Oceanic Plants Are At The Root Of Ecology's Most-Cited Studies

A little more than a year ago, the newsletter Science Watch, published by the Philadelphia-based Institute for Scientific Information (ISI), began conducting analyses of the citation record of ecology and environmental sciences on a regular basis (The Scientist, Feb. 7, 1994, page 15). Since then, the newsletter has found that citation leaders in this burgeoning area come from a wide array of disciplines, including plant science, oceanography, bioremediation and pollution ecology, conservation biology, statistics, and animal behavior. It is interesting to note that, in a recent analysis--using ISI's Science Indicators Database--half of the 10 most-cited papers from 1991-92 were from the field of biological oceanography. Following is the June 1994 Science Watch report, written by Peter D. Moore, a reader in ecology and chairman of human and environmental sciences in the Division of Life Sciences, King's College, London. The report--with an update on the number of citations each of the 10 papers has garnered through September 1994--is presented here with the permission of Science Watch and ISI. The swing of this survey of ecological and environmental citations is distinctly toward the ocean. Papers published during 1991-92 and cited through 1992 show a very strong marine influence, with five of the top 10 having a phytoplankton focus. (Note, however, that Nos. 3 and 10, although bearing 1990 publication dates, did not enter ISI's database until 1991.) The main questions being asked center on the productivity and nutrient cycling of plankton and the ways in which these processes might affect global atmospheric carbon budgets. Two of the papers on phytoplankton productivity (Nos. 1 and 7) report results derived from the Research on the Antarctic Coastal Ecosystem Rates (RACER) program and show that the high productivity one often associates with Antarctic waters is mainly confined to the Antarctic Peninsula coastal zone, other areas being of generally low productivity. The Bransfield Strait, studied by O. Holm-Hansen and B.G. Mitchell (Paper No. 1), lies off the northern tip of the Antarctic Peninsula, and here the greatest phytoplankton biomass was found, especially during the bloom months of December and January. An understanding of what limits the productivity of these waters and controls the blooms is obviously of interest not only to oceanographers, but also to the fishing industry and even to climatologists who still look wistfully to the phytoplankton as a possibly influential carbon sink. But the RACER program, perhaps not surprisingly, fails to come up with a simple answer. Nutrient limitations are not favored as an explanation either of the bloom decline or the low productivity of many Antarctic areas. Temperature may be involved, but the case is unproven. Light may be important, but the bloom declines faster than the light intensities as the Antarctic summer fades. So the answer may lie in the grazing intensity of the zooplankton, which can account for more than 50 percent of the productivity in a season. The significance of grazing is also apparent in the nitrogen cycle of the oceanic surface layers, as shown by the work of P.A. Wheeler and S.A. Kokkinakis in the subarctic Pacific. Here they showed that nitrate (NO3-) ions were between 15 and 40 times more abundant than ammonium (NH4+) ions. This does not appear to be due to efficient microbial nitrification (the oxidation of NH4+ to NO3-), but rather to the preferential uptake of NH4+ by phytoplankton, followed by grazing and then excretion (or decomposition to) ammonium ions. These ions are quickly sequestered once more by the next generation of phytoplankton. The cycle virtually bypasses the nitrate ion. This movement of opinion away from nutrient, or even trace-element, limitation of phytoplankton productivity takes the dream of iron fertilization of the oceans to create an unlimited carbon sink even further into the realms of science fantasy. Two other papers (Nos. 2 and 6) in the top 10 also assist in this process. W.G. Sunda and associates (Paper No. 6) use an experimental approach to examine the iron requirements of different diatom species, one of which--Thallassiosira oceanica--is an open-ocean species and the other--T. pseudonana--an estuarine type. The oceanic species achieved maximum growth at two moles of iron per mole of carbon, while the estuarine species required four times this amount of iron for maximum growth. The early proposals by J.H. Martin and his colleagues (Science Watch, 5[2]:7-8, February 1994) concerning iron limitations were based on the assumption that higher concentrations were optimal for phytoplankton growth. But this may not prove appropriate for open-ocean conditions. Yet one more ferric nail is driven into the coffin of the fertilization/carbon-sink theory by T.-H. Peng and W.S. Broecker (Paper No. 2), who have modeled the atmosphere/ocean carbon flux on the assumption that the iron-limitation hypothesis is correct. They found that the drawdown of carbon from the atmosphere becomes limited by the slow process of vertical mixing within the water. They calculate that in 100 years the atmospheric load of carbon dioxide could be reduced by 10 percent q 5 percent, as a result of enhancing phytoplankton productivity. So, even if the iron limitation proposal is correct (which we have reason to believe is not the case), the impact of mass oceanic fertilization with soluble iron would have little impact on the greenhouse effect. Moving to somewhat drier realms, but sticking with the effects of elevated atmospheric carbon dioxide, R.B. Thomas and B.R. Strain (Paper No. 4) have examined the idea that plant life will grow better in a high-carbon-dioxide world. In general, plants are often limited in their productivity by the availability of carbon dioxide, but these two researchers find that if root growth is restricted, the plant does not respond to increased carbon dioxide. They term this observation "sink-limited feedback inhibition," which basically means that if the plant cannot grow adequately it shuts down its carbon dioxide acquisition, even though gas may be present in higher-than-normal concentrations. It is conceivable that this effect could result in a greenhouse world, especially if root competition were severe. Biotechnology is effectively becoming synonymous with microbial domestication. Two of the highly cited papers on the list (Nos. 5 and 9) indicate the potential value of our bacterial neighbors in cleaning up some of the mess we regularly make of the planet. Both relate to the ability of certain free-living denitrifying bacteria to degrade such toxic molecules as benzene, toluene, and xylene that can con-taminate soils and aquifers as a result of gasoline spills. Denitrifying bacteria operate in anoxic conditions, in which they use nitrate ions rather than oxygen in the respiratory breakdown of organic molecules. These conditions provide bacteria with energy and result in the release of nitrogen gas along with carbon dioxide. P.J. Evans, D.T. Mang, and L.Y. Young (Paper No. 9) have found that a very wide range of soils, sediments, effluents, and sludges contain de-nitrifying bacteria that are able to use toluene, some types of xylene, and benzene as substrates; these bacteria also can completely degrade toluene in culture within three months. S.R. Hutchins and colleagues (Paper No. 5) have conducted similar experiments using cultures from aquifers, and they also conclude that denitrifying bacteria can degrade such aromatic hydrocarbons as toluene. They, however, express caution about the value of such bacteria as cleaning agents because mixtures of aromatic molecules--as found in jet fuels, for example--may lower biodegradation rates overall, owing to the inhibitory influence of some of the mixture's components on denitrification.

Oceanic Plants Are At The Root Of Ecology's Most-Cited Studies

The Scientist Staff

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