Coastal tidal flats are dynamic zones between land and sea that are periodically submerged and exposed by tidal fluctuations (Gao, 2019). These tidal dynamics cause significant variations in environmental factors such as temperature, salinity, and pH (Mandal et al., 2021). Enriched by both marine and terrestrial inputs, tidal flats provide vital ecosystem services, including buffering storm surges, stabilizing shorelines, filtering pollutants, and sequestering carbon. Tidal flats serve as key sites for biogeochemical cycling, harboring diverse and abundant microbial communities (Barbier et al., 2008; Falkowski et al., 2008; Han et al., 2024). Marine sediments (including tidal flats) are estimated to contain approximately 55–86 % of global bacterial and archaeal populations (Hoshino and Inagaki, 2019). The high concentrations of nitrogen, phosphorus, sulfur, and organic matter in tidal flat sediments support microbial activities that are crucial to global elemental cycling (Bauer et al., 2013). For example, in southern China (Guangdong, Guangxi, Hainan, Fujian), dominant microbial taxa in tidal flats include Pseudomonadota, Bacillota, Bacteroidota, Actinomycetota, and Chloroflexota (Tong et al., 2019). The harsh conditions of dehydration, intense solar radiation, and wave impact select microorganisms with strong stress tolerance, making tidal flats as promising sources of specialized functional strains. Indeed, previous studies have isolated pollutant-degrading and carotenoid-producing bacteria from such habitats and explored their bioactive metabolites (Mandal et al., 2021; Liu et al., 2024).

Among the bioactive metabolites from these tidal-flat microorganisms, carotenoids are a unique class of terpenoid compounds characterized by their yellow pigmentation and notable antioxidants, anticancer, and cardiovascular protective properties (Ashokkumar et al., 2023). These properties underpin their widespread use in the food, nutraceutical, and cosmetic industries. Currently, commercial carotenoid production primarily relies on plants and microalgae. However, such approaches face challenges, including low extraction yields, extensive land and time requirements, and sensitivity to environmental conditions, which highlight the need for more efficient and sustainable production strategies (Eun and Lee, 2024).

Furthermore, studies have shown that carotenoids produced by bacteria, particularly in the members of class Alphaproteobacteria from marine habitats, play a key role in protecting cells from photooxidative damage and regulating membrane fluidity (Armstrong, 1997; Stafsnes et al., 2010). Carotenoid biosynthesis is considered as an adaptive response to environmental stressors such as freeze-thaw cycles and intense solar radiation, thereby enhancing microbial survival in extreme habitats like coastal tidal flats (Dieser et al., 2010).

The genus Altererythrobacter (Kwon et al., 2007) and genus Tsuneonella (Xu et al., 2020) are two members of the family Erythrobacteraceae belonging to order Sphingomonadales, class Alphaproteobacteria in the phylum Pseudomonadota. At the time of writing, 9 and 10 validly published species names were included in the genus Altererythrobacter (https://lpsn.dsmz.de/genus/altererythrobacter) and genus Tsuneonella (https://lpsn.dsmz.de/genus/tsuneonella) in LPSN (Parte et al., 2020), respectively. Recently, Wang et al. (2025) proposed a genome-based reclassification of the order Sphingomonadales, which resulted in the reassignment of Altererythrobacter aquiaggeris to the genus Aestuarierythrobacter. Following this taxonomic revision, the genus Altererythrobacter currently comprises eight validly published species names. Most members of genera Altererythrobacter and Tsuneonella were distributed in sediment (Liu et al., 2024; Matsumoto et al., 2011; He et al., 2023; Qu et al., 2019) and water (Zhang et al., 2021, Gao et al., 2019, Lei et al., 2014, Yoon and Ryu, 2019, Kang et al., 2016). Genus Tsuneonella is also distributed in various habitats such as air (Xue et al., 2016), desert soil (Yan et al., 2017) and sea urchin Strongylocentrotus intermedius (Nedashkovskaya et al., 2013). Member of genera Altererythrobacter and Tsuneonella are non-motile, rod-shaped, Gram-stain-negative bacteria, and their major fatty acid (>10 %) are C18:1ω7c and C18:1ω6c. The predominant respiratory quinone of these two genera are ubiquinone-10, and phosphatidylethanolamine, phosphatidylglycerol, diphosphatidylglycerol, sphingoglycolipid are major polar lipids of these two genera.

Many studies have shown that Altererythrobacter possess multiple functional characteristics, including the capacity for aromatic compound degradation (Fan et al., 2022; Su et al., 2022), lignin decomposition (Levy-Booth et al., 2021), and algicidal activity (Li et al., 2016). Notably, genomic analyses of these strains under genus Altererythrobacter have identified the presence of genes associated with astaxanthin biosynthesis (Matsumoto et al., 2011) and carotenoid production (Liu et al., 2024). Previous studies have revealed that the genus Tsuneonella comprises ten species, however only Tsuneonella litorea KCTC 82812T produces carotenoid (He et al., 2023).

In this study, four carotenoid-producing strains designated SYSU HZ0097T, SYSU HZ0136, SYSU LHT278T and SYSU LHT272, were isolated from tidal flat sediment in South China Sea. Genomic annotation identified carotenoid-related genes in these strains, and full-spectrum scanning using a spectrophotometer confirmed the presence of characteristic carotenoid absorption peaks. Polyphasic taxonomic analyses of the family Erythrobacteraceae, confirmed that strains SYSU HZ0097T and SYSU HZ0136, SYSU LHT278T and SYSU LHT272, represent two novel species under the genus Altererythrobacter and genus Tsuneonella, respectively. We hereby proposed the names Altererythrobacter guangdongensis sp. nov. (SYSU HZ0097ᵀ = MCCC 1K09423ᵀ = KCTC 8714ᵀ) and Tsuneonella sediminis sp. nov. (SYSU LHT278ᵀ = MCCC 1K09543ᵀ = KCTC 8862ᵀ) for these two newly discovered species. Our work has also enriched the resources of tidal flat sediment functional bacteria.