How does rising atmospheric CO2 affect marine organisms?

Click to locate material archived on our website by topic

Effects of Elevated CO2 and Nitrogen Supply on a Legumious Tree
Schortemeyer, M., Atkin, O.K., McFarlane, N. and Evans, J.R.  1999.  The impact of elevated atmospheric CO2 and nitrate supply on growth, biomass allocation, nitrogen partitioning and N2 fixation of Acacia melanoxylonAustralian Journal of Plant Physiology 26: 737-774.

What was done
Seedlings of Acacia melanoxylon, a leguminous nitrogen-fixing tree native to south-eastern Australia, were grown hydroponically for six weeks in growth cabinets receiving atmospheric CO2 concentrations of 350 and 700 ppm.  In addition, seedlings received various concentrations of nitrogen, ranging from 3 to 6,400 mmol m-3, to determine if mineral nitrogen availability influences CO2-induced changes in seedling growth or nitrogen fixation.

What was learned
Averaged across all nitrogen treatments, seedlings grown in elevated CO2 displayed net photosynthetic rates that were 22% higher than those of control seedlings.  Moreover, such plants did not exhibit any signs of photosynthetic acclimation.  In addition, the leaves of CO2-enriched seedlings possessed significantly greater amounts of total nonstructural carbohydrates than leaves of plants grown at 350 ppm CO2, regardless of nitrogen treatment.  These positive responses to atmospheric CO2 enrichment likely contributed to the greater final biomass observed in CO2-enriched seedlings, which consistently exhibited twice the biomass of ambiently-grown seedlings in all but the two lowest nitrogen concentrations, where final biomass was unaffected by atmospheric CO2 enrichment.  Biomass also increased with nitrogen supply, and was 5- to 10-fold larger at the highest nitrogen concentration than it was at the lowest nitrogen concentration.

In contrast to the positive effects that elevated CO2 had on photosynthesis, leaf carbohydrate contents, and final seedling biomass, atmospheric CO2 enrichment did not stimulate symbiotic nitrogen fixation, nor did it decrease it, at any level of nitrogen supply.  Indeed, symbiotic nitrogen fixation decreased substantially for both CO2 concentrations with increasing mineral nitrogen supply.  Thus, the amount of nitrogen fixation that occurs in this species is more dependent upon soil nitrogen content than on atmospheric CO2 concentration.

What it means
As the atmospheric CO2 concentration rises, it is likely that Acacia melanoxylon seedlings will exhibit increased photosynthetic rates and leaf carbohydrate contents, regardless of soil nitrogen supply.  In turn, these positive responses to rising CO2 levels will likely cause such seedlings to produce more biomass and increase their overall size, in nearly all but the most nitrogen-deprived soils where this leguminous species exists.  Moreover, such CO2-induced increases in biomass will likely result without any significant changes in the amount of symbiotic nitrogen fixation occurring in this species.

Reviewed 15 May 2000