Ocean acidification primarily impacts the nitrogen cycle by hindering crucial processes like nitrogen fixation, particularly affecting key organisms responsible for converting atmospheric nitrogen into usable forms.
The nitrogen cycle is a fundamental biogeochemical process essential for life in the ocean. It involves the conversion of nitrogen between different forms. One critical step is nitrogen fixation, where nitrogen gas (N₂) from the atmosphere is converted into ammonia (NH₃), which can then be used by marine organisms. This process is largely carried out by certain types of bacteria and cyanobacteria, like Trichodesmium.
Key Impacts of Ocean Acidification
Based on research, ocean acidification has specific negative effects on nitrogen fixation, with significant consequences for marine ecosystems.
- Reduced Iron Uptake: Ocean acidification decreased both Fe uptake by Trichodesmium. Iron is a vital micronutrient for nitrogen-fixing organisms because it is a component of the enzymes required for this process. Lower pH conditions can alter the availability or the organism's ability to acquire iron.
- Decreased Nitrogen Fixation Efficiency: Acidification also decreased the efficiency of nitrogen fixation by Trichodesmium. This means that even if some iron is available, the process of converting N₂ to ammonia becomes less effective under more acidic conditions.
Synergistic Effects and Consequences
The combination of reduced iron uptake and decreased fixation efficiency creates a synergistic effect that compounds the problem:
- Decreased Input of "New" Nitrogen: These two effects may act synergistically to decrease the input of “new” nitrogen into the ocean. "New" nitrogen refers to nitrogen added to the ecosystem from external sources, like atmospheric N₂ fixation. This is crucial for supporting biological production.
- Limiting Primary Production: Nitrogen is often a limiting nutrient for primary production (the creation of organic matter by organisms like phytoplankton) in the open ocean. A reduction in the input of "new" nitrogen due to impaired nitrogen fixation can therefore limit the growth of phytoplankton, impacting the entire food web that relies on them.
In summary, ocean acidification disrupts a critical entry point of nitrogen into the marine ecosystem by impairing the ability of key microbes like Trichodesmium to fix atmospheric nitrogen. This reduced nitrogen availability can then constrain the productivity of the open ocean.
