Seawater carbonate chemistry and growth response in a diatom dominated phytoplankton community from SW Bay of Bengal coastal water

The ongoing increase in surface seawater CO2 level could potentially impact phytoplankton primary production in coastal waters; however, CO2 sensitivity studies on tropical coastal phytoplankton assemblages are rare. The present study investigated the interactive impacts of variable CO2 level, light and zinc addition on the diatom dominated phytoplankton assemblages from the western coastal Bay of Bengal. Increased CO2supply enhanced particulate organic matter (POC) production; a concomitant depletion in δ13CPOM values at elevated CO2 suggested increased CO2 diffusive influx inside the cell. Trace amount of Zn added under low CO2 level accelerated growth probably by accelerating Zn-Carbonic Anhydrase activity which helps in converting bicarbonate ion to CO2. Almost identical values of delta 13CPOM in the low CO2 treated cells grown with and without Zn indicated a low discrimination between 13C and 12C probably due to bicarbonate uptake. These evidences collectively indicated the existence of the carbon concentration mechanisms (CCMs) at low CO2. A minimum growth rate was observed at low CO2 and light limited condition indicating light dependence of CCMs activity. Upon the increase of light and CO2 level, growth response was maximum. The cells grown in the low CO2 levels showed higher light stress (higher values of both diatoxanthin index and the ratio of photo-protective to light-harvesting pigments) that was alleviated by both increasing CO2 supply and Zn addition (probably by efficient light energy utilization in presence of adequate CO2). This is likely that the diatom dominated phytoplankton communities benefited from the increasing CO2 supply and thus may enhance primary production in response to any further increase in coastal water CO2 levels and can have large biogeochemical consequences in the study area.

表层海水二氧化碳（CO₂）水平的持续升高，可能对近岸海域浮游植物的初级生产造成潜在影响；然而，针对热带近岸浮游植物群落的CO₂敏感性研究仍较为匮乏。本研究以孟加拉湾西部近岸海域以硅藻为优势类群的浮游植物群落为研究对象，探究了可变CO₂水平、光照与锌添加三者的交互调控效应。CO₂供给的提升可促进颗粒有机物（POC, Particulate Organic Matter）的生成；升高CO₂条件下伴随的δ¹³C_POM值降低，表明细胞内CO₂的扩散摄入量有所增加。在低CO₂水平下添加微量锌可加速藻类生长，这可能是通过提升锌-碳酸酐酶（Zn-Carbonic Anhydrase）活性实现的——该酶可将碳酸氢根离子转化为CO₂。无论是否添加锌，低CO₂处理组细胞的δ¹³C_POM值几乎一致，这说明其对¹³C与¹²C的分辨能力较低，该现象可能源于群落以碳酸氢根作为主要碳源摄取途径。上述证据共同表明，低CO₂条件下存在碳浓缩机制（CCMs, Carbon Concentration Mechanisms）。在低CO₂且光照受限的条件下，藻类生长速率最低，这证实碳浓缩机制的活性依赖于光照强度。随着光照与CO₂水平的提升，藻类生长响应达到峰值。低CO₂环境中培养的细胞表现出更强的光胁迫（硅藻黄素指数与光保护色素-捕光色素比值均更高），而提升CO₂供给或添加锌均可缓解这一胁迫——这可能得益于充足CO₂条件下光能利用效率的优化。由此推测，以硅藻为优势类群的浮游植物群落将从CO₂水平升高中获益，因此近岸海水CO₂的进一步升高或可促进区域初级生产，并对研究区域产生显著的生物地球化学影响。