  Nanotubular molecular self-aggregates are characterized by a high degree of
symmetry and they are fundamental systems for light-harvesting and energy
transport. While coherent effects are thought to be at the basis of their high
efficiency, the relationship between structure, coherence and functionality is
still an open problem. We analyze natural nanotubes present in Green Sulfur
Bacteria. We show that they have the ability to support macroscopic coherent
states, i.e. delocalized excitonic states coherently spread over many
molecules, even at room temperature.
  Specifically, assuming a canonical thermal state, in natural structures we
find a large thermal coherence length, of the order of 1000 molecules. By
comparing natural structure with other mathematical models, we show that this
macroscopic coherence cannot be explained either by the magnitude of the
nearest-neighbour coupling between the molecules, which would induce a thermal
coherence length of the order of 10 molecules, or by the presence of long-range
interactions between the molecules. Indeed we prove that the existence of
macroscopic coherent states is an emergent property of such structures due to
the interplay between geometry and cooperativity (superradiance and
super-transfer). In order to prove this, we give evidence that the lowest part
of the spectrum of natural systems is determined by a cooperatively enhanced
coupling (super-transfer) between the eigenstates of modular sub-units of the
whole structure. Due to this enhanced coupling strength, the density of states
is lowered close to the ground state, thus boosting the thermal coherence
length.
