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<p>Kelp forests are essential marine ecosystems providing high-density habitats, nurseries, and food for countless species, including fish, invertebrates, and mammals. An increasing number of rafting thalli have been observed recently. This phenomenon may represent a natural dispersal strategy of kelp populations, or alternatively, it may be associated with rising ocean temperatures and the increased frequency and intensity of storm events. In the latter case, one can assume that thalli would be degrading during rafting and be associated with declining physiology. With this rationale in mind, we tested the physiological condition and ultrastructure of floating thalli of giant kelp (<i>Macrocystis pyrifera</i>) and featherboa kelp (<i>Egregia menziesii</i>), collected in July 2023 in La Jolla, California, USA. Both species were photosynthetically active with <i>F</i><sub>v</sub>/<i>F</i><sub>m</sub> values &gt; 0.6, maximum relative electron transport rate of PSII (<i>r</i>ETRII<sub>max</sub>) values between 11 and 16, and total chlorophyll (Chl) contents of ~ 3.0 µmol g<sup>− 1</sup> DW. The ultrastructure was typical for Phaeophyceae with a central medulla region and peripheral meristoderm containing chloroplasts, mitochondria and many Golgi bodies indicative of intact and active cells. However, it is not known for how long the kelps were in raft. After exposure to a strong short term experimental UV stress treatment (high UV/PAR ratio), the Chl <i>a</i> and Chl <i>c</i> contents as well as the Chl <i>a</i>/Chl <i>c</i> ratio did not change. However, <i>r</i>ETRII<sub>max</sub> values decreased by 75% <i>E. menziesii</i> and were close to zero in <i>M. pyrifera</i>. Furthermore, a significantly lower fucoxanthin level was found in <i>M. pyrifera</i>, and the xanthophyll cycle de-epoxidation ratio increased in both species after UV treatment, suggesting the rapid induction of this protection mechanism. Dilated thylakoid membranes and accumulations of plastoglobules were observed after UV stress. These observations suggest that rafting kelp is physiologically active and might contribute to the dispersal in the investigated species.</p>

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Rafting kelp from California – is it alive? Physiological condition, ultrastructure and reaction to high UV treatment

  • 期刊论文

  • 出版模式：

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  • 发表日期：

    2026年4月17日

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  • Andreas Holzinger,
  • Thomas Roach,
  • Charlotte Permann,
  • Dimitri D Deheyn

摘要

Kelp forests are essential marine ecosystems providing high-density habitats, nurseries, and food for countless species, including fish, invertebrates, and mammals. An increasing number of rafting thalli have been observed recently. This phenomenon may represent a natural dispersal strategy of kelp populations, or alternatively, it may be associated with rising ocean temperatures and the increased frequency and intensity of storm events. In the latter case, one can assume that thalli would be degrading during rafting and be associated with declining physiology. With this rationale in mind, we tested the physiological condition and ultrastructure of floating thalli of giant kelp (Macrocystis pyrifera) and featherboa kelp (Egregia menziesii), collected in July 2023 in La Jolla, California, USA. Both species were photosynthetically active with Fv/Fm values > 0.6, maximum relative electron transport rate of PSII (rETRIImax) values between 11 and 16, and total chlorophyll (Chl) contents of ~ 3.0 µmol g− 1 DW. The ultrastructure was typical for Phaeophyceae with a central medulla region and peripheral meristoderm containing chloroplasts, mitochondria and many Golgi bodies indicative of intact and active cells. However, it is not known for how long the kelps were in raft. After exposure to a strong short term experimental UV stress treatment (high UV/PAR ratio), the Chl a and Chl c contents as well as the Chl a/Chl c ratio did not change. However, rETRIImax values decreased by 75% E. menziesii and were close to zero in M. pyrifera. Furthermore, a significantly lower fucoxanthin level was found in M. pyrifera, and the xanthophyll cycle de-epoxidation ratio increased in both species after UV treatment, suggesting the rapid induction of this protection mechanism. Dilated thylakoid membranes and accumulations of plastoglobules were observed after UV stress. These observations suggest that rafting kelp is physiologically active and might contribute to the dispersal in the investigated species.