# The Role of Ammonia-Oxidizing Archaea During Cycling and Animal Introduction in a Newly Commissioned Saltwater Aquarium

## TL;DR

This study shows that ammonia-oxidizing archaea help start up a new saltwater aquarium, but adding animals and food changes the microbial community.

## Contribution

The study identifies ammonia-oxidizing archaea as key players in early nitrogen cycling and shows how animal introduction alters microbial dynamics.

## Key findings

- Ammonia-oxidizing archaea, especially Nitrosopumilus, dominated during the initial nitrogen cycling phase.
- Introducing live animals and foods caused significant shifts in microbial community structure and diversity.
- Seadragon introduction led to the most distinct microbial community composition.

## Abstract

Saltwater aquarium systems rely on microbial communities to help maintain water quality, especially during the critical startup phase of new exhibits. These microbes convert nitrogenous waste—produced by feeding and animal metabolism—into less harmful forms, a process known as nitrogen cycling. However, little is known about how these microbial communities change over time or respond to the introduction of animals and live foods. In this study, we monitored daily microbial community composition and water quality parameters in a newly commissioned public saltwater aquarium over 54 days. We observed that ammonia-oxidizing archaea were the primary microbes responsible for early nitrogen cycling. Once live foods and weedy seadragons were introduced, microbial communities shifted substantially in both composition and diversity. These results demonstrate that animal additions can rapidly alter microbial dynamics, even in well-established systems. Understanding these changes can help aquariums more effectively manage new exhibits by informing the timing of biological inputs and ensuring stable water conditions for animal health and welfare.

Recirculating saltwater aquarium exhibits and systems must be meticulously designed and maintained to support the life of not only resident animals but also stable communities of beneficial microbes that process nitrogenous waste. These nitrifying microbial communities act as biofilters that are critical to system functioning, yet little is known about succession dynamics in system startup and eventual impacts of host-associated microbiota on the aquarium environment. Here, we characterized microbial community dynamics in a large public saltwater aquarium over a 54-day period spanning nitrogen cycling, the addition of live foods (Artermia franciscana and Americamysis bahia), and the introduction of weedy seadragons and sea urchins. Daily water samples were collected for 16S rRNA amplicon sequencing, and routine measurements of temperature, pH, salinity, and nitrogen species (ammonium, nitrite, nitrate) were performed. Results revealed that ammonia-oxidizing archaea (AOA), particularly Nitrosopumilus, were the dominant nitrifiers during startup, peaking in abundance during periods of high ammonium additions. Alpha and beta diversity analyses showed significant changes in microbial community structure following the introduction of live animals, with the seadragon phase exhibiting the most distinct community composition. These findings underscore the importance of AOA in saltwater aquarium nitrification and demonstrate the strong influence of host-associated microbial inputs on microbial succession in closed aquatic systems.