the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 26 Sep 2023
Multifactorial effects of warming, low irradiance, and low salinity on Arctic kelps
Abstract. The Arctic is projected to warm by 2 to 5 °C by the end of the century. Warming causes melting of glaciers, shrinking of the areas covered by sea ice, and increased terrestrial runoff from snowfields and permafrost thawing. Warming, decreasing coastal underwater irradiance, and lower salinity are potentially threatening polar marine organisms, including kelps, that are key species of hard-bottom shallow communities. The present study investigates the physiological responses of four kelp species (Alaria esculenta, Laminaria digitata, Saccharina latissima, and Hedophyllum nigripes) to warming, low irradiance, and low salinity through a perturbation experiment conducted in ex situ mesocosms. Kelps were exposed during six weeks to four experimental treatments: an unmanipulated control, a warming condition mimicking future coastlines unimpacted by glacier melting under the CO2 emission scenario SSP5-8.5, and two multifactorial conditions combining warming, low salinity, and low irradiance reproducing the future coastal Arctic exposed to terrestrial runoff following two CO2 emission scenarios (SSP2-4.5 and SSP5-8.5). The physiological effects on A. esculenta, L. digitata and S. latissima were investigated and gene expression patterns of S. latissima and H. nigripes were analyzed. Specimens of A. esculenta increased their chlorophyll a content when exposed to low irradiance conditions, suggesting that they may be resilient to an increase in glacier and river runoff and become more dominant at greater depths. S. latissima showed a lower carbon:nitrogen (C:N) ratio at higher nitrate concentrations, suggesting coastal erosion and permafrost thawing could benefit the organism in the future Arctic. In contrast, L. digitata showed no responses to the conditions tested on any of the investigated physiological parameters. The gene expressions of H. nigripes and S. latissima underscores their ability and underline temperature as a key influencing factor. Based on these results, it is expected that kelp communities will undergo changes in species composition that will vary at local scale as a function of the changes in environmental drivers. For future research, potential cascading effects on the associated fauna and the whole ecosystem are important to anticipate the ecological, cultural, and economic impacts of climate change in the Arctic.
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RC1: 'Comment on egusphere-2023-1875', Anonymous Referee #1, 03 Oct 2023
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GENERAL COMMENTS
Lebrun et al. present a timely and well-designed study investigating the response of four habitat-building kelp species to a warming Arctic by means of a multi-factorial mesocosm experiment, in which they simulate predicted temperature increases along with decreased irradiance and decreased salinity under increased glacial melt scenarios over six weeks. They identify species-specific responses in physiology (growth, chlorophyll a, carbon:nitrogen ratio) and gene expression, indicating specific acclimation mechanisms and responses, which mainly respond to temperature as a key driver. In contrast to the majority of studies in this field, which often observe short-term stress responses, Lebrun et al. performed a 6-week experiment which allows for organismal acclimation to the novel environment, as evident in the lack of stress responses and stable growth rates across treatments. They therefore show that the kelps’ physiology is capable of acclimating to these interactive environmental effects, potentially allowing their range expansion into newly ice-free areas. The manuscript is well written and the conclusions are drawn based on a thorough analysis of the data. My main suggestions are to include a short description of the significance of the fixed factors in the results section before reporting the pairwise comparisons, and to consider the effect of temperature on enzyme reaction rates in the discussion. Apart from this, I only have minor questions and suggestions. This manuscript will be a valuable contribution to the field and I’m looking forward to seeing the paper published!
SPECIFIC COMMENTS
ABSTRACT
The abstract provides a concise overview over the premise, study design, key results and implications. Only the growth rates are not mentioned in the abstract. I would suggest to mention that growth remained stable for each species across treatments, which in my eyes is one of the most important results showing that Arctic warming may not be detrimental per se.
INTRODUCTION
The introduction is short but concise and presents Arctic kelp forests and the changing environment they are facing. A more general audience might appreciate a short taxonomic classification of kelps, e.g. as large brown algae (Phaeophyceae, Laminariales). Do all four tested species occur together in mixed assemblies or are they restricted to different depths in situ?
METHODS
The methods are described in detail and allow replication, provided that the study by Miller et al., which contains the detailed setup of the mesocosm experiment, will be published before this manuscript. Before the start of the experiment, were the holding tanks maintained at in situ conditions? Why were S. latissima and H. nigripes chosen for the transcriptomic analysis?
RESULTS
The results are presented in a clear and focused manner with good statistical support. However, throughout the reports of the physiological responses, the authors only report pairwise comparisons. I would prefer the paragraphs to begin with an overview of the fixed factor significance (Chi-square tests) to assess significant differences between species and treatments in general, and their interaction, i.e. whether species respond differently to the different treatments, before moving on to pairwise comparisons. The Figures are clear and intuitive, except for the display of significant differences in Figure 3 (A. esculenta), which could be improved. Figure 2C seems to be missing the light blue control treatment.
DISCUSSION
The authors provide a detailed placement of their data within the literature, especially macroalgae physiology, and relate the different responses between species to their resilience towards Arctic warming. A key point to be added to the discussion is the relationship of enzyme activity and temperature, especially with respect to the RNAseq results (down-regulation of stress responses and fundamental cellular machinery). For instance, the combination of cold temperature and high irradiance can induce photoinhibition due to slower reaction rates of crucial enzymes such as RuBisCO, triggering stress responses. In general, to maintain cellular functions, slower reaction rates at cold temperature can be compensated by higher protein expression. The authors mention that future warming may reduce stress responses (ln. 361), to which I would like to add that the tested high temperatures are close to the physiological optima described for the species. Regarding the reduced C:N ratio in S. latissima it may make sense to relate this to the increased growth rate in this species.
ADDITIONAL
At the moment there is no data availability statement. I would advise the authors to archive at least the RNAseq data in a public repository.
MINOR COMMENTS
Ln. 29 – gene expression patterns
Ln. 94 – Here it is described how temperature and salinity were adjusted, so I think it makes sense to add that irradiance was adjusted using filters here, too.
Ln. 109 – with a PAR sensor (LI-COR xyz)
Ln. 111 – the difference between the inner and outer
Ln. 120 – consider adding that the meristem is located above the stipe-blade transition zone for a more general audience
Ln. 125 – only on tfinal?
Ln. 129 – Were the samples kept frozen during extraction?
Ln. 132 – What is the Fa fluorescence? I would prefer if Lorenzen’s formula was reproduced in the text to put F0 and Fa into context.
Ln. 144 – Suggestion: 2 cm above the stipe-blade transition. Base of the stipe to me sounds like it is basally located near the holdfast.
Ln. 149 – with dist0: […] to the hole
Ln. 155 – it might be useful to mention that the protocol combines a CTAB extraction followed with a commercial Qiagen kit.
Ln. 167 – using rnaSPAdes
Ln. 191 – Table C1
Ln. 217 – Consider replacing “different” by “higher than”
Ln. 235 – According to the Chi-square test (Table F1), they are strongly affected. This is likely due to the much faster growth of S. latissima in general, but this should at least be acknowledged shortly.
Ln. 238 – It might be worth mentioning that the growth of S. latissima is higher by an order of magnitude
Ln. 245 – Principal component analysis of global gene expression revealed …
Ln. 248 – classified = functionally annotated?
Ln. 258 – 458 down-regulated genes […] classified down-regulated
Ln. 268 – no negative impacts
Ln. 283 – and lower irradiance treatment
Ln. 290 – Short-term acclimation may not be the right term, it may rather be an existent adaptation to effectively use lower irradiance? Niedzwiedz & Bischof (2023; doi.org/10.1002/lno.12312) show that Arctic A. esculenta has a lower compensation irradiance than S. latissima at 3-7°C.
Ln. 295 – Diehl and Bischof (2021)
Ln. 296 – the chl a content of S. latissima
Ln. 305 – Might the nitrogen limitation be related to the 10x higher growth in S. latissima?
Ln. 314 – higher in the T2 treatment (1.68 …)
Ln. 332 – in the control indicating
Ln. 348 – potentially because increased enzyme reaction rates compensate for the reduced expression (see general comment above)
Ln. 384 – and biotic interactions, see the reduced competition of sporophyte recruitment against A. esculenta in Zacher et al. (2019)
Ln. 393 – acclimation
Ln. 395 – So growth is plastic only in specific seasons? Or does this sentence refer to general seasonal growth patterns?
Ln. 405 – optimum
Ln. 406 – acclimation
Ln. 426 – Is it really more than one co-author on the editorial board?
Ln. 703 / Figure 7 – what is the meaning of the shading behind the bars for T2?Citation: https://doi.org/10.5194/egusphere-2023-1875-RC1
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