Colonization Rates, Annual Temporal Variation, and Some Ecological Interactions of the Microbial Epibiont Community on the Prop Roots of the Red Mangrove Tree, Rhizophora mangle

Within non-limiting substrate (NLS) environments bacteria are able to reach their carrying capacity. Under these conditions microbial communities are bottom-up controlled. In other words, they are mainly responsible for transferring carbon to higher trophic levels, rather then controlling bacterial numbers. One ecosystem where these ecological interactions have been sparsely examined is mangroves. Mangrove trees are highly productive which in part, explains the rich epibiont layer covering their roots. This study provided first information on the microbial epibionts on the prop roots of red mangroves, Rhizophora mangle. Six groups of microbial epibionts were examined for their rate of colonization of the prop roots, their temporal variation within four seasons (June 28, 1999 to May 25, 2000), and their role in carbon transfer to higher trophic levels. Microbes examined included bacteria, cyanobacteria, autotrophic diatoms, amoebae, heterotrophic ciliates, and heterotrophic flagellates (HF). Colonizing microbes on artificial substrates reached densities similar to those on natural prop roots within one to seven days, except for cyanobacteria, which occurred in ten to thirteen days. This rapid colonization was presumably due to the high organic input into this NLS environment. Final numbers of microbial epibionts on the artificial substrata (MEAS) were over two fold higher than abundances of microbial epibionts on the prop roots (MEPR). It is possible that this difference was due to phytotoxins produced by roots or bacterial interactions (ammensalism) occurring within the MEPR. Mean densities of the MEPR throughout the four seasons were 6.9x109 cells g-1 dry weight for the bacteria, 9.8x106 cells g-1 dry weight for the cyanobacteria, and 2.7x6 cells g-1 dry weight for the diatoms. The protozoan mean densities were 7.7x103 cells g-1 dry weight for the amoebae, 4.8 x 103 cells g-1 dry weight for the ciliates, and 2.7x105 cells g-1 dry weight for the HF. Due to the lack of seasonal change in this NLS environment, the temporal patterns for the protozoa were reminiscent of the variability throughout the year shown by the bacteria, cyanobacteria, and/or diatoms. The strongest positive correlations were seen with ciliates and heterotrophic flagellates with suspended bacteria (r=0.64 and r=0.80, respectively). Although, not as strong, amoebae were found to have a moderate positive correlation with diatoms (r=0.44). Based upon predator-prey relationships proposed by Paffenhofer (1998) it is possible that these correlation analyses are indicative of the dominant prey consumed by the protozoa examined. To further support this idea growth studies were done to determine which prey items were the most palatable to the protozoan groups studied. In other words, the more palatable the prey the faster the generation times of the protozoan group. This, in turn, provides information on protozoa predation within the epibiont community and possible carbon to higher trophic levels. Amoebae and ciliates were found to have their fastest generation times when feeding on cyanobacteria (40.9 h gen-1 and 60.7 h gen-1, respectively). Heterotrophic flagellates had their fastest generation times with suspended bacteria (14.5 h gen-1). A pioneering discovery from these growth studies demonstrated that amoebae were the only protozoan group capable of growing able with tightly attached bacteria was amoebae. It was discovered that 50% of epibiont bacteria were composed of attached bacteria (loosely or closely surface associated). This is a high percentage so the role of protozoa in removing attached bacteria was investigated. Based on the mean densities of the protozoan groups on the MEPR throughout the year, amoebae consumed 4.48 x 106 attached bacteria hr-1 ciliates consumed 5.71x106 attached bacteria hr-1, and HF consumed 2.91x107 attached bacteria hr-1. Converting these bacterial numbers into carbon equivalents it was determined that out of the potential attached\u27 bacterial carbon available in the environment amoebae, ciliates and HF are only able to consume: 0.1%, 0.2%, and 0.8%, respectively of this carbon