**1. Introduction**

The Pacific white shrimp (*Litopenaeus vannamei*) is the most important commerciallytraded species in shrimp aquaculture [1]. The culture systems for *L. vannamei* are becoming more intensive, resulting in ammonia and nitrite accumulation within grow-out systems [2,3]. Attention should be given to high ammonia concentrations because the stress can kill shrimp, fish, and other cultured animals [4–6]. Three processes involving ammonia removal from aquaculture systems are photoautotrophic uptake by algae, chemoautotrophic oxidation of ammonia to nitrate, and heterotrophic assimilation of ammonia directly to bacterial protein [7,8]. These three processes occur simultaneously in many aquaculture systems but seldom in equal importance, and changes in carbon sources may cause shifts in the dominant pathway [7].

The chemoautotrophic process is dominant in systems with a low total organic carbon/total nitrogen (C/N) ratio, such as a recirculating aquaculture system, [9]. Increasing the C/N ratio in biofloc technology (BFT) systems by adding organic carbon sources induce a shift in nitrogen use by the bacterial community from chemoautotrophy to heterotrophy [10]. The rate of nitrogen use by heterotrophic bacteria is 10 times faster than that

**Citation:** Xue, Y.; Li, L.; Dong, S.; Gao, Q.; Tian, X. The Effects of Different Carbon Sources on the Production Environment and Breeding Parameters of *Litopenaeus vannamei*. *Water* **2021**, *13*, 3584. https://doi.org/10.3390/w13243584

Academic Editor: Christophe Piscart

Received: 4 November 2021 Accepted: 10 December 2021 Published: 14 December 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

of autotrophic nitrification [11]. Previous studies have demonstrated the efficiency of heterotrophic bacteria in controlling total ammonia nitrogen (TAN) in shrimp and tilapia BFT systems [5,8]. Although the growth of heterotrophic bacteria is stimulated in BFT systems, nitrification occurs in these systems, and the process is important for the long-term removal of TAN originating from the feed in intensive BFT systems [8,12,13]. However, it is unclear whether the nitrification rates differ in different carbon added systems, and whether the nitrification rate changes with the development of the bacterial community in a BFT system.

Molasses is a by-product of sugar production, that was suggested to be an appropriate carbon source to increase the C/N ratio in BFT systems. This water-soluble carbon source needs to be applied frequently with constant supervision to prevent overdosing and starvation of the bacterial floc, which increases the management effort. In recent years, insoluble biodegradable polymers (BDPs), such as poly (3-hydroxybutyric acid-co-3-hydrovaleric acid) (PHBV) have been used as carbon sources and biofilm carriers in aquaculture [14]. The relatively low cost of BDPs and the simple management requirements make them an attractive alternative carbon source for aquaculture [15]. However, the use of different carbon sources may lead to differences in water quality and changes in the bacterial community of a BFT system [6,16].

The conversion of nitrogen and waste recycling in BFT systems strongly depends on the capacity of microbes to assimilate and convert the nutrient waste [17]. The functions of biofloc, as a waste nutrient converting agent and a food source, are related to the microbial community. The changes in the bacterial community after adding carbons can affect the nutrient composition, which is essential for bacteria and cultured animals. Moreover, the relevance of host–microbiota–nutrient interactions is important in aquaculture [18]. Therefore, it is essential to explore the relationships between environmental factors and the microbiota [18].

The present study was designed to evaluate the effects of different carbon sources (molasses and PHBV) on water quality, nitrification rate and growth performance of *L. vannamei* in low water exchange grow-out systems. Moreover, the microbial community, the relationship between microbial structure and water environmental factors, as well as microbial function was investigated to develop a further understanding of the impact of different carbon sources on the microbial community in BFT systems.
