Beneficial :
Algae play an important part in the ecosystem as they being a producer in the food chain.
They contain pigments for photosynthesis and hence about a quarter of CO2 in the atmosphere were uptake by them.
Nowadays bio-fuel products were produced from algae such as Vegetable oil, biogasoline, biomethanol, biodiesel, bio-ethanol, bio-butanol
Those sediments of phytoplankton, diatom as the sea floor since millions years ago could be a source of petroleum, and natural gas that we used nowadays