbiome. Hence, we analyzed the influence of light availability on root exudation and community structure of the root microbiome of three co-occurring seagrass species, Halophila ovalis, Halodule uninervis and Cymodocea serrulata. Plants were grown under four light treatments in mesocosms for 2 weeks; control (100% surface irradiance (SI), medium (40% SI), low (20% SI) and fluctuating light (10 days 20% and 4 days 100%). 16S rDNA amplicon sequencing revealed that microbial diversity, composition and predicted function were strongly influenced by the presence of seagrass roots, such that root microbiomes were unique to each seagrass species. Reduced light availability altered seagrass root exudation, as characterized using fluorescence spectroscopy, and altered the composition of seagrass root microbiomes with a reduction in abundance of potentially beneficial microorganisms. Overall, this study highlights the potential for above-ground light reduction to invoke a cascade of changes from alterations in root exudation to a reduction in putative beneficial microorganisms and, ultimately, confirms the importance of the seagrass root environment – a critical, but often overlooked space.
Highlights
It has long been established that roots of terrestrial plants are colonized by a diverse assemblage of microbes that collectively function as a ‘microbiome.’ These microbiomes are critical for plant growth and health via their influence on biogeochemical cycling and nutrient acquisition, induction of host defense to pathogens and disease and production of plant growth regulators (see reviews by Reinhold-Hurek et al (2015) and Alegria Terrazas et al (2016)
We addressed three hypotheses: (i) the composition and predicted function of the root microbiome will differ among seagrass species and be distinct from the surrounding sediment, (ii) root exudation will be increased by low and/or fluctuating light availability, and (iii) microbial composition and predicted function will be influenced by light availability, but in a seagrass species-specific manner
We found that light reduction altered seagrass root exudation and the composition of seagrass root microbiomes with a decrease in abundance of potentially beneficial microorganisms
Summary
It has long been established that roots of terrestrial plants are colonized by a diverse assemblage of microbes that collectively function as a ‘microbiome.’ These microbiomes are critical for plant growth and health via their influence on biogeochemical cycling and nutrient acquisition, induction of host defense to pathogens and disease and production of plant growth regulators (see reviews by Reinhold-Hurek et al (2015) and Alegria Terrazas et al (2016). Seagrass Root Microbiomes and Light to carry out metabolic functions important for their hosts including nitrogen fixation (Bagwell et al, 2002; GarciasBonet et al, 2016), sulfate reduction and oxidation (Küsel et al, 2006), phosphate solubilisation (Ghosh et al, 2012) and nutrient turnover (Duarte et al, 2005; Trevathan-Tackett et al, 2017). A reduction in light availability reduces photosynthetic performance and impacts negatively on growth, productivity and overall survival of the plant (Biber et al, 2009; Yaakub et al, 2014). Seagrasses avoid negative impacts from short-term light reductions by modifications to their physiology and metabolism; light reduction of more than 1 month may be required to affect growth (Longstaff and Dennison, 1999; Ralph et al, 2007; McMahon et al, 2013). Microbial communities can respond rapidly to disturbance (Allison and Martiny, 2008) and monitoring their composition could prove an effective early indicator of environmental fluctuations and ecosystem change
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