Effects of pCO2 and iron on the elemental composition and cell geometry of the marine diatom Pseudo-nitzschia pseudodelicatissima (Bacillariophyceae)

Partial pressure of CO₂ (pCO₂) and iron availability in seawater show corresponding changes due to biological and anthropogenic activities. The simultaneous change in these factors precludes an understanding of their independent effects on the ecophysiology of phytoplankton. In addition, there is a lack of data regarding the interactive effects of these factors on phytoplankton cellular stoichiometry, which is a key driving factor for the biogeochemical cycling of oceanic nutrients. Here, we investigated the effects of pCO₂ and iron availability on the elemental composition (C, N, P and Si) of the diatom Pseudo-nitzschia pseudodelicatissima (Hasle) Hasle by dilute batch cultures under 4 pCO₂ (~200, ~380, ~600, and ~800 μatm) and 5 dissolved inorganic iron (Fe′; ~5, ~10, ~20, ~50, and ~100 pmol. L⁻¹) conditions. Our experimental procedure successfully overcame the problems associated with simultaneous changes in pCO₂ and Fe′ by independently manipulating carbonate chemistry and iron speciation, which allowed us to evaluate the individual effects of pCO₂ and iron availability. We found that the C:N ratio decreased significantly only with an increase in Fe′, whereas the C:P ratio increased significantly only with an increase in pCO₂. Both Si:C and Si:N ratios decreased with increasing pCO₂ and Fe′. Our results indicate that changes in pCO₂ and iron availability could influence the biogeochemical cycling of nutrients in future oceans with high CO₂ levels, and, similarly, during the time course of phytoplankton blooms. Morever, pCO₂ and iron availability may also have affected oceanic nutrient biogeochemistry in the past, as these conditions have changed markedly over the Earth’s history.

Complex responses of intertidal molluscan embryos to a warming and acidifying ocean in the presence of UV radiation

Climate change and ocean acidification will expose marine organisms to synchronous multiple stressors, with early life stages being potentially most vulnerable to changing environmental conditions. We simultaneously exposed encapsulated molluscan embryos to three abiotic stressors—acidified conditions, elevated temperate, and solar UV radiation in large outdoor water tables in a multifactorial design. Solar UV radiation was modified with plastic filters, while levels of the other factors reflected IPCC predictions for near-future change. We quantified mortality and the rate of embryonic development for a mid-shore littorinid, Bembicium nanum, and low-shore opisthobranch, Dolabrifera brazieri. Outcomes were consistent for these model species with embryos faring significantly better at 26°C than 22°C. Mortality sharply increased at the lowest temperature (22°C) and lowest pH (7.6) examined, producing a significant interaction. Under these conditions mortality approached 100% for each species, representing a 2- to 4-fold increase in mortality relative to warm (26°C) non-acidified conditions. Predictably, development was more rapid at the highest temperature but this again interacted with acidified conditions. Development was slowed under acidified conditions at the lowest temperature. The presence of UV radiation had minimal impact on the outcomes, only slowing development for the littorinid and not interacting with the other factors. Our findings suggest that a warming ocean, at least to a threshold, may compensate for the effects of decreasing pH for some species. It also appears that stressors will interact in complex and unpredictable ways in a changing climate.