Consider, for example, the widespread bleaching of corals that occurred in the Indian and Pacific oceans during the peak warmth of the 1998 El Niño, the deadly effects of which were described at the time as the most extensive ever seen.  So serious was the situation, in fact, that many of the devastated corals were not expected to recover (Normile, 2000); but to the surprise of scientists and laymen alike, barely a year later many of the reefs were recovering, with large amounts of new coral found to be inhabiting what had been made to look like "graveyards" in the wake of the prior year's major marine heat wave.  In describing this phenomenal recovery, Terry Done of the Institute of Marine Science in Cape Ferguson was quoted by Normile as saying that in light of these real-world observations, many people were forced to admit that coral reefs are indeed "more resilient than we had thought."

Another stunning example of marine biota resiliency comes from a report that examined the density and population structure of giant kelp "forests" located near Bahia Tortugas, Baja California, Mexico, both before and after, as well as during, the same 1998 El Niño.  During the height of the extreme warming event, sea surface temperatures (SSTs) were 3°C higher than the previous 10-year average for this region, and they led to the complete disappearance of the giant kelp that had historically inhabited the area.  However, when the SST anomalies subsided, the giant kelp were once again found to be growing there; and from evidence derived from population structure data, plus the rapidity with which the plants had reestablished themselves, Ladah et al. (1999) deduced that "a microscopic stage that was not visible during dive surveys survived the stressful conditions of ENSO and caused the recruitment event, supporting the hypothesis that a bank of microscopic forms can survive conditions stressful to macroscopic algae."  In addition, they stated there was independent evidence to suggest that "microscopic stages may subsist in nature under low light intensities in a semi-dormant state until conditions become favorable."

Further evidence that marine biota are not doomed to extinction by extreme climatic variability comes from the study of Cannariato et al. (1999), who investigated the character, magnitude and speed of responses of benthic foraminifera to millennial-scale climate oscillations manifest in data obtained from an ocean sediment core in the Northeast Pacific that covered the most recent 60,000 years.  Although a number of rapid climatic switches were noted throughout the course of the record, representing periods of what they called "extreme environmental variability," the scientists report that no extinctions were observed, and that the benthic ecosystems "appear to be both resilient and robust in response to rapid and often extreme environmental conditions."  Even though faunal turnovers occurred within decades, they determined they did so "without extinction or speciation."

Another study that led to a similar conclusion was that of Warwick and Turk (2002), who demonstrated that a late Pliocene fossil assemblage of molluscs from St. Erth Pits, north Cornwall, was "not significantly different in biodiversity ... from the present-day regional species pool."  In light of this finding, the two scientists concluded that "predicted changes in climate, by the end of this century, will not affect molluscan biodiversity," although they noted that if the climate warms by 2°C over the next 50 years, "we can expect future latitudinal shifts in the marine biota of 300-600 km."  Their bottom-line conclusion, therefore, was that in coastal marine ecosystems, "widespread extinctions seem unlikely, but changes in community distributions and compositions will inevitably occur."

In a comprehensive study that integrated the findings of several different aspects of this topic, Sarmiento et al. (2004) employed six coupled climate model simulations to determine - to a very rough first approximation - the biological response of the entire global ocean to the climate warming the models simulated for the period between the beginning of the Industrial Revolution and the year 2050.  Based on vertical velocity, maximum winter mixed-layer depth and sea-ice cover, they defined six biomes and calculated how their surface geographies changed in response to calculated changes in global climate.  Then, they used satellite ocean color and climatological observations to develop an empirical model for predicting surface chlorophyll concentrations from the final physical properties of the world's oceans as derived from their global warming simulations, after which they used three primary production algorithms to estimate the response of oceanic primary production to climate warming based on their calculated chlorophyll concentrations.  And the final result?  When all was said and done, the thirteen scientists from Australia, France, Germany, Russia, the United Kingdom and the United States arrived at a global warming-induced increase in global ocean primary production that ranged from 0.7 to 8.1%.

Turning our attention from temperature to atmospheric CO2 concentration, we learn from Wolf-Gladrow et al.'s (1999) review of the direct effects of atmospheric CO2 enrichment on marine phytoplankton, including the consequences of these phenomena for the world's oceanic carbon pump, that the ongoing rise in the air's CO2 content may also benefit the planet's marine biota, producing significant increases in phytoplanktonic growth rates that may even "serve as negative feedbacks to anthropogenic CO2 increase," which subject we treat briefly in our Editorial of 11 Aug 2004 and a bit more comprehensively under the heading of Forcing Factors (Aerosols - Biological: Aquatic) in our Subject Index.

Much the same conclusion was reached by Chen and Gao (2004), who grew the unicellular marine diatom Skeletonema costatum in nutrient-enriched seawater maintained at 20°C under a cycle of 12 hours light (at an intensity of 200 µmol m^-2 s^-1) followed by 12 hours of darkness, while continuously aerating one culture with air of 350 ppm CO2 and another with air of 1000 ppm CO2 and simultaneously measuring a number of physiological parameters related to the diatom's photosynthetic activity.  They report that cell numbers of the alga "increased steadily throughout the light period and they were 1.6 and 2.1 times higher after the 12 h light period for the alga grown at 350 and 1000 ppm CO2, respectively."  They also say that chlorophyll a concentrations in the two CO2 cultures "increased 4.4- and 5.4-fold during the middle 8 h of the light period for the alga grown at 350 and 1000 ppm CO2, respectively," and that "the contents of cellular chlorophyll a were higher for the alga grown at 1000 ppm CO2 than that at 350 ppm CO2."  In addition, they found that the initial slope of the light saturation curve of photosynthesis and the photochemical efficiency of photosystem II "increased with increasing CO2, indicating that the efficiency of light-harvesting and energy conversion in photosynthesis were increased."

Chen and Gao say their experiments "showed that the alga mainly use free CO2 from the medium," and that "S. costatum benefited from CO2 enrichment," noting their data "showed that the light-saturated photosynthesis rate based on cell number, the chlorophyll a content, the photosynthetic chemistry of photosystem II and the efficiency of the light reaction all increased to various degrees with elevated CO2."  And since marine diatoms, in their words, "are responsible for approximately 40% of marine primary productivity," and since S. costatum "is widely distributed in coastal waters all over the world and constitutes a major component of natural assemblages of most marine phytoplankton," we can assume that the responses they observed would be ubiquitous.

In another laboratory study, Yu et al. (2004) grew the marine microalgae Platymonas subcordiformis (Wille) Hazen at ambient levels of atmospheric CO2 concentration and UV-B radiation flux density as well as at elevated levels of 5000 ppm CO2 and UV-B radiation characteristic of that anticipated to result from a 25% stratospheric ozone depletion under clear sky conditions in summer.  By itself, they report that the elevated UV-B treatment "significantly decreased [microalgal] dry weight, photosynthetic rate, chlorophyll a and carotenoid contents," while the elevated CO2 treatment by itself "enhanced dry weight and photosynthetic rate, but chlorophyll a content and carotenoid content had no major difference compared with those of ambient UV-B and ambient CO2."  They also report that elevated UV-B by itself significantly increased the production of the toxic superoxide anion and hydrogen peroxide, as well as malonyldialdehyde, which is an end product of lipid peroxidation, whereas elevated CO2 by itself did just the opposite.  In addition, in the treatment consisting of both elevated UV-B and elevated CO2, the concentrations of these three substances were lower than those observed in the elevated UV-B and ambient CO2 treatment.  Finally, they report that elevated CO2 decreased the levels of several antioxidative enzymes found in the microalgae, reflective of their reduced need for detoxification of reactive oxygen species in the elevated CO2 treatment.

Yu et al. say their results suggest that "CO2 enrichment could reduce oxidative stress of reactive oxygen species to P. subcordiformis, and reduce the lipid peroxidation damage of UV-B to P. subcordiformis."  They also say that "CO2 enrichment showed a protective effect against the oxidative damage of UV-B-induced stress," and, therefore, that "elevated CO2 can … enhance the capacity of stress resistance."  Put more simply, they say in their concluding paragraph that "algae grown under high CO2 would better overcome the adverse impact of environmental stress factor[s] that act via generation of activated oxygen species."

Marine macroalgae are also helped by higher levels of atmospheric CO2.  Gao et al. (1993), for example, grew cultures of the red alga Gracilaria sp. and G. chilensis in vessels enriched with nitrogen and phosphorus that were continuously aerated with either normal air of 350 ppm CO2, air enriched with an extra 650 ppm CO2, or air enriched with an extra 1250 ppm CO2 for a period of 19 days.  Compared to the control treatment, the relative growth enhancements in the + 650-ppm and +1250-ppm CO2 treatments were 20% and 60%, respectively, for G. chilensis, and 130% and 190%, respectively, for Gracilaria sp.  Gao et al. say these results suggest that, "in their natural habitats or cultivation sites, photosynthesis and growth of Gracilaria species are likely to be CO2-limited, especially when the population density is high and water movement is slow."  Hence, as the air's CO2 content continues to rise, these marine marcoalgae should be able to grow ever better; and such may well be the case with many other macroalgae too, for Gao et al. note that "photosynthesis by most macroalgae is probably limited by inorganic carbon sources in natural seawater," citing the studies of Surif and Raven (1989), Maberly (1990), Gao et al. (1991) and Levavasseur et al. (1991) as evidence for the validity of this statement.

At a glance, this latter finding might possibly be construed to imply that corals in a CO2-enriched world may be out-competed by marine macroalgae, which, to quote Langdon et al. (2003), "are not conspicuous on healthy reefs, but due to various anthropogenic pressures … are becoming increasingly abundant."  However, in an experiment they conducted at the Biosphere-2 facility near Oracle, Arizona, USA, where they studied gross primary production and calcification in a macrophyte-dominated ecosystem that had a coral cover of 3%, Langdon et al. obtained evidence that suggests that the ongoing rise in the air's CO2 content will not "hasten the transformation of reef community structure from coral to algal dominance."

In conclusion, then, the results of these several studies would appear to suggest that increases in both air temperature and atmospheric CO2 concentration would be in the best interests of marine plant life, which suggests they would also be in the best interests of marine animal life.

References
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Chen, X. and Gao, K.  2004.  Characterization of diurnal photosynthetic rhythms in the marine diatom Skeletonema costatum grown in synchronous culture under ambient and elevated CO2.  Functional Plant Biology 31: 399-404.

Gao, K., Aruga, Y., Asada, K., Ishihara, T., Akano, T. and Kiyohara, M.  1991.  Enhanced growth of the red alga Porphyra yezoensis Ueda in high CO2 concentrations.  Journal of Applied Phycology 3: 355-362.

Gao, K., Aruga, Y., Asada, K. and Kiyohara, M.  1993.  Influence of enhanced CO2 on growth and photosynthesis of the red algae Gracilaria sp. and G. chilensis.  Journal of Applied Phycology 5: 563-571.

Ladah, L.B., Zertuche-Gonzalez, J.A. and Hernandez-Carmona, G.  1999.  Giant kelp (Macrocystis pyrifera, Phaeophyceae) recruitment near its southern limit in Baja California after mass disappearance during ENSO 1997-1998.  Journal of Phycology 35: 1106-1112.

Langdon, C., Broecker, W.S., Hammond, D.E., Glenn, E., Fitzsimmons, K., Nelson, S.G., Peng, T.-S., Hajdas, I. and Bonani, G.  2003.  Effect of elevated CO2 on the community metabolism of an experimental coral reef.  Global Biogeochemical Cycles 17: 10.1029/2002GB001941.

Levavasseur, G., Edwards, G.E., Osmond, C.B. and Ramus, J.  1991.  Inorganic carbon limitation of photosynthesis in Ulva rotundata (Chlorophyta).  Journal of Phycology 27: 667-672.

Maberly, S.C.  1990.  Exogenous sources of inorganic carbon for photosynthesis by marine macroalgae.  Journal of Phycology 26: 439-449.

Normile, D.  2000.  Global Warming: Some coral bouncing back from El Niño.  Science 288: 941-942.

Sarmiento, J.L., Slater, R., Barber, R., Bopp, L., Doney, S.C., Hirst, A.C., Kleypas, J., Matear, R., Mikolajewicz, U., Monfray, P., Soldatov, V., Spall, S.A. and Stouffer, R.  2004.  Response of ocean ecosystems to climate warming.  Global Biogeochemical Cycles 18: 10.1029/2003GB002134.

Surif, M.B. and Raven, J.A.  1989.  Exogenous inorganic carbon sources for photosynthesis in seawater by members of the Fucales and the Laminariales (Phaeophyta): ecological and taxonomic implications.  Oecologia 78: 97-103.

Warwick, R.M. and Turk, S.M.  2002.  Predicting climate change effects on marine biodiversity: comparison of recent and fossil molluscan death assemblages.  Journal of the Marine Biological Association of the United Kingdom 82: 847-850.

Wolf-Gladrow, D.A., Riebesell, U., Burkhardt, S. and Bijma, J.  1999.  Direct effects of CO2 concentration on growth and isotopic composition of marine plankton.  Tellus 51B: 461-476.

Yu, J., Tang, X-X., Zhang, P-Y., Tian, J-Y. and Cai, H-J.  2004.  Effects of CO2 enrichment on photosynthesis, lipid peroxidation and activities of antioxidative enzymes of Platymonas subcordiformis subjected to UV-B radiation stress.  Acta Botanica Sinica 46: 682-690.