Spatial variability in iron nutritional status of large diatoms in the Sea of Okhotsk with special reference to the Amur River discharge

[摘要] The Sea of Okhotsk is known as one of the most biologically productiveregions among the world's oceans, and its productivity is supported in partby the discharge of iron (Fe)-rich water from the Amur River. However, littleis known about the effect of riverine-derived Fe input on the physiology ofthe large diatoms which often flourish in surface waters of the productivecontinental shelf region. We conducted diatom-specific immunochemicalferredoxin (Fd) and flavodoxin (Fld) assays in order to investigate thespatial variability of Fe nutritional status in the microplankton-sized(20–200 μm; hereafter micro-sized) diatoms. The Fd index, definedas the proportion of Fd to the sum of Fd plus Fld accumulations in the cells,was used to assess their Fe nutritional status. Additionally, activechlorophyll fluorescence measurements using pulse–amplitude-modulated (PAM)fluorometry were carried out to obtain the maximum photochemical quantumefficiency (F_v/F_m) of photosystem II for thetotal micro-sized phytoplankton assemblages including diatoms. During ourobservations in the summer of 2006, the micro-sized diatoms were relativelyabundant (> 10μg C L⁻¹) in the neritic region, andformed a massive bloom in Sakhalin Bay near the mouth of the Amur River.Values of the Fd index and F_v/F_m were high(>0.9 and >0.65, respectively) near the river mouth, indicating thatFe was sufficient for growth of the diatoms. However, in oceanic waters ofthe Sea of Okhotsk, the diatom Fd index declined as cellular Fld accumulationincreased. These results suggest that there was a distinct gradient in Fenutritional status in the micro-sized diatoms from near the Amur River mouthto open waters in the Sea of Okhotsk. A significant correlation betweendissolved Fe (D-Fe) concentration and the Fd index was found in waters offSakhalin Island, indicating that D-Fe was a key factor for thephotophysiology of this diatom size class. In the vicinity of the KurilIslands between the Sea of Okhotsk and the Pacific Ocean, micro-sized diatomsonly accumulated Fld (i.e., Fd index = 0), despite strong vertical mixingconsistent with elevated surface D-Fe levels (>0.4 nM). Since higher Fequotas are generally required for diatoms growing under low-light conditions,the micro-sized diatoms off the Kuril Islands possibly encountered Fe andlight co-limitations. The differential expressions of Fd and Fld inmicro-sized diatoms helped us to understand how these organisms respond to Feavailability in the Sea of Okhotsk in connection with the Amur Riverdischarge.

[学科分类] 地球化学与岩石