Abstract
The marine microbial community plays a fundamental role in controlling atmospheric carbon dioxide (CO2) by the drawdown and subsequent incorporation into biomass as either particulate or dissolved organic matter, thus driving oceanic carbon pumps for long-term storage in the ocean interior. Microbial growth and activity is sustained and ultimately controlled by the availability of vital nutrients, nitrogen (N) and phosphorus (P), whilst progressing climate change with associated rise in temperature, changes in water chemistry and increasing ocean acidification imposes additional pressure on the viability of the marine microbiome. Simultaneously, progressive anthropogenic pressures such as increasing emissions of greenhouse gases, changes in environmental conditions and perturbations towards nutrient resources are further constraining the microbial community and thus ultimately the biogeochemical cycling of nutrients and carbon.This thesis provides perspectives of two exemplary oceanic regions, the subtropical North Atlantic and a temperate coastal setting, both important sites for biogeochemical cycling whilst facing a multitude of anthropogenically induced climate change. This study presents high spatial and temporal resolution data to characterise the contemporary ocean and to provide a perspective of each setting under conditions of future oceans. Therefore, a suite of biogeochemical parameters was collected and experimental incubation studies were conducted in the subtropical North Atlantic via a ship-based sampling campaign along a west- east transect on a nominal latitude of 24 °N and a long-term monitoring station was set up at Langstone Harbour, UK, supplemented by a multi-stressor incubation setup.
The contemporary subtropical North Atlantic was observed with different biogeochemical regimes, whilst occupied by small-celled cyanobacteria, dominated by Prochlorococcus and Synechococcus, and heterotrophic bacteria driving the cycling of nutrients and carbon. As such, the boundary systems accounted for substantial carbon sequestration by direct export of particulate organic matter, whilst carbon storage in the central gyre system was driven by the microbial alteration of organic matter enriched in carbon relative to nitrogen and phosphorus. Here, where inorganic nutrient supply was scarce, alternative sources from organic compounds drive the biogeochemical functioning of the system. The incubations conducted throughout the subtropical North Atlantic simulated nutrient resources in a P-limited future ocean and revealed a shift in the microbial community structure from a Prochlorococcus dominated to a community dominated by larger cell-sized members such as Synechococcus and picoeukaryotes, whilst the system shifted towards net autotrophy. As a consequence, a shift from the particulate towards the dissolved fraction in the allocation of nutrients was observed, ultimately increasing the efficiency of the microbial carbon pump by altering the stoichiometric ratios of organic matter produced.
The contemporary biogeochemical functioning of the coastal setting was found to be largely influenced by seasonal fluctuations in nutrient availability and environmental conditions. Here, larger eukaryotic organisms such as diatoms drive most of the export of nutrients and carbon via formation of particulate organic matter. Fluctuations in environmental conditions such as temperature, light and nutrient availability do not only account for changes in the efficiency of the biogeochemical cycling of nutrients and carbon, but also control the viability of the microbial community responsible, largely dominated by larger cell-sized eukaryotic organisms. As such, export of nutrients and carbon is mediated by the formation of particulate matter, whilst conditions of future coastal oceans with increased temperature and acidification displayed a shift towards a smaller cell-sized microbial community. As a consequence, the formation of particulate organic matter, and the ecosystem biogeochemical activity is hindered, resulting in a reduction in particle export, exacerbated by decreased stability and adaptation of the microbial community. The decreased microbial resilience towards additional anthropogenic stressors such as nutrient pollutions and marine heatwaves results in a decreased efficiency of coastal carbon sequestration.
This thesis, although containing limitations, which are addressed throughout, highlights significant shifts in the microbial community structure in response to future climate change related and anthropogenic perturbations. Associated impacts of a shifting microbial community on the biogeochemical cycling of nutrients and carbon and subsequent export efficiency are addressed throughout. Whilst implications of this remain broadly unknown on a global and larger temporal scale, this thesis highlights the need for further investigation and calls for an intensification of observational and experimental efforts incorporating multiple cumulative occurring stressors, whereby methodological approaches on assessing systematic responses should focus on ecosystem rather than single species level.
| Date of Award | 8 Nov 2023 |
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| Original language | English |
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| Supervisor | Sarah Reynolds (Supervisor), Federica Ragazzola (Supervisor) & Michelle Hale (Supervisor) |