Congrats to Natalia Erazo for her first first-authored publication in the lab! Her paper, Sensitivity of the mangrove-estuarine microbial community to aquaculture effluent, appears in a special issue of the journal iScience. The publication is the culmination of our 2017 field effort in the Cayapas-Mataje and Muisne regions of Ecuador.
Ecuador is ground zero for mangrove deforestation for shrimp aquaculture. Most of Ecuador’s coastline is in fact completely stripped of mangroves. The biogeochemical consequences of this aren’t hard to imagine. Mangrove forests contain a significant amount of carbon in living biomass and in the sediment. Aquaculture ponds, by contrast, contain a large amount of nitrogen as a result of copious additions of nitrogen-rich shrimp feed. The balance of C to N is one of the fundamental stoichiometric relationships in aquatic chemistry. When it shifts all kinds of interesting things start to happen.
The one place in Ecuador where you can find large areas of mangroves is the Cayapas-Mataje Ecological Reserve. CMER is in fact the largest contiguous mangrove forest on the Pacific coast of Latin America. Its status comes from an interesting combination of social and economic factors that left this part of Ecuador relatively undeveloped until recently. There is shrimp aquaculture in the reserve, but it’s nowhere near as expansive as in Muisne and other ex-mangrove sites in Ecuador.
Natalia leveraged the different levels of disturbance present in Cayapas-Mataje, and between Cayapas-Mataje and Muisne, to explore what the impact of all this aquaculture activity is on microbial community structure. After all it’s really the microbial community that responds to and drives the biogeochemistry, so understanding the sensitivity of these communities to the changing conditions gives us insight into how the system is changing as a whole.
By using our paprica pipeline Natalia was able to evaluate changes in microbial community structure, predicted genomic content, and key genome features across the disturbance gradient. A nitrogen excess (relative to phosphorous) was associated with bacteria with larger genomes and more 16S rRNA gene copies, indicative of a more copiotrophic or fast-growing population. This has implications for how carbon is turned over or retained at the higher levels of disturbance.
Different microbial metabolisms are also associated with the level of disturbance. The figure above shows the distribution of predicted metabolic pathways associated with nitrogen metabolism. Nitrogen fixation, a feature of microbial symbionts of many plants, is less abundant at high levels of disturbance, while pathways associated with denitrification are more abundant. The interesting thing about this is that these samples are restricted to the mangroves themselves – the high disturbance samples don’t reflect the actual aquaculture ponds – so these changes reflect altered processes in the remaining stands of mangroves. The loss of beneficial, symbiotic bacteria and elevated abundance of putative shellfish pathogens suggests the impacts of aquaculture are not limited to the physical removal of mangrove trees and associated release of carbon.
