Influence of organic and microbial load on the microbiome of sewage sludge anaerobic reactors

Resumo

A complex and dynamic microbial community is responsible to carry out the anaerobic digestion (AD) process. Many factors can affect its composition and structure. Among them, the type of substrate stands out. The organic matter concentration and composition are the main drivers of changes in these communities. Furthermore, the presence of a high load of viable microorganisms in the substrate can also influence the AD microbiome. The main goal of this thesis was to study the influence of the organic and microbial load on the microbiome of sewage sludge anaerobic reactors. As well, the microbial correlation of AD microbiome during different conditions. During chapter 1 and 2 a general introduction and the analytical methods used during all experimental work were presented. In Chapters 3 and 4, replicated anaerobic sewage sludge reactors were subjected to organic overloading. As a consequence, the volatile fatty acids accumulated within the first 24 hours, while methane production started to decrease after 30 h. The time response of the microbial community was shorter (3 h) in relation to changes in physic-chemical parameters (15 h). An increase in the dynamics of the bacterial community and in the relative abundances of Tenericutes (Acholeplasma), Erysipelotrichaceae, Methanoculleus and Woesearchaeota were observed before any change in monitoring parameters. However, the greatest shifts in the microbiome occurred simultaneously with the changes in the physic-chemical parameters. For instance, the increase of hydrogenotrophic methanogens and the decrease of acetoclastic Archaea were observed when propionic acid reached values over 5 g / L during the first 24 h of organic overloading. The changing environment during the organic overloading caused a temporary succession of functionally redundant microorganisms, suggesting a niche specialization. In addition, there were several mutualistic and competitive relationships between groups of microorganisms, although most of the details of these mechanisms remain hidden due to the lack of information on the genetic and physiological capacities of many microbial groups in the AD microbiome. The volatile fatty acids increase not always is an adverse environment for Methanosaeta. The reactor acidosis can occur, at least transiently, without the decrease in the abundance of this methanogen. In general, during the organic overloading, the interrelations of the species highlight that this deterministic event drove the dynamics of the microbial community of AD. The continuous entrance of microorganisms into a sewage sludge reactor was investigated during chapter 5. The microbial load did not affect the performance of the AD process and methane production. Likewise, no significant changes were observed in bacterial community. However, the immigration of microorganisms seems to influence the structure and composition of the archaeal community by affecting the proportion of the type of interaction (co-presence / mutual exclusion) among microorganisms. The most of incoming microorganisms died in the sewage sludge anaerobic reactors microbial communities. Some Bacteria were able to survive in the microbiome of the reactors. However, they were in a very low number of sequences read. Besides, none of the immigrant microorganisms was enriched in the anaerobic reactors microbial communities. The results and knowledge about the dynamics of the AD microbiome acquired in this thesis are very important to improve the management and efficiency of the AD process.