  A simple model to study cooperation is the two-species symbiotic contact
process (2SCP), in which two different species spread on a graph and interact
by a reduced death rate if both occupy the same vertex, representing a
symbiotic interaction. The 2SCP is known to exhibit a complex behavior with a
rich phase diagram, including continuous and discontinuous transitions between
the active phase and extinction. In this work, we advance the understanding of
the phase transition of the 2SCP on uncorrelated networks by developing a
heterogeneous mean-field (HMF) theory, in which the heterogeneity of contacts
is explicitly reckoned. The HMF theory for networks with power-law degree
distributions shows that the region of bistability (active and inactive phases)
in the phase diagram shrinks as the heterogeneity level is increased by
reducing the degree exponent. Finite-size analysis reveals a complex behavior
where a pseudo discontinuous transition at a finite-size can be converted into
a continuous one in the thermodynamic limit, depending on degree exponent and
symbiotic coupling. The theoretical results are supported by extensive
numerical simulations.
