Non-photosynthetic enhancement of growth by high CO 2 level in the nitrophilic seaweed Ulva rigida C. Agardh (Chlorophyta)

Abstract

The effects of increased CO2 levels (10,000 microl l(-1)) in cultures of the green nitrophilic macroalga Ulva rigida C. Agardh were tested under conditions of N saturation and N limitation, using nitrate as the only N source. Enrichment with CO2 enhanced growth, while net photosynthesis, gross photosynthesis, dark respiration rates and soluble protein content decreased. The internal C pool remained constant at high CO2, while the assimilated C that was released to the external medium was less than half the values obtained under ambient CO2 levels. This higher retention of C provided the source for extra biomass production under N saturation. In N-sufficient thalli, nitrate-uptake rate and the activity of nitrate reductase (EC 1.6.6.1) increased under high CO2 levels. This did not affect the N content or the internal C:N balance, implying that the extra N-assimilation capacity led to the production of new biomass in proportion to C. Growth enhancement by increased level of CO2 was entirely dependent on the enhancement effect of CO2 on N-assimilation rates. The increase in nitrate reductase activity at high CO2 was not related to soluble carbohydrates or internal C. This indicates that the regulation of N assimilation by CO2 in U. rigida might involve a different pathway from that proposed for higher plants. The role of organic C release as an effective regulatory mechanism maintaining the internal C:N balance in response to different CO2 levels is discussed.

Growth, physiological and biochemical responses of Meliaceae species - Azadirachta indica and Melia dubia to elevated CO2 concentrations

“…This is in accordance with previous studies carried out under elevated CO 2 [47][48][49][50]. Plants lose the ability to take up soil nitrate (most common form of nitrogen) and convert it into organic compounds such as proteins at enriched CO 2 conditions.…”

Divergent responses in growth and nutritional quality of coastal macroalgae to the combination of increased pCO2 and nutrients

“…Therefore, in our system, nutrients are the key driver of C:N ratios in Ulva and Fucus. While we observed higher concentrations of N within Ulva tissues grown under high pCO 2 treated individuals, the difference was not significant and our results do not match Gordillo et al (2001) and Xu and Gao (2012), who found that high pCO 2 facilitates nitrate uptake in U. rigida and U.…”

“…or Fucus vesiculosus. Although adding carbon is counterintuitive to decreasing the C:N ratio, Gordillo et al (2001) found that increased pCO 2 resulted in increased uptake of nitrate in U. rigida, thus lowering the tissue C:N ratio. While our results don't show decreased C:N with high pCO 2 , analysis of Ulva tissues grown in biculture and under high pCO 2 show an uptick in nitrogen concentration.…”

“…As such, ocean acidification may ultimately end up contributing to the timing, frequency, and duration of macroalgal blooms. Ocean acidification has been shown to not only enhance growth rates in some algal species but also may enhance the uptake of nutrients (Gordillo et al, 2001), increasing growth rates indirectly. In addition, acidification can alleviate the cold-driven temperature stress in a species of red algae, resulting in higher growth rates in colder waters .…”

Seasonal variations of Fucus vesiculosus fertility under ocean acidification and warming in the western Baltic Sea

“…High CO 2 and/or dissolved inorganic carbon (DIC) conditions stimulate growth and photosynthesis of different non-calcifying seaweeds (Gordillo et al 2001, Nygård and Dring 2008, Wu et al 2008, Olischläger et al 2012, Saderne 2012, Koch et al 2013). In addition to the effect of enhanced CO 2 on growth and photosynthesis of seaweeds, reproduction may be impacted by the DIC concentration in seawater.…”