Microbial biomass and productivity in seagrass beds

Abstract

Different methods for measuring the rates of processes mediated by bacteria in sediments and the rates of bacterial cell production have been compared. In addition, net production of the seagrass Zostera capricorni and bacterial production were compared and some interrelationships with the N cycle discussed. Seagrass productivity was estimated by measuring the plastochrome interval using a leaf stapling technique. The average productivity over 4 seasons was 1.28 .+-. 0.28 g C m-2 day-1 (mean .+-. standard deviation, n = 4). Bacterial productivity was measured 5 times throughout a year using the rate of tritiated thymidine incorporated into DNA. Average values were 33 .+-. 12 mg C m-2 day-1 for sediment and 23 .+-. 4 for water column (n = 5). Spatial variability between samples was greater than seasonal variation for both seagrass productivity and bacterial productivity. Bacterial productivity also was measured using the rate of 32P incorporated into phospholipid. The values were comparable to those obtained with tritiated thymidine. The rate of sulfate reduction was 10 mmol SO4 = m-2 day-1. The rate of methanogenesis was low, being 5.6 mg CH4 produced m-2 day-1. A comparison of C flux measured using rates of sulfate reduction and DNA synthesis indicated that anaerobic processes were predominate in these sediments. An analysis of microbial biomass and community structure, using techniques of phospholipid analysis, showed that bacteria were predominant members of the microbial biomass and that of these, strictly anaerobic bacteria were the main components. Ammonia concentration in interstitial water varied from 23-71 .mu.M. Estimates of the amount of ammonia required by seagrass showed that the ammonia would turn over about once per day. Rapid recycling of N by bacteria and bacterial grazers is probably important.