How does rising atmospheric CO2 affect marine organisms?

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Effects of Elevated CO2 on an Epiphytic Fern
Ong, B.-L., Koh, C.K-K. and Wee, Y.-C.  1998.  Effects of CO2 on growth and photosynthesis of Pyrrosia piloselloides (L.) Price gametophytes.  Photosynthetica 35: 21-27.

What was done
The authors germinated and grew an epiphytic fern (Pyrrosia piloselloides) in small containers at atmospheric CO2 concentrations of 0, 219, 350, 515, and 3360 ppm to study the effects of elevated CO2 on the fern's photosynthesis and growth.  Whereas most CO2 enrichment experiments investigate responses of seed-bearing vascular plants to elevated CO2, this one investigated responses of a seedless vascular plant that is less adapted to terrestrial habitats than its seed-producing relatives.

What was learned
Spores that were deprived of CO2 germinated two days after the germination of spores exposed to CO2 concentrations ranging from 219 to 3360 ppm.  By 17 days after sowing, however, germination was maximum in all CO2 treatments, with little difference exhibited between spores exposed to CO2 concentrations greater than 219 ppm.

After germination, elevated CO2 positively impacted the photosynthesis of the resulting gametophytes.  After 40 days of CO2 exposure, for example, light-saturated rates of net photosynthesis were 22% and 114% greater at 515 and 3360 ppm, respectively, than they were at 350 ppm.  Over time, elevated CO2 induced photosynthetic acclimation in these plants, but in a concentration-dependent manner.  After 100 days of exposure to elevated CO2, for example, the photosynthetic stimulation in plants grown at 515 ppm CO2 had dropped to 10%, which represents a 50% decline relative to their original stimulation.  However, this photosynthetic adjustment was much less at the super-enriched CO2 concentration of 3360 ppm, for plants grown in this treatment reduced their original photosynthetic enhancement by only 10%.

As part of their acclamatory response to elevated CO2, gametophytes exposed to 515 and 3360 ppm CO2 reallocated limiting resources away from their photosynthetic apparatus, as indicated by an 11 and 28% reduction in their tissue chlorophyll contents, respectively.  Despite these reductions, resulting from an optimization of resources at elevated CO2 concentrations, total gametophytic dry mass at 515 and 3360 ppm was still 43 and 214% greater, respectively, than it was at ambient CO2 at physiological maturity (100 and 80 days for plants grown at 515 and 3360 ppm CO2, respectively).

What it means
In the future, it is likely that the rising CO2 content of the air will promote the photosynthesis and growth of ferns, which are considered more primitive forms of terrestrial plant life than earth's more numerous seed-bearing plants.  Thus, it is likely that ferns will continue to maintain their presence in many ecosystems across the globe.  Indeed, the authors concluded that the "sum responses of Pyrrosia piloselloides gametophytes to elevated CO2 concentration suggest greater success against competitors in the future environment, enabling this fern to continue to establish itself in a future world with high atmospheric CO2."

Reviewed 15 March 2000