Diversity of CO2-concentrating mechanisms and responses to CO2 concentration in marine and freshwater diatoms

Abstract

The presence of CO2-concentrating mechanisms (CCMs) is believed to be one of the characteristics that allows diatoms to thrive in many environments and to be major contributors to global productivity. Here, the type of CCM and the responses to variable CO2 concentration were studied in marine and freshwater diatoms. At 400 ppm, there was a large diversity in physiological and biochemical mechanisms among the species. While Phaeodactylum tricornutum mainly used HCO3-, Thalassiosira pseudonana mainly used CO2. Carbonic anhydrase was an important component of the CCM in all species and C4 metabolism was absent, even with T. weissflogii. For all species, at 20 000 ppm, the affinity for dissolved inorganic carbon was lower than at 400 ppm CO2 and the reliance on CO2 was higher. Despite the difference in availability of inorganic carbon in marine and fresh waters, there were only small differences in CCMs between species from the two environments, and Navicula pelliculosa behaved similarly when grown in the two environments. The results suggest that species-specific differences are great, and more important than environmental differences in determining the nature and effectiveness of the CCM in diatoms.

Storage Compound Accumulation in Diatoms as Response to Elevated CO2 Concentration

“…− [43], which might explain the high effect observed on growth in this diatom at high CO 2 concentration.…”

“…Interestingly and in contrast to seawater condition, N. pelliculosa cultured in freshwater medium had decreased lipid productivity at high CO 2 compared to low CO 2 (Figure 2d). Previous work showed that, at high CO 2 , the affinity for CO 2 (shown as a reduced half-saturation constant) is 7.5-fold higher in N. pelliculosa cultured in seawater than in freshwater, although the overall CO 2 -concentrating mechanisms (CCMs) are not different in these two conditions [43]; thus, the higher lipid accumulation observed in cells grown in seawater medium at high CO 2 might be more related to a change in the carbon conversion/metabolism than to a change in CO 2 uptake. N. pelliculosa is a diatom species with a high tolerance to salinity as it can grow in fresh water and marine media.…”

Insights on the Functions and Ecophysiological Relevance of the Diverse Carbonic Anhydrases in Microalgae

“…This C4 metabolism is present in some aquatic plants (e.g., Hydrilla verticillata and Ottelia alismoides [41][42][43]) and in some green macroalgae (e.g., Ulva prolifera [44]). In contrast, although C4 photosynthesis has been proposed in the diatom Thalassiosira weissflogii (now called Conticribra weissflogii), more recent data indicate that this type of CCM is not widespread in microalgae [38,[45][46][47][48][49][50].…”

The intracellular distribution of inorganic carbon fixing enzymes does not support the presence of a C4 pathway in the diatom Phaeodactylum tricornutum

“…These findings strongly indicate that neither PEPC nor PEPCK are involved in an active CCM. Moreover, the work by Clement et al (2017) revealed that even though the PEPC activity in P. tricornutum increased under low CO 2 concentrations, the overall PEPC:RubisCO ratio remains below one. Furthermore, they show that neither the PPDK nor the NAD-ME activity are affected by changing CO 2 concentrations which leads to the conclusion that P. tricornutum does not perform a biochemical CCM.…”

“…Accordingly, this mechanism is also referred to as the C4 pathway or C4-type CCM (Reinfelder 2011). The carbon source (CO 2 or HCO 3 − ), the Ci uptake system (solute carrier 4 transporter or external CAs) and the extent to which the C i is actively transported and subsequently accumulated inside the cells can be species-specific and might differ in the same species under different environmental conditions such as high and low CO 2 concentrations (Tortell 2000;Clement et al 2017;Tsuji et al 2017a).…”