Arctic sea ice algae differ markedly from phytoplankton in their ecophysiological characteristics

Abstract

Photophysiological and biochemical characteristics were investigated in natural communities of Arctic sea ice algae and phytoplankton to understand their respective responses towards variable irradiance and nutrient regimes. This study revealed large differences in photosynthetic efficiency and capacity between the 2 types of algal assemblages. Sea ice algal assemblages clearly displayed increased photoprotective energy dissipation under the highest daily average irradiance levels (>8 µmol photons m-2 s-1). In contrast, phytoplankton assemblages were generally light-limited within the same irradiance ranges. Furthermore, phytoplankton assemblages exhibited more efficient carbon assimilation rates in the low irradiance range compared to sea ice algae, possibly explaining the ability of phytoplankton to generate substantial under-ice blooms. They were also able to readily adjust and increase their carbon production to higher irradiances. The Arctic is warming more rapidly than any other oceanic region on the planet, and as a consequence, irradiance levels experienced by microalgae are expected to increase due to declining ice thickness and snow cover, as well as enhanced stratification. The results of this study suggest that sea ice algae may have less capacity to adapt to the expected environmental changes compared to phytoplankton. We therefore anticipate a change in sea ice-based vs. pelagic primary production with respect to timing and quantity in a future Arctic. The clearly distinct responses of sea ice algae vs. phytoplankton need to be incorporated into model scenarios of current and future Arctic algal blooms and considered when predicting implications for the entire ecosystem and associated biogeochemical fluxes.

Shifts in growth light optima among diatom species support their succession during the spring bloom in the Arctic

“…Considering N. frigida dominance in Arctic sea‐ice (≈60% of cell counts in our data (Figure 1d), see also Poulin et al, 2011), it is tempting to suggest its photoadaptive plasticity reflects a near‐optimal balance between strong resilience against stress with limited potential gross productivity in in situ sea‐ice conditions. Intriguingly, while N. frigida is clearly less productive and more photoinhibited under supersaturating gE (also visible in sympagic/planktonic F. cylindrus ) than planktonic species, no evidence supports ice‐related photoadaptation as being advantageous under lower growth light intensity (see the initial slope of light‐limited growth rate (Figure 2e), PgE under low gE (Figure 5b) and similar conclusions reached by Kvernvik et al (2021). Arctic spring time means low light intensities but also very short photoperiods, and variations in diatoms' growth rates as a function of photoperiod are species dependent (Li et al, 2017; Shatwell et al, 2013).…”

“…This paper is the second in a work series in which we investigate whether sequential taxa dominance of certain ecological niches over the Arctic seasonal light continuum is linked to interspecific photoadaptation divergences. Our previous report (Croteau et al, 2021) and Kvernvik et al (2020, 2021) concluded that open‐water Arctic diatom species are better equipped to cope with light stress than are sympagic species. To build upon these findings, we coupled field data from the Green Edge project 2015–2016 campaigns, which set out to unravel the seasonal dynamics steering Arctic microalgae spring blooms [see Green Edge Special Feature: Babin, 2019; Massicotte et al (2020)], to the investigation of growth‐light responses of five Arctic diatom species representing dominant groups from contrasting light environments over the spring‐to‐summer transition (Lafond et al, 2019; Figure 1).…”

“…This paper is the second in a work series in which we investigate whether sequential taxa dominance of certain ecological niches over the Arctic seasonal light continuum is linked to interspecific photoadaptation divergences. Our previous report (Croteau et al, 2021) and Kvernvik et al (2020Kvernvik et al ( , 2021 to the investigation of growth-light responses of five Arctic diatom species representing dominant groups from contrasting light environments over the spring-to-summer transition (Lafond et al, 2019; Figure 1). This refined resolution of Arctic diatoms light adaptation reveals that species-specific ascending growth light optima mostly align with their sequential seasonal dominance with profound low-light versus high-light specialization between sympagic and open-water succession endmembers.…”

“…With this objective, we grew five ecologically relevant Arctic (or bipolar) diatom species under a range of limiting, saturating and supersaturating light typical of their respective habitats, and documented changes in their growth rates, photophysiology and carbon fixation capabilities. Most knowledge on Arctic diatom photoadaption at the moment is (i) inferred from correlation‐based field observations (Kvernvik et al, 2021; Lewis et al, 2019; Schuback et al, 2017), (ii) assembled from independent studies targeting diverse photoadaptative traits in different polar species [reviewed by Lacour et al (2017)] and/or (iii) extrapolated from laboratory studies in which most taxa occur only in Antarctic (Kulk et al, 2019; Petrou et al, 2011; Strzepek et al, 2019). However, we lack studies describing dose–response relationships between environmental factors and diverse species dominating distinct niches over seasonal dynamics required by production models and ecological theory.…”

Pelagic and ice‐associated microalgae under elevated light and pCO2: Contrasting physiological strategies in two Arctic diatoms

“…The ability of microalgae to cope with different kinds of photosynthetic stress has been shaped over evolutionary timescales by the selective pressures experienced in the habitats they occupy. Accordingly, ice-associated algae typically struggle to cope with the high light (HL) intensities and the variability that occurs in open ocean waters (e.g., Kvernvik et al 2020Kvernvik et al , 2021Croteau et al 2021). Pelagic species, on the other hand, have developed plastic photosynthesis apparatuses and effective protective mechanisms to thrive under such light regimes (Lacour et al 2018;Kvernvik et al 2020;White et al 2020).…”