Particulate organic matter: Vector for carbon export, indicator for aquaculture impact and microbial hotspot


A large proportion of global carbon lies within the ocean. Here, the particulate fraction (compounds >0.45 µm) can serve as vectors for horizontal and vertical carbon export, indicators for aquaculture impact and microbial habitats, hotspots and refuges.
In Eastern Boundary Currents, such as the Canary Current system off NW Africa, upwelling of nutrient-rich waters stimulates primary productivity. This results in substantial horizontal and vertical export of particulate organic matter (POM). Meanwhile, heterotrophic bacteria are involved in the degradation of the organic matter. On the one hand this can result in a less efficient carbon sequestration but on the other hand this may stimulate food webs in the surface waters. Thereby, the bacterial community composition (BCC) is a major factor, influencing organic carbon (from here on carbon) turnover capacities. Especially due to global climate change, the vertical carbon export, termed "biological carbon pump" (BCP) is of great interest. However, despite the important role of bacteria in determining the efficiency of the BCP, little is known about the BCC off NW Africa. Therefore, the first objective of this thesis is the description of the free-living (FL) and particle-attached (PA) BCC off NW Africa and the identification of potential key players in carbon degradation. Results reveal high relative abundances of particle-degrading bacterial groups, suggesting a key role of these bacteria in the turnover of carbon. Furthermore, simulated sinking of POM resulted in a shift in the attached BCC, which was not observed in simulated horizontal POM export. As a comparable shift has previously been shown to lead to reduced respiration rates, future research might reveal another possible microbial mechanism, involved in increasing vertical carbon export efficiencies.

Coastal aquaculture practices in the tropics are often accompanied by eutrophication and high POM loads in their effluents. Here, the horizontal export of high amounts of POM can have detrimental effects on adjacent ecosystems (e.g. seagrass beds and coral reefs) and their functioning. The vertical export of POM, especially in shallow coastal regions, often causes the accumulation of thick layers of organic matter on the sea floor. In both cases, bacterial carbon degradation is stimulated by high POM concentrations, leading to a substantial drawdown of O2, at times even hypoxia. In Bolinao, Philippines, recurring hypoxic events in the water column lead to major fish kills. Nevertheless, so far POM-attached microbial respiration rates have not been quantified, yet. Therefore, another aim of this study was the quantification of PA microbial respiration rates to assess if their activity can contribute to recurring water column hypoxia in Bolinao. Results reveal high carbon specific respiration rates, indicating that PA microbial carbon degradation has the potential to contribute to water column hypoxia. Furthermore, due to direct impact of aquaculture practices on the POM (e.g. due to feeding), POM characteristics, such as carbon/nitrogen (C/ N) ratio, stable isotope values, POM concentrations, sinking velocities, bacterial alpha diversity and their BCC, may reflect the aquaculture impact. Therefore, we tested the possibility if POM characteristics can serve as indicators for aquaculture impact on coastal ecosystems. Results indicate a strong signal of aquaculture activities of POM characteristics, rendering them ideal indicators for aquaculture impact.

Despite the major ecological differences between the two study sites of this thesis, POM always served as a substrate for PA bacteria. However, microbial particle dynamics are very complex. Motility and chemotaxis allow certain bacterial groups to find and attach to substrates (POM). Some bacteria only loosely associate and detach again, if they encounter other hotspots in their vicinity. However, so far it is unknown how differences in substrate availability in the natural environment affect this process. Therefore, we aimed at determining the similarities of FL and PA bacteria in waters with high (deep-chlorophyll maximum) vs. low (meso- to ultraoligotrophic waters) substrate availabilities. Results indicate that FL and PA bacteria are more similar at the DCM. This may indicate that substrate availability stimulates the exchange of PA bacteria between substrates, because to reach the next hotspot, they have to cross the FL fraction. Additionally, also vertical and horizontal export may result in exchange of PA bacteria, because the substrate-attached bacteria are subjected to different water masses and hence surrounding BCC. Therefore, we examined if the BCC of substrate-attached bacteria is also in exchange with surrounding BCC during horizontal and vertical export. Although we observed that the majority of BCC was determined at the beginning, we identified changes in substrate-attached bacteria when subjected to different water masses, which indicates exchange (e.g. due to scavenging) with the surrounding sea water BCC. Overall, the results presented in this thesis advance our understanding regarding the microbial component of the BCP in Eastern Boundary Currents, about biogeochemical processes on aquaculture-derived POM and about microbial particle dynamics. Furthermore, they indicate how processes occurring on the microscale are relevant for current global challenges, such as climate change and the degradation of coastal ecosystems.