Characterization of curly branch of Gracilariopsis lemaneiformis (Rhodophyta) at morphological, physiological, and molecular levels

Abstract

Gracilariopsis lemaneiformis is an important commercial macroalgae. However, G. lemaneiformis strains with curly branches (CB), located at tertiary order with different phenotype when compared with those with normal branches (NB), were found in the cultivation region. This study investigated CB of G. lemaneiformis. The results revealed that CB exhibited loosely arranged epidermal cells with decreased number of layers; thickened cell wall of medullary cells; reduced plastids with loose lamellas; and lower specific growth rate, pigment contents, chlorophyll fluorescence parameters, and nucleic acid contents when compared with NB. Genes involved in plastid, pyrimidine, and purine biosynthesis as well as nitrogen metabolism were enriched, whereas those encoding small heat shock proteins (sHSPs) in the vicinity of transposable elements (TEs) were evidently down-regulated in CB. The morphological changes in CB might be caused by the alteration at microstructural level. The photosystem II (PSII) in CB presented weak light energy utilization ability and electron transfer activity, compared with that in NB, and the photosynthetic structure of CB may be damaged, which further exerted negative effect on photosynthetic intensity. The reduced plastid structure may be one of the reasons for the decreased photosynthetic intensity of CB. Genes encoding glutamine synthetase involved in nitrogen metabolism were down-regulated in CB, resulting in decrease in nucleic acids synthesis. Under certain stress condition, TEs in the vicinity of sHSPs genes were activated, leading to a decrease in sHSPs, causing loss of algal flexibility to cope with changes in the external environment. These results provided valuable information about irregular branches formation in G. lemaneiformis.