Assessing the impact of macroalgal blooms on microbial communities and nitrogen cycling within estuarine sediment

Abstract

Microbial communities are the drivers of biogeochemical cycles. These communities are relied upon for converting nutrients into bioavailable forms and also removing excess anthropogenic nutrient pollution from the environment. Estuarine sediments play a vital role in filtering out excess nitrogen however, many estuarine ecosystems are becoming degraded due to high nutrient input and subsequent growth of bloom macroalgae. A variety of methods were employed within this thesis to gather data from environmental DNA and RNA, such as 16S rRNA gene sequencing, metagenomics techniques such as whole genome sequencing and transcriptomics, along with quantitative methods such as droplet digital PCR were used to assess the impact of nutrient induced macroalgal blooms on sediment microbial community structure and also microbial community function, with a focus on nitrogen cycling. Results from this thesis show small changes in microbial community composition and functioning with short term algal cover (10 days), these changes become greater with an extended duration of experimental cover (28 days). We observed changes in in the relative contribution of nitrification and denitrification to overall community functioning via transcriptomic methods after 10 days, and significant quantitative changes in target genes for nitrification and denitrification after 28 days. Long-term cover also significantly alters communities in comparison to non-impacted sediment. Targeted genes for nitrification – ammonia monooxygenase (amoA), and denitrification – nitrous oxide reductase (nosZ), also showed differing responses to seasonal factors. Expression of nosZ showed no obvious seasonal trend while amoA showed a strong response to season. Finally there were differing effects on nitrification and denitrification between the two type of macroalga studied. Ulva, but not Gracilaria, appeared to have a significant effect on gene expression, reducing the relative contribution of denitrification and nitrification, while increasing that of dissimilatory nitrate reduction to ammonia. However, Gracilaria had a stronger impact on geochemistry than Ulva, with greatly reduced N availability at covered sites observed. This thesis advances the understanding of the impact of macroalgal blooms and the extent to which blooms disrupt vital biogeochemical cycles in the short and long-term, as well as with season and how the type of macroalga can lead to differing responses.