Diversity of Gracilariaceae (Gracilariales, Rhodophyta) Across Distinct Ecosystems in Zhanjiang, China: A Foundation for Screening Potential Cultivable Species in Southern China

Abstract

This study was conducted to assess the diversity of Gracilariaceae species across various coastal ecosystems in Zhanjiang, Guangdong Province, China, and identify species suitable for large-scale cultivation in the southern coastal waters of China. The diversity and seasonal and spatial distribution patterns of Gracilariaceae species in different ecosystems were systematically analyzed, and taxonomic studies were performed on species with disputed taxonomic identities using morphological and multi-gene marker techniques to clarify their classification status. Species richness was higher, but individual species coverage was lower in open ecosystems (e.g., tidal pools) compared to enclosed ecosystems (e.g., mangroves, seagrass beds, saltwater ponds), and both factors showed significant seasonal variation. Conversely, enclosed ecosystems had lower species richness, higher species coverage, and minimal seasonal variation. The presence of Gracilaria fisheri in China was recorded for the first time, and the taxonomic status of G. hainanensis was systematically evaluated. Based on taxonomic evaluations and a review of the literature, G. changii and G. firma were proposed to be synonymous. In total, eight Gracilariaceae species were identified during the survey. Among them, Gracilariopsis heteroclada, G. fisheri, G. edulis, and G. hainanensis were identified as potential candidates for large-scale cultivation in the southern coastal waters of China. This study advanced the understanding of the taxonomy and ecology of Gracilariaceae species in the Zhanjiang region and provided a scientific foundation for the conservation and industrial development of Gracilariaceae resources.

Keywords: agarophytes; coastal ecosystems; molecular markers; morphology; newly recorded species; species richness.

FIGURE 1

Study sites and habitats. The map displays the four representative coastal ecosystems (represented by colored dots) selected for in‐depth investigation in Zhanjiang, Guangdong Province, China. The habitat images on the right, from top to bottom, show the following locations: Seagrass beds in Haiwei (HW, 20°26′20″ N, 109°57′10″ E); mangroves of Techeng Island (TCI, 21°9′9″ N, 110°26′39″ E); saltwater ponds of Wushi (WSS, 20°33′51″ N, 109°50′11″ E); and tidal pools of Wushi (WST, 20°29′57″ N, 109°50′3″ E).

FIGURE 2

Morphology of Gracilaria fisheri with its habitat. (A) Thallus morphology; (B) Cross‐section of thallus. (C) Enlarged view of thallus cross‐section. (D) Cystocarps densely distributed on the thallus surface. (E) External view of a cystocarp. (F) Cross‐section of a cystocarp. (G) Cross‐section of the cystocarp wall. (H) Carpospores; (I) Absorbent filaments. (J) Surface view of spermatangia. (K) Cross‐section of spermatangia. (L) Surface view of tetrasporangia. (M) Cross‐section of tetrasporangia. (N) Habitat of G. fisheri , growing in tidal pools. Scale bars: A = 4 cm; B, E = 400 μm; C = 100 μm; D = 2 cm; F = 200 μm; G, J, L = 40 μm; H = 150 μm; I, K = 20 μm; M = 50 μm.

FIGURE 3

Morphology of Gracilaria hainanensis with its habitat. (A) Thallus morphology. (B) Cross‐section of thallus. (C) Enlarged view of thallus cross‐section. (D) Cystocarps scattered on the thallus surface. (E) Highly constricted branch bases. (F) External view of a cystocarp. (G) Cross‐section of a cystocarp. (H) Cross‐section of the cystocarp wall. (I) Absorbent filaments. (J) Carpospores. (K) Surface view of tetrasporangia. (L) Cross‐section of tetrasporangia. (M) Habitat of G. hainanensis , with branches entwining around mangrove roots. Scale bars: A = 4 cm; B, E, F = 400 μm; C, H, I, J, L = 40 μm; D = 2 cm; G = 200 μm; K = 20 μm.

FIGURE 4

Maximum likelihood phylogenetic tree of Gracilariaceae species based on rbcL gene sequences. The scale bar indicates evolutionary distance. Bold branches represent sequences newly amplified in this study. Node values represent bootstrap support, shown as NJ/ML/BI from left to right. Only support values with Bayesian posterior probabilities ≥ 0.7 or bootstrap values (ML and NJ) ≥ 70 are displayed.

FIGURE 5

Maximum likelihood phylogenetic tree of Gracilariaceae species based on Cox1 gene sequences. The scale bar indicates evolutionary distance. Bold branches represent sequences newly amplified in this study. Node values represent bootstrap support, shown as NJ/ML/BI from left to right. Only support values with Bayesian posterior probabilities ≥ 0.7 or bootstrap values (ML and NJ) ≥ 70 are displayed.

FIGURE 6

Boxplots of species diversity indices across different ecosystems and seasons. (A) Shannon diversity indices. (B) Simpson diversity indices. Boxes represent the interquartile range of the data, with the central line indicating the median and the dots indicating the mean.

FIGURE 7

Species coverage heatmap for Gracilariaceae species across ecosystems and seasons. Species coverage data were averaged for each sampling location and season. The color gradient ranges from purple to yellow, representing increasing species coverage, with purple indicating lower coverage and yellow indicating higher coverage values.