Amazing story, Sue! I had no doubts about how your scientific approach would translate into powerful horror narrative, and I was not mistaken! The algae, absent any form of consciousness or volition, ultimately become a force of nature, one before which we puny humans are completely helpless. Lovecraftian.

The one line I have quarrels about is your scene transition after paragraph 17. By using “A bit later” to convey the passing of time, you created an abrupt stop that hindered the pacing of an otherwise fluent and dynamic narrative. Had you noted that time difference with a milder conjunctive construction (such as adding “After analyzing Mrs. Meyer’s blood” or something along those lines to the start of the following parragraph), the cadence of your story would have remained untainted.

Despite that minor hiccup, your text was truly a pleasure to read.

Best of lucks in the contest!

All best,
Jean