Bacterial response to alkalinity enhancement in intertidal environments: results of one-year field experiment

Marine Carbon Dioxide Removal (mCDR), in particular alkalinity enhancement, is considered a promising measure to increase oceanic uptake and long-term storage of CO₂ from the atmosphere. This may be affected by spreading of fine-grained, mafic minerals and rocks in coastal areas, where the weathering of these substrates produces excess alkalinity and thus increases the CO₂ consumption. Before marine alkalinity enhancement can be considered as a large-scale mCDR measure, the biogeochemical and ecological impacts are to be evaluated under natural conditions in field experiments. The response of the bacterial community to alkalinity enhancement is of prime importance, because of their high biomass, low trophic levels, and relevance for the nutrient cycle.

An experiment was installed and monitored for one-year in the intertidal pioneer vegetation zone of the saltmarsh at Ria Formosa Coastal Lagoon, southern Portugal. The experimental plot comprised three replicate deployments of fine and coarse-grained olivine and basalt, and an untreated control site. The pore water properties (e.g., temperature, salinity, pH, alkalinity) of the substrates and the control were analysed every month. Sediment samples were collected from each treatment and the control every three months, starting the day after substrate deployment in September 2022 until June 2023, thus covering a one-year seasonal cycle. Bacterial dynamics were monitored using a metagenomic full-length 16S gene approach conducted by AppGenomics Lda, Faro. DNA was extracted from the sediment samples. The 16S region was amplified and sequenced using the Oxford Nanopore Technologies (ONT) Promethion P2 solo sequencerlibrary and equipments. After quality control and filtering, the generated reads were analysed regarding the taxonomic content using the Kraken2 and Bracken tools coupled with the database RefSeq 16S database of NCBI. The results were analysed using the Phyloseq R package. Proteobacteria (54 to 36%), Bacteroidota (23 to 7%), Cyanobacteria (19 to 3%) and Planctomycetota (16 to 8%) were the most abundant phyla in all samples. Bacteriodota increased in abundance with high alkalinities in the treatments with fine olivine while the Proteobacteria were suppressed by the high pH and ensuing alkalinities in the olivine treatments. There was no response to the basalt treatments, although alkalinity was also increased compared to the control. The bacterial Shannon Diversity Index (H) of the four treatments and the control ranged from 5.66 to 6.44 and no significant differences on the bacterial diversities in the different treatments were found.

Acknowledgement. Research supported by the Portuguese Science Foundation, with the projects RECAP - PTDC/CTA-CLI/1065/2021 (https://doi.org/10.54499/PTDC/CTA-CLI/1065/2021), UID/00350/2020CIMA (https://doi.org/10.54499/UIDP/00350/2020, https://doi.org/10.54499/UIDB/00350/2020), LA/P/0069/2020ARNET and contracts DL57/2016/CP1361/CT0009, CEECINST/00052/2021/CP2792/CT0012.