| 08 May 2026
Light availability controls multi-decadal trends in crustose coralline algal cell elongation
Abstract. Crustose coralline algae (CCA) are globally distributed calcifying macroalgae that can grow as free-living rhodoliths, act as ecosystem engineers by creating complex three-dimensional habitats, and contribute to the carbon cycle. The dimensions of cells in the carbonate skeleton influence rhodolith structural integrity and, consequently, the ecological functions these rhodoliths provide, yet CCA cellular responses to environmental change remain poorly resolved. This study quantifies multi-decadal variability in skeletal cell dimensions of the rhodolith-forming CCA Sporolithon nodosum from New Zealand and evaluates the relative importance of sea surface radiation (SSR), sea surface temperature (SST) and sea surface CO2 partial pressure (pCO2) as potential drivers. The length and width of 2975 cells were measured along a 38-year transect (1985–2022) using stitched scanning electron microscope images. Cell length declined significantly over time (R2adj = 0.105, p = 0.027), whereas cell width showed no temporal trend (R2adj = -0.019, p = 0.583). Among the environmental variables, SSR was the strongest predictor of cell length (R2adj = 0.235, p = 0.001), while SST and pCO2 explained comparatively little variance. These results identify light availability as a primary correlate of cell elongation in S. nodosum, consistent with a role for irradiance-driven changes in photosynthetic energy supply. Because long-term SSR trends are spatially heterogeneous, light-mediated shifts in CCA cell dimensions – and their potential implications for rhodolith structure and ecosystem function – are likely to vary regionally. This study highlights the central role of light availability in shaping CCA cell morphology under changing surface-ocean conditions and motivates multi-site comparisons to assess broader ecosystem implications.
General Comments
The paper provides a robust dataset ( ∼ 40 years) of cell measurements from one protuberance of a rhodolith built by Sporolithon nodosum, collected at a water depth of 5 meters on the Whangaparāoa Peninsula in New Zealand.
The study aims at reconstructing cell size variation (cell length and width, annual growth, and number of cells per growth increment) during time and correlate such variation with environmental parameters (SSR, SST, and pCO₂, derived from different sources during the time span of the study) to identify which environmental parameter control size-trend over time.
This topic is of great interest to the scientific community studying coralline algae biomineralization processes. Despite the available literature, much effort is still needed to describe this phenomenon trying to define a descriptive model between environmental parameters and growth trends. Differences exist among species, and the wide bathymetric distribution of CCA implies the need for several specific case studies. Therefore, I believe this paper will contribute to improve our knowledge of the topic.
The authors followed rigorous laboratory methods and provided robust statistics to support their findings. Furthermore, all data and methods are available in an open repository. The only limitation is that the authors provided data from a single protuberance of a single specimen. I suggested therefore to be more cautious in "Discussion" when exposing your interpretation.
Specific comments have been included in the attached PDF.
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