The arrowheads indicate electron-dense structures that ocur in the cytoplasm and preferentially associate with the inner face of the cytoplasmic membrane.
The nature of the structures is unknown, but we hypothesisze that these may be carboxysome-like structures.Polyhedral structures believed to be carboxysomes shave been observed in Nitrosomonas sp. (Wullenweber et al. 1977), but have not been reported in N. europaea.
The structures in N. europaea lack the polyhedral morphology typical of carboxysomes.
Also, these structures are prevelant only in growing cells, suggesting that their formation is growth-regulated.