how microbes can prevent invasive success

Invasive species can cause dramatic changes to environments by out competing native species. A lack of understanding of the processes that allow species to invade successfully makes it difficult to manage their establishment and their spread. There is some understanding on the mechanisms underpinning the establishment of invasive terrestrial plants but these tend to focus on the direct effects on above ground processes. There is growing evidence to show that soil microbes have some part in controlling invasive success in terrestrial ecosystems, but this is yet to be tested in marine ecosystems.

Microbial communities can have positive and negative effects on invasive success. 

Microorganisms in marine sediments can exert strong control over ecological processes by controlling things like nutrient availability and sediment chemistry which in turn effects marine macrophytes. These processes differ between interacting invasive and native species.

Caulerpa taxifolia is an invasive green alga that forms high density beds in sediments outside sea grass beds and has severe impacts on native fauna within the sediments. It can outperform native sea grasses and it is thought its success comes from the alga’s ability to modify chemical and physiological sediment properties. C. taxifolia grows amongst seagrass and is often dense immediately next to it. Zostera capricorni commonly occurs as meadows in mud and sand.

Sediments were collected from Coral Bay, Austrailia. Samples were taken from sites with 100% cover of Zostera capricorni, 100% cover of Caulerpa taxifolia and taken from sediments with o% alga cover as a control. Each of these samples were then split in half with half being autoclaved to create an inactive microbe free sediment type. Commercially available sediment was also purchased and used as an inactive control to account for any modification autoclaving could have on the sediment. Detrus samples from each site were ground into a paste and added to the sediment samples. Fragments of each species were placed into each experimental condition; the biomass of each fragment was used to measure fragment growth. To assess microbe activity in the sediments 16s rRNA was amplified using PCR and Operational taxonomic units (OTU) were created using MOTHUR.

The study found that each sediment type was characterised by the same OTU’s but with contrasting abundance. The most abundant being Gamma and Delta-proteobacteria. Phylum Bacteroidetes and phylum Chlorofexi were also abundant, these bacteria are common in estuarine and marine sediments and carry out aerobic and anaerobic nutrient cycling including nitrogen, sulfur and iron. C. taxifolia sediments were found to contain bacteria associated with the reduction of sulfate, sulphite, thiosulfate and sulfur in anaerobic environments. Z.capricorni sediments had bacteria accosciated with the oxidation of sulfur in aerobic environments producing sulfates.

Z. Capricorni sea grass sediments likely have bacterial communites that are delicately balanced between aerobic and anaerobic sulfur cycling which may provide an unfavourable environment for C. taxifolia. C. taxifolia may be able to take hold on declining sea beds due decaying detrus making the sediments anaerobic, the absence of Z. capricorni rhizoids may also lead to a decrease of oxygen being put into the sediments. This lack of oxygen would be favourable to sulfate reducing bacteria already present in the sediments which would cause an increase in sulphides. This would make the environment favourable for C. taxifolia. The invasion of C. taxifolia in disturbed sea beds accelerates the decline of seagrasses by modifying the sediments and increasing sulphide levels. C. taxifolia can only invade sea beds where there is already a microbial community present as results showed that C. taxifolia fragment growth decreased in sediments absent of microbes.

In conclusion. Z. capricorni and C. taxifolia has similar bacterial communities but in varying levels of density. Z. capricorni has rhizoids which provide oxygen for aerobic sulfur cycling. C. taxifolia is associated with microbes that use anaerobic sulfur cycling. These differences in sediment chemistry prevent C. taxifolia from invading. However, if the Z. capricorni seagrass bed is damaged, the breakdown of detrus along with the absence of rhizoids causes the sediment to become anaerobic. This allows anaerobic sulfur reducing bacteria to increase making the sediments inhabitable for C. taxifolia. The more C. taxifolia invades the more it changes the sediments and Z. capricorni declines. C. taxifolia can only invade where microbial communities are already established.

This paper has shown that microbial communities have an important role in sediment function like terrestrial ecosystems. Microorganisms appear to indirectly modify the establishment and growth of invasive macrophytes in marine sediments. This is an important step to understanding how marine sediments can provide biotic resistance to invasive species it also improves our knowledge of how coastal ecosystem pressures could allow invasive species to take hold. Further study could allow us to identify the importance of microorganisms in identifying the risk to native species by invasive species. There could also be other terrestrial studies which could be applied to marine environments. I think that this was a good paper apart from the layout which I feel affects the flow of how you read it.

https://www.nature.com/articles/s41598-017-10231-2.pdf

Gribben et al., (2017) ‘Microbial communities in marine sediments modify success of an invasive macrophyte’, Scientific Reports 7: 9845. DOI:10.1038/s41598-017-10231-2