The discovery of tens of billions of barrels of oil in fields far off the coast of Rio de Janeiro was billed as one of the biggest finds of this century when it was announced in 2006.

Many hoped it would deliver a bonanza for education and health and make Brazil one of world’s major economies.

But with the country’s biggest energy company, Petrobras, mired in debt and scandal, the low price of oil and the dangers of a second Deepwater Horizon, the viability of this massive undertaking has never been under more scrutiny.

On the surface, this 412 sq km body of water dotted with more than 100 islands is picture-postcard Brazil, a symbol of the country’s immense natural wealth and beauty and a backdrop for Rio’s world heritage tourist sites.

But oil-fuelled development is transforming this iconic landscape into a petrochemical and service centre for the oil and gas fields more than 200km offshore in the Campos and Santos basins.

While this has delighted local politicians and businesses, Alexandre Anderson and his fellow fishermen say their territory, the bay environment and the aquatic food supply are the victims.

Alexandre, the leader of a traditional community in Rio’s Guanabara Bay, has devoted his life to defending local fisheries against what he sees as the encroachment of the oil industry.

Challenging the petro-chemical industry in Brazil is dangerous.

Alexandre has been shot twice, arrested 12 times and had 28 lawsuits filed against him. Several members of his fishing union were murdered during a pipeline protest a few years ago. Petrobras denies any connection with the deaths, but Alexandre says his friends were killed by guards and militias linked to the company.

He’s now under a federal protection programme, but he still patrols the bay in search of oil spills, illegal construction and other environmental crimes.

“Guanabara Bay is a sacred place for us. We respect it like a church,” he says. “But the bay is becoming an industrial hub in a disgusting and criminal way.”

Few Cariocas, as Rio’s residents are known, are aware of the extent of Petrobras’ sprawl across the land and water.

It started in 1961 and has accelerated dramatically since the discovery of the pre-salt fields. To demonstrate, Alexandre takes The Guardian on patrol. Starting from his quiet fishing village in Magé, we first pass by a propene and ethylene terminal, which began operating in 2008, then the Iesa shipyard where several oil tankers are being refitted.

Further along the shore, Alexandre points out the vast Duque de Caxias refinery, which stretches across the horizon, belching murky smoke from six chimneys, then a liquified natural gas terminal and a smaller refinery on Redonda Island, which is undergoing expansion.

Near Rio-Niterói bridge, we spy the famous tourist sites - Sugar Loaf Mountain and the Christ the Redeemer statue - but closer to the shore are storage tanks, huge warehouses, factories and shipyards - almost all of which are geared towards the oil industry: General Electric pipes and cables, a huge drilling platform waiting for a refit, a cluster of shiny-new mini refineries waiting to be loaded on to floating platforms by an enormous crane painted in the canary yellow and green colours of the national flag, the national team and the national oil company.

Guanabara, it seems, has become Petrobras Bay.

Altogether, there are at least two refineries, four terminals, four shipyards, as well as countless storage tanks, support ships, service factories and underwater pipelines.

The waters - now deemed too dirty to swim in - are like the forecourt of a giant petrol station.

Instead of cars lining up at the pumps, there are 30 huge tankers waiting to load or unload oil and gas at the terminals. A huge new refinery will soon be added to the list. Petrobras’s Comperj plant is now under construction close to the northeast - and currently least spoiled - corner of the bay. It will be the destination for gas from the pre-salt fields once a new pipeline is completed later this year.

Alexandre vows to fight this threat.

“Once Comperj is finished, the impact of Petrobras will spread right across the bay. We hope that refinery won’t be viable because of the low price of oil, because of the struggle by people like us, and because of the growing clamour in society about the environment,” he says.