Iron availability limits well-being of fish in open oceans?

Maanantai 16.9.2019 klo 11.46 - Mikko Nikinmaa

Iron is essential for production of haemoglobin and many enzymes (e.g. cytochromes, which include most detoxification enzymes). It is known that iron availability is very low in some parts of the open ocean, notably the Southern Seas, areas around Antarctica. It is also known that iron availability limits phytoplankton growth in the oceans. A recent article in Frontiers in Marine Science (Galbraith et al (2019) Growth Limitation of Marine Fish by Low Iron Availability in the Open Ocean. Frontiers Mar Sci 6:509) argues quite convincingly that this may be the case also for fish. One of the good points made by the authors is that the icefish without haemoglobin have only evolved in the iron-poor waters of Antarctica, not in the much iron-richer Arctic waters, although the temperatures in both are the same, and the low temperature is usually used as the reason for the possibility to the evolution of icefish devoid of haemoglobin. If the evolution has been just a random event, the question remains: why has not this random event taken place in the Arctic? Now, because the low temperature allows it, the haemoglobinless icefish benefits immensely from not having to produce haemoglobin. Compared to fish with haemoglobin, its iron requirements are about 20 times smaller, enabling it to live comfortably in iron-poor environment.

If iron availability limits the well-being of fish, it also causes problems with pollutants requiring cytochrome-dependent detoxification. Usually the detoxification system produces more cytochromes upon toxic insults. However, this may be difficult or impossible for species living in iron-poor areas. If this is the case, pollution of the Southern Seas may be a much more serious for fish inhabiting the area than, e.g. for fish living in North Atlantic.