Progress in Fishery Sciences (Oct 2023)
Composition of Epiphytic Microbial Communities in Gametophytes and Sporophytes of Undaria pinnatifida
Abstract
Undaria pinnatifida is a brown alga with high economic value. Its annual production is second only to Saccharina japonica among economically important brown algae in China. Due to climate change and increased cultivation density, the occurrence of diseases in cultivated seaweeds has become more frequent and serious in recent years. Most diseases are directly or indirectly related to the interactions between the seaweed host and the epiphytic microorganisms such as bacteria. There is a close relationship between algae and epiphytic microorganisms. When the phycosphere niche maintains a dynamic balance, the two rely positively on each other growing and developing together. When the balance is disturbed, the structure of the epiphytic microbial community may change, possibly resulting in algal diseases. The absence of certain microbes may also lead to the failure of key biological processes such as the morphogenesis of the host algae. Therefore, understanding the composition of the epiphytic microbial community is of great significance for the study of the interaction between U. pinnatifida and epiphytic microorganisms and the better control of U. pinnatifida diseases. In addition, stock resources of U. pinnatifida are usually conserved in the form of gametophytes, which can persist for a long time under controlled conditions. Hence, understanding the composition of the epiphytic microbial community will also facilitate the development of efficient conservation methods and help remove microbial contaminations when axenic cultures need to be established. The life history of U. pinnatifida consists of the alternation between heteromorphic macroscopic sporophytes and microscopic gametophytes. The stark morphological and physiological differences between these two phases suggest that the composition of epiphytic microbial communities between them is likely different. However, studies on the composition of epiphytic microbial communities, especially comparison studies between sporophytes and gametophytes in U. pinnatifida are limited. The advent of high-throughput sequencing provides robust and efficient tools for studying the composition and relative abundance of the microbial community associated with the algae. In the present study, sporophytes and gametophytes (each with three biological replicates) of U. pinnatifida derived from the cultivated population in Dalian China were selected as the study objects. After DNA extraction and PCR amplification of the v3–v4 region of 16S rRNA gene and the v4 region of 18S rRNA gene, Illumina HiSeq 2500 high-throughput sequencing platform was used to sequence these specific regions. We identified and classified the composition of epiphytic microbial communities of the gametophytes and sporophytes of U. pinnatifida based on the sequencing results. A total of 446 932 effective reads were obtained through 16S rRNA gene sequencing. The raw reads have been submitted to the GenBank database (https://www.ncbi.nlm.nih.gov/) with the accession number PRJNA823903. The bacterial community composition of gametophyte and sporophyte was revealed to be significantly different, and the diversity of the bacterial community in gametophyte samples was higher than that in sporophyte samples. In gametophytes, Proteobacteria (66.67%) was the most dominant phylum, followed by Bacteroidetes (13.48%) and Cyanobacteria (11.13%). At the class level, Alphaproteobacteria (34.58%) was the most abundant, followed by Gammaproteobacteria (31.01%), Bacteroidia (13.16%), and Oxyphotobacteria (11.13%). Cyanobacteria (95.67%) was predominantly detected in sporophytes, followed by Actinobacteria (1.65%) and Firmicutes (1.48%). The distribution of Oxyphotobacteria, Alphaproteobacteria, Bacteroidia, Gammaproteobacteria, Negativicutes, OM190, Acidimicrobiia, Erysipelotrichia, Planctomycetacia, and Verrucomicrobiae were found to be different between gametophyte and sporophyte samples, among which OM190, Acidimicrobiia and Planctomycetacia were unique to gametophytes. The genus-level bacterial groups detected in gametophyte samples were Lewinella (10.06%), Leucothrix (5.99%), Sulfitobacter (4.06%), Bifidobacterium (0.02%), while Bifidobacterium accounted for 1.41% of the bacterial genus of sporophyte samples. There were 57.37% and 95.68% uncultured bacterium in gametophytes and sporophytes, respectively. We obtained a total of 473 770 effective reads through 18S rRNA gene sequencing. A major share (97.22%) of microeukaryotes in gametophytes were unclassified, while in sporophytes, the number was 94.95%. Streptophyta, Intramacronucleata, Basidiomycota, Apicomplexa, Arthropoda, Bacillariophyta, Chordata, Gastrotricha, Ascomycota, and Mucoromycota were detected both in the gametophytes and sporophytes. Among them, Basidiomycota, Ascomycota, and Mucoromycota belong to fungi. The community abundance of sporophyte samples was higher than that of gametophytes. In gametophyte samples, except for Phaeophyceae (88.77%) to which U. pinnatifida belongs, Copepoda of Arthropoda, Mediophyceae of Bacillariophyta, Mammalia of Chordata, Prostomatea of Intramacronucleata, and Liliopsida were dominant, with a proportion of 0.62%, 0.50%, 0.25%, 0.23%, and 0.23%, respectively. In addition to Phaeophyceae (94.49%), Conoidasida of Protozoa, Agaricomycetes of Basidiomycota, Spirotrichea of Intramacronucleata and Mammalia of Chordata were predominant in sporophytes, accounting for 0.91%, 0.83%, 0.51% and 0.24% of OTUs, respectively. Chytridiomycetes, Nassophorea, Colpodea, Tremellomycetes, Sordariomycetes, Conoidasida, Agaricomycetes, Arachnida, and Chromadorea were only detected in the sporophytes, and there was a significant difference in Spirotrichea abundance between gametophytes and sporophytes. In conclusion, the composition of epiphytic microbial communities and the relative abundance of different bacteria and microeukaryotes in the sporophytes and gametophytes of U. pinnatifida were determined through high-throughput sequencing of the amplicons of the v3–v4 region of 16S rRNA gene and the v4 region of 18S rRNA gene. Remarkable differences were revealed between the two life stages, indicating that their growth and development are associated with different microbial communities. The results of this study provide valuable information for sustainable cultivation and stock culture conservation of this important kelp species.
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