Populations of the temperate seagrass, Zostera marina L. (eelgrass), often exist as discontinuous beds in estuaries, harbors, and bays where they can reproduce sexually or vegetatively through donal propagation. We examined the genetic structure of three geographically and morphologically distinct populations from central California (Elkhorn Slough, Tomales Bay, and Del Monte Beach), using multilocus restriction fragment length polymorphisms (DNA fingerprints Seagrasses form extensive meadows along the shores of all but the polar seas (1). Where they are found, these submerged marine angiosperms structure nearshore food webs and are highly productive (2,3). Seagrass systems worldwide serve as important and often critical habitats for a broad diversity of invertebrate and fish species, many of which are economically important (3, 4). In addition, seagrasses protect coastlines by minimizing erosion; increasing sedimentation, leading to enhanced recycling of nutrients; and improving water clarity (5).Zostera marina L., or eelgrass, is the dominant seagrass species in temperate waters and can achieve production rates exceeding 4 g of carbon m-2 day-l (2). Eelgrass reproduces both sexually and vegetatively and can colonize to depths of 30 m in clear waters but typically is restricted to shallow or intertidal depths in many estuaries (1-3, 6, 7). Although eelgrass is ecologically successful in very low-light environments (<100 ILmol of quanta m-2.s1l; refs. 6 and 7), the reduction in light penetration found in most industrialized coastal regions has severely restricted the depth distribution and abundance of eelgrass and other seagrass species (8-11).Losses worldwide of seagrass beds have accelerated at alarming rates in the last two decades because of physical disturbance (e.g., dredging, coastline development, fishing practices) and water quality deterioration most often realized as enhanced light attenuation by the water column because of particle loading, eutrophication, and nuisance algal blooms (8-11). Though some of the proximal causes for seagrass loss are increasingly evident (8)(9)(10)(11)(12), the importance of genetic diversity and gene flow for resource stability is unknown. The poor knowledge of the minimal habitat requirements for seagrass growth, colonization and establishment mechanisms, genetic diversity, and reproductive modes requisite for the maintenance of ecologically successful populations hinders the development of sound management criteria (see ref. 13).Previous investigations examining isozyme polymorphisms revealed essentially no genetic diversity within populations and a low level of genetic distinction between geographically disjunct populations of eelgrass (14,15). These findings, in conjunction with the known vigorous rhizomatous growth of this and other seagrass species, have led to the notion (3, 15) that the wide distribution and general ecological success of seagrasses are based upon a vegetative growth strategy. Consequently, a high degree of genetic similarity within pop...

The role of N,-fixing microheterotrophs in marine nitrogen budgets is poorly understood, primarily due to the absence of appropriate technology for assessing the in situ distribution and abundance of these microorganisms. We describe here the application of immunofluorescence microscopy for identifying and characterizing marine N,-fixing microorganisms. Polyclonal antibodies raised against the Fe-S protein of the nitrogenase enzyme complex from the bacterium Rhodospirillum rubrum were demonstrated to recognize nitrogenase from both marine and freshwater N,-fixing bacteria and cyanobacteria. Initial tests of the antibodies were conducted with immunoblots of cell lysates and indicated monospccificity for nitrogenase. We also conducted immunofluoresccnce assays on paraformaldehyde-fixed whole bacterial cells made permeable with lysozyme, allowing microscopic examination and identification of cells containing the enzyme. Immunofluorescence microscopy also revealed the localization of nitrogenase within cyanobacterial heterocysts or filaments. The approach and techniques described here provide reliable means to determine the presence, relative abundance, and distribution of N,-fixing microorganisms in natural microbial assemblages.