restricted access IV.9 Seascape Microbial Ecology: Habitat Structure, Biodiversity, and Ecosystem Function
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IV.9 Seascape Microbial Ecology: Habitat Structure, Biodiversity, and Ecosystem Function David M. Karl and Ricardo M. Letelier OUTLINE 1. Introduction 2. Seascape structure, variability, and function 3. Assessments of microbial ‘‘species’’ diversity and function 4. The ocean genome 5. Ecotype variability and resource competition 6. The streamlined genome of SAR 11 7. Station ALOHA: A microbial observatory in the open sea 8. Conclusion Seascapes are marine analogs of landscapes in the terrestrial biosphere, namely the physical, chemical, and biological elements that collectively define a particular marine habitat. The field of seascape ecology, also referred to as ecological geography of the sea, seeks fundamental understanding of spatial and temporal variability in habitat structure and its relationships to ecosystem function, including solar energy capture and dissipation, trophic interactions and their effects on nutrient dynamics, and patterns and controls of biodiversity. Implicit in the study of seascape ecology is an interest in the management of global resources through the development of new theory, the establishment of long-term ecological observation programs, and the dissemination of knowledge to society at large. GLOSSARY euphotic zone. Upper portion of the ocean where there is sufficient light to support net photosynthesis, usually the upper 0–200 m in the clearest ocean water genome. The complete assembly of genes present in a given organism, coded by specific nucleotide sequences of DNA, that determines its taxonomic structure, metabolic characteristics, behavior, and ecological function microorganism. The smallest form of life (1mg m–3 ) as a result of net plant growth. The NPSG is the central region characterized by low ambient NO3  concentrations (500 mM). However, the relative stability of the triple bond of N2 renders this form inert to all but a few specialized N2- fixing microbes, dubbed diazotrophs. N2 fixation in most open-ocean ecosystems is solar powered, and most diazotrophs are cyanobacteria. Diazotrophs require an ample supply of iron (Fe), which is an obligate cofactor for the enzyme nitrogenase. The Fe supply to the surface waters of most open-ocean habitats is via atmospheric dust delivery, and thus Fe flux varies considerably with geographic location and distance from dust sources (e.g., deserts). Furthermore, in order for N2-dependent net growth to occur, diazotrophs require a suite of macro- and micronutrients, especially phosphate. If light, Fe, and phosphate are present in excess, phototrophic diazotrophs would have a competitive advantage in N-limited seascapes, and N2 fixation may be a significant pathway for the introduction of new N into the ecosystem. When the HOT program began, N2 fixation was not considered to be a significant process in the NPSG. Seascape Microbial Ecology 495 Indeed, the climax community paradigm for this seascape circa 1970 was one of a time-independent, nitratecontrolled , eukaryote-dominated, low-productivity biome . Whereas nitrate resupply from deep waters was considered the ultimate source of new N in the historical view, we now recognize N2 fixation as an approximately equal new N flux pathway (table 2). N2 fixation can be viewed as a keystone ecological process in the N-stressed NPSG, supplying new nitrogen via a delivery pathway that is independent of turbulence . The impact of diazotrophs is disproportional to their relatively low abundance. With the possible exception of stochastic bloom events, diazotroph biomass rarely exceeds a few percent, at most, of phytoplankton carbon in these habitats. However, the removal of all N2 fixers from the NPSG would likely lead to a significant decrease in phytoplankton biomass, net primary production, fish production, CO2 sequestration , and a corresponding reduction of the export of carbon and energy to the mesopelagic and deep sea, with attendant ecological consequences—the hallmark of an ecological keystone. We currently recognize at least three fundamentally different groups of diazotrophs at Station ALOHA (see table 3): (1) small, free-living unicellular cyanobacteria (Crocosphaera-like), (2) large filamentous and colonial morphologies of the cyanobacterium Trichodesmium, and (3) Richelia-like cyanobacteria living as ecto- and endosymbionts with several species of large aggregateforming diatoms (e.g., Rhizosolenia, Hemiaulus). The N2 fixed by each of these groups has a different impact on the ecology and biogeochemistry of the NPSG, despite the fact that all belong to the same diazotroph guild (table 3). In addition to alleviating N stress, N2 fixation-based organic matter production enhances the sequestration of CO2 because the import of N2 is decoupled from the delivery of deep water nitrate that also contains a high concentration of CO2. Gravitational settling of N2-based particulate organic matter pumps excess carbon...