한국연안의 미생물 생태 생지화학: 기후변화에 따른 생태계 반응 예측

Abstract

Heterotrophic bacterial production, respiration, and growth efficiency associated with upwelling intensity in the Ulleung Basin, East Sea We investigated bacterial production (BP) and respiration (BR), as well as the physico-chemical properties of the water column, to elucidate the effect of upwelling on heterotrophic bacterial metabolic activities and growth efficiency (BGE) in July 2012 and May 2013 in the Ulleung Basin (UB), East/Japan Sea. The upwelled conditions were characterized by higher chlorophyll-a (Chl-a) concentrations resulting from the upward shift of the nitracline compared to that of the non-upwelled condition. Analyses of the size fractions of Chl-a and pigment composition revealed that large size phytoplankton (> 20 μm), mainly consisting of diatoms, appeared to be the major phytoplankton component. BP and BR were significantly correlated with Chl-a (P < 0.001), but the correlations with temperature were not significant (P > 0.05). These results suggest that bacterial metabolic activities are stimulated by the availability of organic resources enhanced by upwelling in the UB. Further statistical analysis showed that the difference in BP and BGE with variations in upwelling intensity were significant (P = 0.018 for BP, P = 0.035 for BGE), but the difference in BR was not significant (P = 0.321). These results suggest that metabolic energy is partitioned more for BP under a strong upwelling condition, i.e. high nutrient and Chl-a conditions. In contrast, the energy generated via respiration was partitioned more for maintaining metabolism rather than for biomass production under weakly or non-upwelled conditions, i.e. stratified and low Chl-a conditions. Overall, our results suggest that any changes in upwelling intensity would significantly affect the carbon cycle associated with the fate of primary production, and the role of the microbial loop in the UB where changes in the intensity and frequency of upwelling associated with climatic changes are in progress. Heterotrophic bacterial production and growth efficiency in Gyeonggi Bay, Yellow Sea In estuaries and coastal ecosystems where an uncoupling between primary production (PP) and bacterial production (BP) is often reported, the ratio of BP/PP and bacterial growth efficiency (BGE) are key parameters to understanding bacterial influence through the microbial food web. We investigated chlorophyll-a (Chl-a), PP, BP, bacterial respiration (BR), and BGE in the shallow and turbid coastal estuary of Gyeonggi Bay (GB) in the Yellow Sea. BGE was generally lower than 0.26 (average 0.12, range 0.02–0.26), which suggested that more than 70% of the bacterial carbon demand (BCD) occurs through BR. The BP/PP ratio ranged from 0.13 to 6.88, and the high BP/PP ratio (>1) observed October–February suggested that bacteria consumed dissolved organic carbon in excess of photosynthetically fixed carbon. In comparison to the early 1990s, the annual average water temperature has increased and the size of phytoplankton has decreased in Gyeonggi Bay. The contributions of small phytoplankton to total phytoplankton and the BP/PP ratio were also higher than in the early 1990s. Our results suggested that along with climate change, bacterial roles and the microbial food web becomes increasingly significant in GB. Sulfate Reduction and Sulfur Cycles at Two Seagrass Beds Inhabited by Cold Affinity Zostera marina and Warm Affinity Halophila nipponica in Temperate Coastal Waters To evaluate the impact of invading seagrass on biogeochemical processes associated with sulfur cycles, we investigated the geochemical properties and sulfate reduction rates (SRRs) in sediments inhabited by invasive warm affinity Halophila nipponica and indigenous cold affinity Zostera marina. A more positive relationship between SRR and below-ground biomass (BGB) was observed at the H. nipponica bed (SRR = 0.6809 × BGB – 4.3162, r2 = 0.9878, p = 0.0006) than at the Z. marina bed (SRR = 0.3470 × BGB – 4.0341, r2 = 0.7082, p = 0.0357). These results suggested that SR was more stimulated by the dissolved organic carbon (DOC) exuded from the roots of H. nipponica than by the DOC released from the roots of Z. marina. Despite the enhanced SR in spring-summer, the relatively lower proportion (average, 20%) of acid volatile sulfur (AVS) in total reduced sulfur and the strong correlation between total oxalate-extractable Fe (Fe(oxal)) and chromium reducible sulfur (CRS = 0.2321 × Total Fe(oxal) + 1.8180, r2 = 0.3344, p = 0.0076) in the sediments suggested the rapid re-oxidation of sulfide and precipitation of sulfide with Fe. The turnover rate of the AVS at the H. nipponica bed (0.13 d-1) was 2.5 times lower than that at the Z. marina bed (0.33 d-1). Together with lower AVS turnover, the stronger correlation of SRR to BGB in the H. nipponica bed suggests that the extension of H. nipponica resulting from the warming of seawater might provoke more sulfide accumulation in coastal sediments.