Program

The ocean absorbs approximately 30 % of anthropogenic CO₂ (IPCC, 2013), and plays a significant role in global changes. Increase in the partial pressure of CO₂ (pCO₂) results in bicarbonate ion and hydrogen ion concentrations, and a decrease in carbonate ion concentration, which is the known "ocean acidification (OA)". However, in eutrophic waters, microbial processes, such as respiration, turned to be more important in OA, which is associated with hypoxia as well as carbon sequestration through the biological pump (BP) and the microbial carbon pump (MCP). These issues should therefore be taken together for integrated study and management. OA inhibits phytoplankton calcification which would decrease the ballast effect of calcium carbonate, decreasing the vertical transport of particulate organic carbon (POC) through the BP. Shifts in phytoplankton community structure have occurred under higher pCO₂ that could impact the composition and bioavailability of the dissolved organic carbon (DOC) produced (related to the MCP). Although there is no clear impact of OA on bacterial abundance, gross- and cell-specific bacterial production are usually stimulated by high pCO₂, and may reduce POC flux through the BP but enhance the efficiency of the MCP, as higher pCO₂ condition enhances DOC: POC production ratio (Kim et al. (2011). Generally, an increase in nutrient supply is expected to lead to an increase in primary production, POC, and consequently an increase of the BP. However, high nutrient concentrations could have a negative impact on the MCP, as primary production, bacterial respiration and bacterial growth efficiency  respond differently to enhanced nutrients (Jiao et al., 2014). Once phytoplankton populations increase with increasing nutrients to the maximum, light shading effect becomes important and primary production starts to decrease, while heterotrophic bacterial growth keeps increasing as DOC is in adequate supply. During this process, oxygen consumption due to respiration can eventually lead to hypoxia. A MCP mediated by sulfate-reducing or iron-reducing microbes under hypoxic or anoxic conditions may have facilitated the accumulation of authigenic carbonate derived from DOC in sediments or bottom water, which may have played an important role in the global carbon budget through Earth’s history.