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EFFECTS OF LIGHT REDUCTION ON GROWTH OF THE SUBMERGED MACROPHYTE VALLISNERIA AMERICANA AND THE COMMUNITY OF ROOT-ASSOCIATED HETEROTROPHIC BACTERIA
Kurtz, J C., D F. Yates, J M. Macauley, R L. Quarles, F J. Genthner, C A. Chancy, AND R D. Devereux. EFFECTS OF LIGHT REDUCTION ON GROWTH OF THE SUBMERGED MACROPHYTE VALLISNERIA AMERICANA AND THE COMMUNITY OF ROOT-ASSOCIATED HETEROTROPHIC BACTERIA. JOURNAL OF EXPERIMENTAL MARINE BIOLOGY AND ECOLOGY 291(2):199-218, (2003).
A shading experiment was conducted over a growing season to measure the effects of light reduction on Vallisneria americana in Perdido Bay on the Florida-Alabama border, and to determine the response of heterotrophic bacteria in the rhizosphere. Plants subjected to 92% light reduction showed the most pronounced effects in chlorophyll a concentration, above and below ground biomass, and leaf dimensions. The results and field observations further suggested the V. americanalife cycle, as exhibited in temperate waters, was impaired. Heterotrophic bacteria were enumerated and identified (i) from the roots and sediments of fully
illuminated plants and from unvegetated sediments at three intervals and (ii) from the roots of plants that had been subjected to 92% light reduction for three months. Up to two orders of magnitude greater numbers of bacteria were enumerated from root samples than sediment samples on a dry weight basis. Bacteria enumerated from the roots of plants subjected to light reduction (1.3 ? 1.1 x 108 CFU g-1) were significantly higher than numbers of bacteria enumerated from the roots of fully illuminated plants (4.8 ? 1.8 x 107 g-1 in the summer) or sediment samples (1.4 ? 0.03 x 106 g-1). This suggests the roots of seagrasses stressed by light reduction provided more nutrients for bacterial growth. Higher percentages of Gram-negative bacteria were isolated from roots (up to 85% in the fall) than sediments (0-15%). Examination of isolates for traits characteristic of rhizosphere bacteria (siderophore production, formation of the phytohormone indole-3-acetic acid, and antifungal activity) did not show a clear distinction between root-associated and sediment isolates. Taxonomic identifications of root associated bacteria based on MIDI analysis of fatty acid methyl esters were consistent with bacteria known to be associated with other plants or found at oxic-anoxic interfaces. In addition, the bacterial identifications showed most species were found only with roots or only in sediments. Relative to the sediment samples, the higher numbers of bacteria and the higher incidence of Gram-negative bacteria associated with V. americana roots, together with the bacterial identifications, suggests selection of a rhizosphere bacterial community.