Are mitochondria needed for eukaryotes?

There has been a lot of articles the later years that conclude that eukaryotes need mitochondria to produce enough energy. The need for energy has been related to the number of genes. It is especially Nick Lane that has claimed that genes depend on membrane area. I have earlier shown that there is no such dependence for eukaryotes. It is true that under oxic conditions membranes are essential for energy production. But it also holds when there are other electron acceptors, e.g. iron ions present. Membranes are also useful when hydrogen can be used as an energy source, for reducing carbon dioxide. But when no such external electron acceptor or electron donor is available, then there is no need for membrane area for metabolism. Metabolism is then dependent of available volume for the enzymes, not membrane area. And there is no limitation to the size of cells, so under anaerobic conditions, provided there are metabolites available, eukaryote cells can be large if that is practical of other reasons.

It is correct that the mitochondrion is a key to producing much energy from each molecule of consumed glucose. But the membraneous structures that produce energy in mitochondria are voluminous, so there is not much difference between the energy yield per volume in anaerobic and aerobic organisms. In fact, the membranous processes have limitations, even in human muscle cells. They are optimized for aerobic respiration, but when we are lifting heavy weights, then our muscle cells will use anaerobic fermentation, producing lactic acid.

Fermentation gives higher effect, due to the full utilization of the cell volume, while respiration gives more energy per volume of fuel. So the difference is more about how much fuel is needed to drive the cells. It should also be added that the aerobic cells need a continuous import of oxygen, which is in itself a limitation factor.

There are in fact a lot of anaerobic eukaryotes even today, although the access to such conditions is limited. Some of these have other organelles or no organelles at all. The hypothesis is that all eukaryotes once had mitochondria. But if they were the key to supporting an eukaryotic cell organization, how could they then be lost?

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