Need energy to convert the algae into anything, need good yields, and then if you burn it’s just back into the air as co2. There is a reason that all the big investors in algae to biofuels have given up, they don’t produce enough fatty oils per gram to make it cost effective. It could be work and be useful but it’s still really far from being productive enough to continue developing the technology. It’s just tough to engineer really old organisms to do things they don’t do naturally at the levels we need to be productive. Modern efforts are moving to electric vehicles and away from combustion. The future is boats and planes burning fuel and cars and trucks running electric.