Effects of CO2 and iron availability on rbcL gene expression in Bering Sea diatoms

Iron (Fe) can limit phytoplankton productivity in approximately 40% of the global ocean, including in high-nutrient, low-chlorophyll (HNLC) waters. However, there is little information available on the impact of CO2-induced seawater acidification on natural phytoplankton assemblages in HNLC regions. We therefore conducted an on-deck experiment manipulating CO2 and Fe using Fe-deficient Bering Sea water during the summer of 2009. The concentrations of CO2 in the incubation bottles were set at 380 and 600 ppm in the non-Fe-added (control) bottles and 180, 380, 600, and 1000 ppm in the Fe-added bottles. The phytoplankton assemblages were primarily composed of diatoms followed by haptophytes in all incubation bottles as estimated by pigment signatures throughout the 5-day (control) or 6-day (Fe-added treatment) incubation period. At the end of incubation, the relative contribution of diatoms to chlorophyll a biomass was significantly higher in the 380 ppm CO2 treatment than in the 600 ppm treatment in the controls, whereas minimal changes were found in the Fe-added treatments. These results indicate that, under Fe-deficient conditions, the growth of diatoms could be negatively affected by the increase in CO2 availability. To further support this finding, we estimated the expression and phylogeny of rbcL (which encodes the large subunit of RuBisCO) mRNA in diatoms by quantitative reverse transcription polymerase chain reaction (PCR) and clone library techniques, respectively. Interestingly, regardless of Fe availability, the transcript abundance of rbcL decreased in the high CO2 treatments (600 and 1000 ppm). The present study suggests that the projected future increase in seawater pCO2 could reduce the RuBisCO transcription of diatoms, resulting in a decrease in primary productivity and a shift in the food web structure of the Bering Sea.

全球约40%的海洋海域（包括高营养盐低叶绿素（HNLC）海域）中，铁（Fe）可限制浮游植物生产力。然而，目前针对二氧化碳诱导的海水酸化对HNLC海域自然浮游植物群落影响的研究数据仍较为匮乏。因此，我们于2009年夏季开展了甲板原位实验，以缺铁的白令海水为实验材料，对二氧化碳与铁浓度进行调控。培养瓶的二氧化碳浓度设置如下：未添加铁的对照组培养瓶为380 ppm与600 ppm，添加铁的实验组培养瓶为180、380、600及1000 ppm。通过整个培养周期（对照组为5天，添加铁的实验组为6天）的色素标记物分析可知，所有培养瓶中的浮游植物群落均以硅藻为主，其次为定鞭金藻。培养结束时，对照组中380 ppm二氧化碳处理组的硅藻对叶绿素a生物量的相对占比显著高于600 ppm处理组，而添加铁的实验组未观察到显著变化。上述结果表明，在缺铁条件下，二氧化碳可利用性升高会对硅藻的生长产生负面影响。为进一步佐证该发现，我们分别通过定量反转录聚合酶链式反应（PCR）与克隆文库技术，对硅藻中编码核酮糖-1,5-二磷酸羧化酶/加氧酶（RuBisCO）大亚基的rbcL基因的表达情况与系统发育进行了分析。有趣的是，无论铁供应状况如何，高二氧化碳处理组（600与1000 ppm）的rbcL转录本丰度均出现下降。本研究表明，未来预测的海水分压二氧化碳升高可能会降低硅藻的RuBisCO转录水平，进而导致白令海的初级生产力下降，并引发其食物网结构发生改变。