ScienceWeek

MICROBIOLOGY: ON DEEP-SEA MICROBIAL FAUNA

The following points are made by Edward F. DeLong (Science 2004 306:2198):

1) The apparent paucity of deep-sea biota led the 19th-century biologist Edward Forbes (1815-1854) to question the very existence of life at depths greater than 550 m. Subsequent oceanographic expeditions soon laid Forbes' "azoic theory" to rest, with discoveries of a diverse and abundant marine fauna flourishing in the greatest depths of the oceans. In parallel ways, contemporary microbial surveys are expanding the range of known habitats where microbial life thrives. D'Hondt et al [1] have reported evidence for metabolically diverse and active microbial communities buried deep within marine sediments nearly 0.5 km below the seafloor. Using chemical clues hidden deep within marine sediment cores, these investigators infer how subseafloor microbes eat and breathe [1]. They suggest that certain microbial activities deviate substantially from standard models [2] of microbial metabolism in subseafloor sediments.

2) How important are the microbial communities buried deep within the marine sediments that overlay two-thirds of Earth's surface? Counting microbes under the microscope (which does not distinguish living from dead organisms) reveals that substantial numbers of microbes must exist in deep seafloor sediments [3]. Quantitative estimates indicate that the vast majority of these sediment-associated microbes (97% or so) reside in the upper 600 m of sediment [3,4]. Microbial cell numbers range from 10^(8) cells per gram of sediment just below the seafloor, to about 10^(4) cells per gram of sediment 0.5 km deep in the subsurface [3]. This substantial subsurface microbial biomass raises a number of interesting questions. Do these microbes represent well-preserved remnants of a microbial burial at sea? Alternatively, do these organisms thrive actively in the subsurface and, if so, what do they eat and how do they breathe? Does microbial activity vary with the depth and geochemical gradients found deep within the sediments? D'Hondt et al (1) begin to answer these questions with their analyses of deep-sea sediment cores recovered from the equatorial Pacific Ocean off the coast of Peru. Some of their conclusions are rather unexpected.

3) Comparative analyses of the geochemistry of subseafloor sediment cores is providing new insights into subsurface microbial life. The sediment cores collected by D'Hondt et al [1] were sampled to depths of 420 m. Samples include those from the Peruvian shelf, the Peru Trench, and further offshore from open-ocean sediments. Similar to previous studies [3], D'Hondt et al [1] discovered remarkable numbers of microbes in sediment samples, which decreased with increasing sediment depth. These investigators also measured potential respiratory electron acceptors (oxidants), including sulfate and nitrate. The flux of these oxidants can serve as markers of specific microbial activities, because certain microbes use them to respire in the absence of oxygen. The occurrence and distribution of other microbial metabolic by-products -- carbon dioxide, ammonia, sulfide, methane, manganese, and iron -- also provide metrics of microbial activity. Profiles of these biologically processed compounds paint a picture of how microbial activities may be partitioned in the deep sediment, and serve as indicators of which metabolic pathways are crucial.

References (abridged):

1. S. D'Hondt et al., Science 306, 2216 (2004)

2. D. E. Canfield et al., Mar. Geol. 113, 27 (1993)

3. R. J. Parkes et al., Nature 371, 410 (1994)

4. W. B. Whitman, D. C. Coleman, W. J. Wiebe, Proc. Natl. Acad. Sci. U.S.A. 95, 6578 (1998)

5. N. R. Pace, Science 276, [734] (1997)

Science http://www.sciencemag.org

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ON THE CHEMISTRY AND BIOLOGY OF THE OCEANS

Notes by ScienceWeek:

The combination of vast areas of liquid water on its surface together with a high concentration of free molecular oxygen in its atmosphere is unique to Earth in this solar system. Calculations based on *ultraviolet absorption cross sections indicate that whereas direct photolysis of water could have produced small amounts of O(sub2), almost all of the gas was produced by biological systems through the photobiologically catalyzed oxidation of the liquid.

The following points are made by Falkowski et al (Science 1998 281:200):

1) Changes in oceanic primary production, linked to changes in the network of global biogeochemical cycles, have profoundly influenced the geochemistry of Earth for over 3 billion years.

2) In the contemporary ocean, photosynthetic *carbon fixation by marine phytoplankton leads to the formation of approximately 45 gigatons of organic carbon per year, of which 16 gigatons are exported to the ocean interior.

3) Changes in the magnitude of total and export production can strongly influence atmospheric CO(sub2) levels (and hence climate) on geological time scales, as well as set upper bounds for sustainable fisheries harvest.

4) Because the average turnover time of phytoplankton carbon in the ocean is on the order of a week or less, total and export production are extremely sensitive to external forcing, and consequently are seldom in steady state.

5) Elucidating the biogeochemical controls and feedbacks on primary production is essential to understanding how oceanic biota responded to and affected natural climate variability in the geological past, and to understanding how oceanic biota will respond in the coming decades to changes influenced by human activities.

Science http://www.sciencemag.org

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Notes by ScienceWeek:

ultraviolet absorption cross sections: The ratios of the amount of energy removed from incident UV by absorption to the total energy of incident UV. In other words, in this context, a measure of how much energy is (was) actually available for direct photolysis of liquid water.

carbon fixation: Refers to the process of converting the carbon in a substance into a form usable by an organism. For example, the conversion of the carbon in CO(sub2) into organic carbon (the carbon in organic compounds).

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ECOLOGY: ON OCEANIC PLANKTON DIVERSITY

The following points are made by P.J. Morin and J.W. Fox (Nature 2004 429:813):

1) Much of what we know about how the diversity of life varies across environments comes from studies of large, "charismatic" terrestrial organisms that typically attract the attention of ecologists(1). These studies show that diversity often peaks at intermediate levels of productivity, where productivity describes the rate of energy capture and its transformation into biomass by organisms. Little is known about whether similar patterns of diversity and productivity hold for the much smaller organisms that predominate in the world's oceans, and there are suggestions that some ecological patterns of single-celled organisms might differ in important ways from those of larger ones(2).

2) Irigoien et al(3) have reported that the algae --phytoplankton -- supporting food webs in the oceans, Earth's largest ecological realm, exhibit a unimodal diversity-productivity pattern similar to that described for many other systems. Revealing the unimodal pattern required the compilation and analysis of a database of algal species composition and biomass (where biomass serves as a reasonable surrogate for productivity) in more than 350 samples collected from oceans around the world. Although several other diversity-productivity patterns exist(1), the unimodal pattern appears repeatedly for a variety of organisms in different circumstances. In this respect, ecology exhibits some important generalities, which can now be extended to the oceans. The analysis also emphasizes the value of the new field of "ecoinformatics" in uncovering patterns that emerge only from extensive data sets collected over large temporal and spatial scales.

3) Several factors could make phytoplankton diversity reach a peak at intermediate levels of productivity. Community history(4), spatial niche differentiation(5) and competition for multiple resources can all produce unimodal patterns. Community history refers to the stochastic, sequential arrival of species at a local site from a surrounding regional pool of potential community members. The arrival sequence can affect diversity (for instance, weaker competitors might persist if they arrive and become established before other species), and can interact with productivity to produce a variety of diversity-productivity relationships, including a unimodal pattern(4). Spatial niche differentiation occurs when local sites contain different microhabitats (spatial niches), such as a lake surface or water column, to which species are differentially adapted, and which vary in their ability to support reproduction.

4) Diversity is maximized in sites that are sufficiently productive to allow reproduction in all microhabitats but are not so productive that the species adapted to the best microhabitat produces sufficient offspring to "swamp" other species(5). The hypothesis of competition for multiple resources suggests that shading within dense algal blooms probably increases with productivity, so that phytoplankton at high-productivity sites compete more strongly for light than for nutrients. Superior competitors for nutrients or light would respectively dominate low- and high-productivity sites, whereas sites of intermediate productivity would support a diverse mixture of phytoplankton.

References (abridged):

1. Waide, R. B. et al. Annu. Rev. Ecol. Syst. 30, 257-301 (1999)

2. Finlay, B. J. Science 296, 1061-1063 (2002)

3. Irigoien, X., Huisman, J. & Harris, R. P. Nature 429, 863-867 (2004)

4. Fukami, T. & Morin, P. J. Nature 424, 423-426 (2003)

5. Kassen, R., Buckling, A., Bell, G. & Rainey, P. B. Nature 406, 508-512 (2000)

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