Tuna/Microbes

Question

Dear Rabbi Abadi, 1. I once heard a rabbi say that tuna requires a hechsher b/c: a number of species of fish are sold as "tuna" (which he said is not really a species in and of itself), several of these species are not kosher fish, and the mashgichim pay the men on the boat x dollars for each non-kosher fish they spot, which are then discarded. Is this true? Is this a problem? 2. As I understand it, microbes, like bacteria, live by the tens of thousands on most food and actually all over the place. They are considered life (they are capable of replicating themselves by dividing) by biologists, and they are not considered animals or plants. My questions are: (i) are they kosher or are they the creepy crawlers the Torah prohibits. (ii) if kosher, what about algae, e.g. spirulina, found in supplements. (iii) even if kosher, when one walks or moves on shabbes one is likely killing several each time. Are they even "life" the killing of which would be problematic? I don't mean to sound like I am from outer space, but these questions seem usually to go under the radar for a variety of reasons. Thank you,