Hi Newtonian: There is likely a layer of the upper Venus atmosphere where your flying algae would find the temperature optimum. The sulpheric acid clouds are at somewhat lower altiudes, so the acid is not a problem for the algae. The algae could be supplyed water by crashing small comets near the Equator of Venus. This would also supply the trace minerals that the algae need. If the algae used one percent of the solar energy that falls on the upper atmosphere of Venus to covert carbon dioxide to free oxygen, the carbon dioxide would fall from 90% to perhaps 0.04% in 60,000 years. That is a long time to send a weekly comet and seed with algae perhaps monthly, but there is a fair chance of success, if we do a few other things. The prevailing winds in the Venus upper atmosphere are toward the poles, so the algae will use considerable solar energy to avoid being sucked into the polar regions, where there is not enough light for efficent photosynthesis because it shaded by the cloud of flying algae closer to the equator. The weakend algae would then be sucked down the polar downdraft. The air is compressed as it decends. Compressing air makes the air hot, coverting the algae to algae charcoal = very fine carbon dust. We need to keep the polar down draft from wandering and put up multiple 1000 mile snow fences, to keep the surface wind from blowing the algae charcoal toward the equator. We will crash a weekly small iron asteroid at each pole, beginning pehaps the end of the first century. The iron dust will mix with the algae charcoal, to help hold it in place and to absorb most of the free oxygen, otherwise the dunes of algae charcoal will catch fire and return carbon dioxide to the atmosphere. Iron dust will rust at less than 1% oxygen, if water vapor is present. We may have to provide extra comets to keep the polar humidity high enough to rust the iron dust. Unfortunately sulpheric acid really likes water, so some our comet water will dilute sulpheric acid. After abou