I've been developing a planet for an original scifi setting and I'm not sure if it's scientifically plausible. Said planet is in a state of global algae infestation across all major oceans, resulting from unchecked industrial activity several million years before the present. The few surviving fauna were either hardy extremophiles or formed a symbiotic relationship with the algae, such as undergoing kleptoplasty. These organisms became the dominant lifeforms, speciating into a relatively diverse ecosystem, albeit not as complex as the previous generation. The ecological shift was long enough ago for a moderately intelligent animal species, comparable to corvids or chimpanzees, to evolve into a sapient one with at least a neolithic level of technology.
My main concern is whether or not this infested state would last long enough for this to occur. Algae blooms on Earth typically only last a few weeks before the algae runs out of sustenance, suggesting a relatively quick recovery. I previously considered increased volcanic activity, another byproduct of global warming, as a new source of greenhouse gases and nitrates. However, I'm not sure if this would be enough to perpetuate the bloom for millions of years.
What mechanism would allow a planet to remain in an algae-infested state for an extended period?
Background info/criteria:
- The planet's physical characteristics differ from Earth, being twice as massive with a 25% larger radius. Its surface gravity is about 28% higher, resulting in flatter terrain and overall smaller landmasses due to heavy erosion. Its rotational period is 43.125 hours, with an axial tilt of 6.2°. It is tidally locked to a mars-sized companion, creating a permanent 800m tidal bulge. As a result, its landmasses are concentrated on opposite hemispheres perpendicular to the companion planet.
- The main causes of the planet's current state include: unchecked greenhouse gas emissions, namely CO2 and methane; rampant fracking and mineral harvest, deep enough into the planet's crust to cause mild instability and increase volcanic activity; excessive use of fertilizer and dumping organic waste, such as sewage.
- Currently, the planet's average surface temperature is about 25°C, 10 degrees hotter than it was before the collapse. There is no surface ice due to a lack of major polar landmasses with high mountains. Tropical regions exceed 30°C, consisting of scorched, barren landmasses and desert archipelagoes. The algal bloom encompasses all bodies of water, including major oceans and smaller inland bodies such as lakes and rivers. The current generation of fauna descended from the few hardy organisms which survived the collapse and mass-deoxygenation. Many of them are kleptoplastic, absorbing the chloroplasts from the algae to undergo photosynthesis and produce their own oxygen. Others are simply adapted to anoxic conditions, including abyssal sea life.
- This state should last at least 10 million years, and the mechanism perpetuating it can't be actively sustained by sapients. The planet's state is the result of sapient actions, but they went extinct shortly after the collapse. Whether or not the current sapients can perpetuate the bloom is irrelevant since they've only recently become intellient enough to do so.
