The Influence of Natural Seagrass Bed Sediments and the Microbial Communities therein on Z. marina Growth and Survival for Restoration Purposes.
Supervisor: Emma Ransome (Imperial College London)
Seagrass meadows are valuable marine habitats that are declining globally at a rate of 1.5% per year due to anthropogenic pressures and natural diseases. Some species are particularly affected. The most abundant species of the temperate North Atlantic, Zostera marina, has declined in coverage by over 50% in Europe since the 1860’s. The Ocean Conservation Trust (OCT), the UK’s leading expert in seagrass restoration, has contributed to restoration efforts by growing Z. marina from seeds in an indoor facility. However, they have observed inconsistent germination rates and plant health deterioration during the indoor growth period. There is potential to improve this with the use microbial probiotics as is done in agriculture. While putatively beneficial, seagrass-associated microbes have been identified, their use has rarely been investigated, and never in an facility such as that of the OCT.

To this end, an 83-day experiment was conducted testing the impact of natural seagrass bed sediments and the microbial communities therein on the growth and survival of Z. marina in the OCT’s indoor facility. Seeds were planted in microbe-free (autoclaved) and microbe-containing natural sediments collected from the Cawsands and Drakes Island seagrass meadows in the UK. The non-sterile, horticultural sand typically used by the OCT was used as a contrasting and control treatment. The effects thereof were assessed on Z. marina germination success, shoot loss, and leaf abundance. Furthermore, sediment microbial communities were cultured throughout to provide insight into the application of probiotics in such facilities.

This study found that growing Z. marina in natural sediments had no effect on leaf abundance, but significantly increased germination success and plant survival, in contrast to the horticultural sand. Germination success increased from 4.85 ± 3.48% in the sand treatment to 11.7 ± 2.9 % and 12.80 ± 4.64% in the microbe-free Cawsands and Drakes Island sediments, respectively. The subsequent deterioration of plant health, likely due to temperature increases and algae cover, showed a decrease in shoot mortality from 83.2 ± 18.5% in the sand to 55.3 ± 9.5% and 67.0 ± 10.9% in the microbe-free Cawsands and Drakes Island sediments, respectively. The presence of a microbial community within the natural sediments, however, had no effect on leaf abundance and shoot mortality, but significantly decreased germination success (to 9.2 ± 5.0% and 8.9 ± 5.4% for Cawsands and Drakes Island, respectively). The culturing of bacteria at different timepoints throughout the experiment showed that colony abundance and richness significantly increased in natural sediments and significantly decreased with the presence of a natural sediment microbial community, highlighting differing microbial community coalescence processes. Despite expectations that natural seagrass microbial communities would benefit Z. marina growth, this study illustrates the need for a better understanding of such processes in indoor growth facilities for their successful implementation. However, the use of natural sediments proved to have clear benefits for Z. marina growth.