海洋における生元素の循環と微生物代謝の相互作用

抄録

Use of a stable isotope ¹⁵N to study microbial processes associated with nitrogen dynamics in marine environments has several advantages including estimates of in situ rates of nitrogen metabolisms and of transfer of nitrogen within ecosystem. First part of this review summarizes application of ¹⁵N methodology to evaluate microbial processes of ammonification, ammonium assimilation, nitrification, and denitrification in marine sediment as well as in water column. ¹⁵N nitrate isotope dilution was successfully applied to show the coupling between sedimentary nitrification and denitrification, which occurs efficiently in the aerobic-anaerobic micro-sites in the vicinity of organic detritus deposition. In global scale, the amount of denitrified nitrogen in coastal sediments estimated by ¹⁵N method is comparable to that in the whole water column, indicating its global importance as the oceanic sink of available nitrogen. Microbial populations in the upper ocean also play a significant role on nitrogen dynanics including ammonium regeneration and its assimilation. In Antarctic coastal waters of spring bloom condition, bacterial net production of ammonium was observed, although they also assimilated ammonium simultaneously. High concentration of amino acids possibly supplied from spring bloom would support this bacterial metabolism. At the later part, I describe our recent finding of non-living organic particles of sub-micron size range (0.38∼1.0 μm) in the ocean, abundance of which is in the order of 10 million per ml at the surface layer. Since the abundance of those particles is positively correlated with chl a and other biological parameters, we assume its biological origin. Laboratory experiments and field observations show several biological processes of their production including grazing of micro flagellates on bacteria, lysis of bacteria by viral infection and exudates of bloomphytoplankton. Also, flexible and fragile natures of those particles suggest that they are marine snow type organic aggregates of submicron seze. This finding fulfills the previous gap between dissolbed and particulate organic materials in the ocean, and suggests an importance of "detritus ladder" in marine ecosystem.