Broad Phylogenetic Diversity Associated with Nitrogen Loss through Sulfur Oxidation in a Large Public Marine Aquarium.

Denitrification by sulfur-oxidizing bacteria is an effective nitrate removal strategy in engineered aquatic systems. However, the community taxonomic and metabolic diversity of sulfur-driven denitrification (SDN) systems, as well as the relationship between nitrate removal and SDN community structure, remains underexplored. This is particularly true for SDN reactors applied to marine aquaria, despite the increasing use of this technology to supplement filtration. We applied 16S rRNA gene, metagenomic, and metatranscriptomic analyses to explore the microbial basis of SDN reactors operating on Georgia Aquarium's Ocean Voyager, the largest indoor closed-system seawater exhibit in the United States. The exhibit's two SDN systems vary in water retention time and nitrate removal efficiency. The systems also support significantly different microbial communities. These communities contain canonical SDN bacteria, including a strain related to Thiobacillus thioparus that dominates the system with the higher water retention time and nitrate removal but is effectively absent from the other system. Both systems contain a wide diversity of other microbes whose metagenome-assembled genomes contain genes of SDN metabolism. These include hundreds of strains of the epsilonproteobacterium Sulfurimonas, as well as gammaproteobacterial sulfur oxidizers of the Thiotrichales and Chromatiales, and a relative of Sedimenticolathiotaurini with complete denitrification potential. The SDN genes are transcribed and the taxonomic richness of the transcript pool varies markedly among the enzymatic steps, with some steps dominated by transcripts from noncanonical SDN taxa. These results indicate complex and variable SDN communities that may involve chemical dependencies among taxa as well as the potential for altering community structure to optimize nitrate removal.IMPORTANCE Engineered aquatic systems such as aquaria and aquaculture facilities have large societal value. Ensuring the health of animals in these systems requires understanding how microorganisms contribute to chemical cycling and waste removal. Focusing on the largest seawater aquarium in the United States, we explore the microbial communities in specialized reactors designed to remove excess nitrogen through the metabolic activity of sulfur-consuming microbes. We show that the diversity of microbes in these reactors is both high and highly variable, with distinct community types associated with significant differences in nitrogen removal rate. We also show that the genes encoding the metabolic steps of nitrogen removal are distributed broadly throughout community members, suggesting that the chemical transformations in this system are likely a result of microbes relying on other microbes. These results provide a framework for future studies exploring the contributions of different community members, both in waste removal and in structuring microbial biodiversity.

Microbiome Dynamics in a Large Artificial Seawater Aquarium.

Artificial habitats for animals have high commercial and societal value. Microbial communities (microbiomes) in such habitats may play ecological roles similar to those in nature. However, this hypothesis remains largely untested. Georgia Aquarium's Ocean Voyager (OV) exhibit is a closed-system aquatic habitat that mimics the oligotrophic open ocean and houses thousands of large marine animals, including fish, sea turtles, and whale sharks. We present a 14-month time series characterizing the OV water column microbiome. The composition and stability of the microbiome differed from those of natural marine environments with similar chemical features. The composition shifted dramatically over the span of 2 weeks and was characterized by bloom events featuring members of two heterotrophic bacterial lineages with cosmopolitan distributions in the oceans. The relative abundances of these lineages were inversely correlated, suggesting an overlap in ecological niches. Transcript mapping to metagenome-assembled genomes (MAGs) of these taxa identified unique characteristics, including the presence and activity of genes for the synthesis and degradation of cyanophycin, an amino acid polymer linked to environmental stress and found frequently in cyanobacteria but rarely in heterotrophic bacteria. The dominant MAGs also contained and transcribed plasmid-associated sequences, suggesting a role for conjugation in adaptation to the OV environment. These findings indicate a highly dynamic microbiome despite the stability of the physical and chemical parameters of the water column. Characterizing how such fluctuations affect microbial function may inform our understanding of animal health in closed aquaculture systems.IMPORTANCE Public aquariums play important societal roles, for example, by promoting science education and helping conserve biodiversity. The health of aquarium animals depends on interactions with the surrounding microbiome. However, the extent to which aquariums recreate a stable and natural microbial ecosystem is uncertain. This study describes the taxonomic composition of the water column microbiome over 14 months in a large indoor aquatic habitat, the Ocean Voyager exhibit at the Georgia Aquarium. Despite stable water column conditions, the exhibit experienced blooms in which the abundance of a single bacterial strain increased to over 65% of the community. Genome analysis indicated that the OV's dominant strains share unique adaptations, notably genes for storage polymers associated with environmental stress. These results, interpreted alongside data from natural ocean systems and another artificial seawater aquarium, suggest a highly dynamic aquarium microbiome and raise questions of how microbiome stability may affect the ecological health of the habitat.