Phenotypic plasticity and biogeographic variation in physiology of habitat‐forming seaweed: response to temperature and nitrate

Abstract

Southeastern Australian waters are warming at nearly four times the global average rate (~0.7°C · century(-1) ) driven by strengthening incursions of the warm oligotrophic East Australian Current. The growth rate hypothesis (GRH) predicts that nutrient depletion will impact more severely on seaweeds at high latitudes with compressed growth seasons. This study investigates the effects of temperature and nutrients on the ecophysiology of the habitat-forming seaweed Phyllospora comosa in a laboratory experiment using temperature (12°C, 17°C, 22°C) and nutrient (0.5, 1.0, 3.0 μM NO3 (-) ) scenarios representative of observed variation among geographic regions. Changes in growth, photosynthetic characteristics (via chlorophyll fluorescence), pigment content, tissue chemistry (δ(13) C, % C, % N, C:N) and nucleic acid characteristics (absolute RNA and DNA, RNA:DNA ratios) were determined in seaweeds derived from cool, high-latitude and warm, low-latitude portions of the species' range. Performance of P. comosa was unaffected by nitrate availability but was strongly temperature-dependent, with photosynthetic efficiency, growth, and survival significantly impaired at 22°C. While some physiological processes (photosynthesis, nucleic acid, and accessory pigment synthesis) responded rapidly to temperature, others (C/N dynamics, carbon concentrating processes) were largely invariant and biogeographic variation in these characteristics may only occur through genetic adaptation. No link was detected between nutrient availability, RNA synthesis and growth, and the GRH was not supported in this species. While P. comosa at high latitudes may be less susceptible to oligotrophy than predicted by the GRH, warming water temperatures will have deleterious effects on this species across its range unless rapid adaptation is possible.

Distribution models predict large contractions of habitat‐forming seaweeds in response to ocean warming

“…() and Flukes et al. () found no effects of nitrate but strong negative effects of current summer temperatures (22°C) on growth and survival of E. radiata and P. comosa , concluding that long‐term warming would have severe population‐level effects. Consequently, although physiological data are only available for very few of the modelled species, it appears they currently fill their thermal niches, at least within their biogeographic provinces (cf.…”

“…These results also align with experimental data demonstrating that temperature is the dominant factor limiting the performance and survival of Phyllospora comosa, Scytothalia dorycarpa, Sargassum spp. and Ecklonia radiata compared to non-climatic factors (Flukes, Wright, & Johnson, 2015;Mabin et al, 2013;Provost et al, 2017;Xiao et al, 2015). For example, Mabin et al (2013) Total length of temperate coastline was estimated to be 26,730 km.…”

Family‐level variation in early life‐cycle traits of kelp

“…The photosystem of juvenile brown algae possess rapid phenotypic buffering (plasticity; Flukes et al. ; C. Mabin, unpub. data) which suggests that environmental factors influence F v / F m .…”

Forgotten underwater forests: The key role of fucoids on Australian temperate reefs

“…Phyllospora photosynthetic efficiency, growth, and survival appear to be negatively correlated with higher summer temperatures (22°C; Flukes, Wright, & Johnson, ) and in NSW, plants have higher δ 13 C, photosynthetic capacity (rETRmax), and concentrations of chl c and fucoxanthin compared to Tasmania. However, as with morphology (Peters, ), these traits appear to be highly plastic and rapidly converge under similar environmental conditions (Flukes et al., ). Supporting this finding, Weigner () found no latitudinal pattern in Phyllospora tissue chemistry along a 5 degree latitudinal gradient in NSW.…”

“…It has been experimentally demonstrated that Scytothalia can acclimate in response to UVB and increased its light absorption efficiency in the UV bands by upregulating synthesis of photoprotective compounds (Xiao et al., ). Similarly, Phyllospora physiology is also highly plastic and rapidly responds to change in abiotic conditions (Flukes et al., ). Certainly, thresholds exist beyond which these species will be unable to compensate, and these thresholds are likely to be determined by a range of factors including dispersal capacity and population connectivity mediating genetic diversity and capacity to adapt (Wernberg et al., in review).…”