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This paper differentiates two molecular subtypes of Ulva prolifera based on ITS and 5S rDNA analyses and compares their morphological, growth, and photosynthetic responses under natural light and temperature conditions. The work suggests evolved adaptive traits that may contribute to the increasing scale of green tide outbreaks

Overview

This paper provides a comprehensive analysis of two subtypes of the dominant green tide species Ulva prolifera. By employing molecular identification using ITS and 5S rDNA markers, the authors distinguish between what they term 5S-I and 5S-II subtypes. The study compares samples from 2008 and 2021 to reveal morphological, growth, and photosynthetic changes over time that suggest adaptive evolutionary responses to fluctuations in natural light and temperature conditions .

Methods and Experimental Design

• Molecular Approach: The study uses ITS and 5S rDNA sequences to construct maximum likelihood phylogenetic trees. This revealed that strains designated as QD-7 and I08-1 represent two distinct subtypes (5S-II and 5S-I, respectively). This molecular differentiation supports the concept of ecotypic divergence in response to environmental pressures
• Comparative Culture Observations: Samples from 2008 and 2021 were cultured under natural conditions to measure growth rates, morphological traits (such as branching complexity and pigmentation), and photosynthetic parameters (including Fv/Fm, NPQ, and ETRmax). The comparison elucidates that 2008 seedlings grew faster and had greener pigmentation, while 2021 populations exhibited enhanced branching complexity, which is linked to stress adaptation and a strategy to optimize light capture and photoprotection under high-stress environments.
• Photosynthetic Measurements: The study monitored key chlorophyll fluorescence parameters that serve as indicators of photosynthetic efficiency and stress responses. Notably, adaptations in the Chl a/Chl b ratio and NPQ values were observed, suggesting that each subtype modulates its light-harvesting machinery in response to temperature and light variations.

Key Findings

• Adaptive Morphology and Growth: The subtype identified as I08-1 (5S-I) displays a dark green, cypress-like morphology with dense small branches that may lead to light self-shading but indicate enhanced stress tolerance. Conversely, subtype QD-7 (5S-II) shows faster growth and traits favoring fragmentation-based reproduction, contributing to its dominance in disturbed environments
• Photosynthetic Acclimation: Differential modulation of photosynthetic pigments and energy dissipation mechanisms suggests that U. prolifera optimizes its photochemical processes to cope with environmental stress. These findings are critical as they imply that adaptive modifications in photosynthetic performance have likely contributed to the increased scale and persistence of green tide events in the Southern Yellow Sea.
• Ecological Implications: The study underlines that the evolved adaptive traits of the different subtypes may serve as a mechanism by which green tide outbreaks are maintained and even expanded, reinforcing the need for targeted control and prevention strategies in coastal management.

Limitations and Future Directions

While providing robust molecular and physiological data, the study could benefit from additional intermediate time points to better map the evolutionary trajectory of these adaptations. A more detailed statistical analysis of the variability and potential confounding environmental factors would further strengthen the conclusions. Expanding the study to include multi-regional comparisons could assess the generality of these adaptive strategies beyond the Southern Yellow Sea

Conclusions

The integration of molecular techniques with detailed physiological measurements makes this study a significant contribution to understanding the adaptive mechanisms of Ulva prolifera in response to environmental stressors. The demonstrated divergence between subtypes supports the hypothesis that evolving adaptive traits are a key factor in green tide persistence and expansion in the Southern Yellow Sea