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Nitrogen enters the ocean through precipitation, runoff, or as N 2 from the atmosphere. Nitrogen cannot be utilized by phytoplankton as N 2 so it must undergo nitrogen fixation which is performed predominantly by cyanobacteria. [82] Without supplies of fixed nitrogen entering the marine cycle, the fixed nitrogen would be used up in about 2000 ...
Open ocean areas with no oxygen have grown more than 1.7 million square miles in the last 50 years, and coastal waters have seen a tenfold increase in low-oxygen areas in the same time. [34] Measurement of dissolved oxygen in coastal and open ocean waters for the past 50 years has revealed a marked decline in oxygen content.
In general, nitrogen tends to be a limiting ocean nutrient, but in HNLC regions it is never significantly depleted. [1] [2] Instead, these regions tend to be limited by low concentrations of metabolizable iron. [1] Iron is a critical phytoplankton micronutrient necessary for enzyme catalysis and electron transport. [3] [4]
Whales feed at deeper levels in the ocean where krill is found, but return regularly to the surface to breathe. There whales defecate a liquid rich in nitrogen and iron. Instead of sinking, the liquid stays at the surface where phytoplankton consume it. In the Gulf of Maine, the whale pump provides more nitrogen than the rivers.
Another important class of iron–sulfur proteins is the ferredoxins, which have multiple iron atoms. Transferrin does not belong to either of these classes. [5] The ability of sea mussels to maintain their grip on rocks in the ocean is facilitated by their use of organometallic iron-based bonds in their protein-rich cuticles.
Another cause of concern is the sheer amount of urea needed to capture the same amount of carbon as eq. iron fertilization. The nitrogen to iron ratio in a typical algae cell is 16:0.0001, meaning that for every iron atom added to the ocean a substantial larger amount of carbon is captured compared to adding one atom of nitrogen. [35]
The iron oxides were denser than water and fell to the ocean floor forming banded iron formations (BIF). [19] Over time, rising oxygen levels removed increasing amounts of iron from the ocean. BIFs have been a key source of iron ore in modern times. [20] [21]
Ocean iron fertilization is an example of a geoengineering technique that involves intentional introduction of iron-rich deposits into oceans, and is aimed to enhance biological productivity of organisms in ocean waters in order to increase carbon dioxide (CO 2) uptake from the atmosphere, possibly resulting in mitigating its global warming effects.