The root of influence: root-associated bacterial communities alter resource allocation in seagrass seedlings

• Published in: Frontiers in Marine Science 2023-11, Vol.10
• Main Authors: Randell, Anthony S., Tanner, Jason E., Wos-Oxley, Melissa L., Catalano, Sarah R., Keppel, Gunnar, Oxley, Andrew P. A.
• Format: Article
• Language: English
• Subjects: Bacteria; bacterial communities; Deoxyribonucleic acid; DNA; Gene libraries; Genetic resources; Growth; Harbors; Harbours; Leaves; meadow sediment; Meadows; Microorganisms; Plant tissues; Posidonia; Propagation; Resource allocation; Restoration; Rhizoplane; Rhizoplane microorganisms; Rhizosphere; Rhizosphere microorganisms; Roots; rRNA 16S; Sand; Sea grasses; seagrass restoration; Seawater; Sediment; Sediment samples; Sediments; Seedlings; Starch; Substrates; Survival; Water analysis; Water sampling
• ISSN: 2296-7745; 2296-7745
• DOI: 10.3389/fmars.2023.1278837

Seagrass roots harbour diverse assemblages of microorganisms that likely benefit the growth and survival of meadows. Yet, restoration efforts rarely consider their effect on developing seagrass seedlings. Sediment origin should determine the types of rhizosphere and root-colonising (rhizoplane) microorganisms and, therefore, should influence the performance of seedlings during restoration, particularly for slow growing climax species of seagrass like Posidonia. Recent Posidonia restoration attempts in South Australia used commercially sourced ‘play pit sand’ for seedling propagation and planting, but have been impacted by high mortality. More natural substrates like seagrass meadow sediment have not been previously investigated for suitability over conventional substrates with regard to seedling growth and survival. To assess the relevance of seagrass associated microorganisms in the growth of Posidonia angustifolia seedlings, we investigated the bacterial communities from tank-raised seedlings propagated in autoclave treated and untreated ‘play pit sand’ and a natural meadow mix (comprising beach sand and meadow sediment) over a 12-week period. Growth measurements (numbers and total length of roots and leaves, root diameter, seedling weight, starch reserves) and samples for bacterial community analysis were taken at 4 and 12 weeks. Bacterial assemblages were surveyed from DNA extracts from bulk and rhizosphere sediments and root tissues, as well as from swabs of P. angustifolia fruit, established meadow sediment and water samples prior to seedling propagation, by constructing Illumina 16S rRNA gene libraries. Overall, growth measurements did not vary significantly between sediment type or treatment. However, treatment, sediment type and seed length did significantly influence the proportional growth of roots versus leaves (as expressed as a pseudo root:shoot ratio). Seedlings from meadow mix invested more in leaves, regardless of treatment, when compared to play sand. Autoclave treatment increased investment in roots for play sand but increased the investment in leaves for meadow mix. Bacterial communities differed significantly between sediments and between sample types (bulk and rhizosphere sediments and roots), with the roots from meadow mix seedlings containing an increased abundance of various potentially beneficial bacterial taxa. Such changes appear to affect the early development of seedlings, a finding that is pertinent to future seagrass restora