Browsing by Subject "Marine Science"

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  • Camarena Gómez, María Teresa (Helsingin yliopisto, 2019)
    Global warming is one of the most alarming pressures affecting marine ecosystems worldwide. One of the indirect effects of the increasing surface-water temperature is the change in phytoplankton community composition, shifting in some ecosystems from diatom predominance towards the dinoflagellate predominance or co-occurrence with diatoms during blooms. These distinct phytoplankton groups vary in the quality and/or quantity of the dissolved organic matter (DOM) they release, which may have contrasting effects on the associated bacterioplankton communities, in terms of structure and function, and also on the carbon flux passing through the microbial loop. The main objective of this thesis was to assess the effect of diatoms and dinoflagellates on shaping the bacterial community composition and dynamics in different ecosystems in which either one or both of these two groups dominate the phytoplankton bloom, such as the Baltic Sea and the Humboldt Current System (HCS) off Chile. This was achieved by conducting both experimental and field studies in these areas. Phytoplankton community composition and the stage of the bloom phase clearly affected to the bacterial community composition and dynamics in both ecosystems. Alphaproteobacteria, dominated by SAR11 and Rhodobacteraceae, was the most abundant bacterial class in all studies. The oligotrophic SAR11 dominated in pre-bloom conditions and was associated with dinoflagellates. In contrast, copiotrophic bacteria belonging to the classes Flavobacteriia, Gammaproteobacteria, Betaproteobacteria and the family Rhodobacteraceae (genera Loktanella, Planktotalea, Planktomarina and Amylibacter) were associated with diatom species such as Achnanthes taeniata, Chaetoceros spp., Skeletonema costatum and Thalassiosira levanderi in the Baltic Sea and with Thalassiosira spp. in the HCS. In addition, in the Baltic Sea, bacterial communities dominated by copiotrophs had higher bacterial production rates than in SAR11 dominated bacterial communities. Hence, the diatom-released DOM boosted the development of more productive bacterial communities during phytoplankton blooms. Further differences in the bacterial community composition were driven by the different salinities in these two ecosystems; Betaproteobacteria, Planctomycetes and Actinobacteria were more abundant in the brackish Baltic Sea than in the HCS. The results of this thesis highlight that the shift to the dinoflagellate dominance or co-occurrence with diatoms may affect the bacterial community composition and activity during bloom events. Certain diatom species promote the growth of copiotrophic bacteria, which contribute largely to high bacterial production rates and recycling of carbon. In contrast, the increase in dinoflagellate abundance associated with global warming may potentially change the pelagic remineralization of organic matter, which could reduce the carbon flux to higher trophic levels.