Effects ofCO₂enrichment on photosynthesis, growth, and nitrogen metabolism of the seagrassZostera noltii

Abstract

Seagrass ecosystems are expected to benefit from the global increase in CO2 in the ocean because the photosynthetic rate of these plants may be Ci-limited at the current CO2 level. As well, it is expected that lower external pH will facilitate the nitrate uptake of seagrasses if nitrate is cotransported with H+ across the membrane as in terrestrial plants. Here, we investigate the effects of CO2 enrichment on both carbon and nitrogen metabolism of the seagrass Zostera noltii in a mesocosm experiment where plants were exposed for 5 months to two experimental CO2 concentrations (360 and 700 ppm). Both the maximum photosynthetic rate (Pm) and photosynthetic efficiency (α) were higher (1.3- and 4.1-fold, respectively) in plants exposed to CO2-enriched conditions. On the other hand, no significant effects of CO2 enrichment on leaf growth rates were observed, probably due to nitrogen limitation as revealed by the low nitrogen content of leaves. The leaf ammonium uptake rate and glutamine synthetase activity were not significantly affected by increased CO2 concentrations. On the other hand, the leaf nitrate uptake rate of plants exposed to CO2-enriched conditions was fourfold lower than the uptake of plants exposed to current CO2 level, suggesting that in the seagrass Z. noltii nitrate is not cotransported with H+ as in terrestrial plants. In contrast, the activity of nitrate reductase was threefold higher in plant leaves grown at high-CO2 concentrations. Our results suggest that the global effects of CO2 on seagrass production may be spatially heterogeneous and depend on the specific nitrogen availability of each system. Under a CO2 increase scenario, the natural levels of nutrients will probably become limiting for Z. noltii. This potential limitation becomes more relevant because the expected positive effect of CO2 increase on nitrate uptake rate was not confirmed.

Short Term CO2 Enrichment Increases Carbon Sequestration of Air-Exposed Intertidal Communities of a Coastal Lagoon

“…The short-term gas exchange measurements with CO 2 enrichment conditions clearly indicated that Z. noltei community production at around the same irradiance level increased 1.5-fold. Higher community production under CO 2 enrichment suggest that community production of Z. noltei are carbon limited under the current atmospheric CO 2 concentration, confirming the same conclusions previously obtained for Z. noltei (Alexandre et al, 2012) and for Z. marina (Beer and Koch, 1996;Zimmerman et al, 1997;Invers et al, 2001). These reports are on the photosynthetic response of individual plants rather than whole community, but it is expected that the increase in photosynthetic rate of individual plants is reflected on the higher organizational level of the community production.…”

“…This was also observed in Z. marina (Zimmerman et al, 1997 and Z. noltei (Alexandre et al, 2012) where increase in CO 2 concentrations increased the light using capacity of individual plants.…”

“…Most of the studies on the effects of elevated CO 2 levels on seagrasses have been on the responses of productivity and light requirements (Beer and Koch, 1996;Thom, 1996;Zimmerman et al, 1997;Palacios and Zimmerman, 2007), photosynthetic and leaf growth rates (Alexandre et al, 2012) and the uptake rate of ammonium and nitrate (Short and Neckles, 1999;Alexandre et al, 2012;Ow et al, 2016). Results so far have suggested that, seagrass meadows can utilize the increased CO 2 concentration from the water column and enhance their photosynthetic activity and community metabolism (Frankignoulle and Disteche, 1984;Frankignoulle and Bouquegneau, 1990;Invers et al, 2001).…”

Response of key stress-related genes of the seagrass <i>Posidonia oceanica</i> in the vicinity of submarine volcanic vents

“…On the other hand, in a long-term experiment, there was no effect of increasing CO 2 levels on the aboveground productivity of Zostera marina (Palacios and Zimmerman, 2007), as opposed to belowground. Alexandre et al (2012) showed that the net photosynthetic rate of Zostera noltii was positively affected by the CO 2 enrichment of the seawater, but they did not observe an increase in leaf growth rates.…”

Twenty-first century climate change and submerged aquatic vegetation in a temperate estuary: the case of Chesapeake Bay

“…Under such conditions, photosynthetic carbon assimilation is increased and photorespiration, which can reduce photosynthetic capacity of eelgrass by 40%, is decreased (e.g., Thom 1995;Zimmerman et al 1997;Palacios and Zimmerman 2007;Alexandre et al 2012;Buapet et al 2013). The most convincing evidence has been provided by Zimmerman and coworkers who have simulated coastal acidification in manipulative experiments with eelgrass for nearly two decades.…”