In contrast to diatoms, cryptophytes are susceptible to iron limitation, but not to ocean acidification

Abstract

Previous field studies in the Southern Ocean (SO) indicated an increased occurrence and dominance of cryptophytes over diatoms due to climate change. To gain a better mechanistic understanding of how the two ecologically important SO phytoplankton groups cope with ocean acidification (OA) and iron (Fe) availability, we chose two common representatives of Antarctic waters, the cryptophyte Geminigera cryophila and the diatom Pseudo-nitzschia subcurvata. Both species were grown at 2 C under different pCO 2 (400 vs. 900 μatm) and Fe (0.6 vs. 1.2 nM) conditions. For P. subcurvata, an additional high pCO 2 level was applied (1400 μatm). At ambient pCO 2 under low Fe supply, growth of G. cryophila almost stopped while it remained unaffected in P. subcurvata. Under high Fe conditions, OA was not beneficial for P.subcurvata, but stimulated growth and carbon production of G. cryophila. Under low Fe supply, P. subcurvata coped much better with OA than the cryptophyte, but invested more energy into photoacclimation. Our study reveals that Fe limitation was detrimental for the growth of G. cryophila and suppressed the positive OA effect. The diatom was efficient in coping with low Fe, but was stressed by OA while both factors together strongly impacted its growth. The distinct physiological response of both species to OA and Fe limitation explains their occurrence in the field. Based on our results, Fe availability is an important modulator of OA effects on SO phytoplankton, with different implications on the occurrence of cryptophytes and diatoms in the future.

Physiological response of an Antarctic cryptophyte to increasing temperature, CO2, and irradiance

“…4A,D). Our observations are consistent with the results from the same G. cryophila strain showing either no (20 and 200 μ mol photons m −2 s −1 , respectively; Trimborn et al 2019) or a modest stimulatory (100 μ mol photons m −2 s −1 ; Camoying et al 2022) high pCO 2 effect in conjunction with low and medium irradiance.…”

“…Consistent with the results of previous studies (Trimborn et al 2019; Camoying et al 2022), the Antarctic cryptophyte G. cryophila was not affected by high pCO 2 . In line with the observations of Trimborn et al (2019) at ambient temperature (2°C), exposure to high light irrespective of the applied pCO 2 impacted growth and POC production of G. cryophila .…”

“…The reasons for this are not yet clear. On the other hand, it was reported that G. cryophila was not negatively affected by high pCO 2 (Trimborn et al 2019; Camoying et al 2022), similar to what has been observed for cryptophytes in general when natural phytoplankton were grown under elevated pCO 2 levels (Sommer et al 2015; Schulz et al 2017; Donahue et al 2019). This is in line with the findings that cryptophytes are able to grow within a broad pH range (i.e., at both lower and upper extreme of the pH spectrum, Weisse and Stadler 2006; Gaillard et al 2020).…”

“…At the end of the experiment, pigment samples were collected by filtering the cultures onto GF/F filters (~ 0.6 μ m, 25 mm; Whatman) and directly freezing them in liquid nitrogen. After storage at −80°C, the extraction and Chl a and chlorophyll c 2 (Chl c 2 ) analysis was done as described in Camoying et al (2022). To calculate cellular quotas, Chl a and Chl c 2 contents were normalized to filtered volume and cell densities.…”

The interacting effect of prolonged darkness and temperature on photophysiological characteristics of three Antarctic phytoplankton species

“…Numerous studies have aimed to explain how irradiance and other factors influences the composition of Southern Ocean phytoplankton (e.g., Alderkamp et al, 2012; Arrigo et al, 2010; Camoying et al, 2022; Trimborn et al, 2019). In addition to their findings, fitness after long darkness may be a factor that influences early spring phytoplankton composition due to species‐specific responses to long darkness.…”