The Seagrass Holobiont: What We Know and What We Still Need to Disclose for Its Possible Use as an Ecological Indicator

Abstract

Microbes and seagrass establish symbiotic relationships constituting a functional unit called the holobiont that reacts as a whole to environmental changes. Recent studies have shown that the seagrass microbial associated community varies according to host species, environmental conditions and the host’s health status, suggesting that the microbial communities respond rapidly to environmental disturbances and changes. These changes, dynamics of which are still far from being clear, could represent a sensitive monitoring tool and ecological indicator to detect early stages of seagrass stress. In this review, the state of art on seagrass holobiont is discussed in this perspective, with the aim of disentangling the influence of different factors in shaping it. As an example, we expand on the widely studied Halophila stipulacea’s associated microbial community, highlighting the changing and the constant components of the associated microbes, in different environmental conditions. These studies represent a pivotal contribution to understanding the holobiont’s dynamics and variability pattern, and to the potential development of ecological/ecotoxicological indices. The influences of the host’s physiological and environmental status in changing the seagrass holobiont, alongside the bioinformatic tools for data analysis, are key topics that need to be deepened, in order to use the seagrass-microbial interactions as a source of ecological information.

Spatial and Species Variations of Bacterial Community Structure and Putative Function in Seagrass Rhizosphere Sediment

“…Seagrass holobionts have been reported to play essential roles in the cycle of sulfur, nitrogen, and carbon, at both microbial structural and functional levels [ 11 , 13 ], and Ugarelli et al (2019) [ 51 ] reported that the seagrass plant and its microbiome were highly interlinked in the cycle of sulfur, nitrogen, and carbon. Likewise, FAPROTAX analysis revealed that many microbes in the seagrass rhizosphere of coral reef ecosystems participated in these processes ( Figure S3 ).…”

“…FAPROTAX dataset is a manually constructed database that maps prokaryotic taxa (e.g., genera or species) to putative functions based on the literature on cultured representatives [ 40 ]. The core microbial community in this study was defined as those present in all samples, including all three replicates [ 11 ], and their network analysis was further analyzed based on a random matrix theory (RMT)-based method according to Deng [ 41 ] and Luo [ 42 ]. Network analysis of the core microbial community was processed using by molecular Ecological Network Analysis Pipeline ( (accessed on 17 August 2020)) and visualized in an open-source software platform Cytoscape (v.3.6.0).…”

“…Seagrass holobiont could execute many functions that may benefit their hosts [ 9 , 10 , 11 ]. Its structure and function are key to seagrass productivity, health, and the biogeochemical cycle of carbon, nitrogen, and sulfur cycle [ 11 , 12 , 13 ].…”

“…Seagrass holobiont could execute many functions that may benefit their hosts [ 9 , 10 , 11 ]. Its structure and function are key to seagrass productivity, health, and the biogeochemical cycle of carbon, nitrogen, and sulfur cycle [ 11 , 12 , 13 ]. The seagrass rhizosphere microbe refers to the microbe that inhabits in the narrow zone surrounding and could be influenced by seagrass roots [ 13 , 14 ].…”

“…Moreover, the rhizosphere microbiome plays crucial roles in affecting plant health, such as nutrient uptake, preventing colonization by pathogens, and modulating host immunity, etc. [ 11 , 13 , 15 ]. The predominant bacteria in the rhizosphere of seagrass Zostera marina , Z.noltii , and Cymodocea nodosa were Proteobacteria, Bacteroidetes, Chloroflexi, Planctomycetes, Actinobacteria, and Acidobacteria [ 16 ].…”