Towards the end of the second video, they are trying to use a refrigeratormagnet to attract the cells.
Unlike the other magnets, refrigerator magnets are made by creating thin
stripes of alternating north and south poles. This allows the relatively weak
magnets (ferrite in a rubber matrix) to stick to the fridge. Unless the poles
are close together, the rubber magnets can't generate enough force to stick.
You can confirm this by rubbing the north pole of a strong NdFeB magnet
all around one side of the rubber magnet. That will make that side south,
and the other side north. It will no longer stick to the fridge.
The bacteria swim along the field lines. The magnetosomes are not actualy
being pulled by the magnet to drag the bugs around, the bugs just use them
as a compass to tell up from down (they don't care about north and south).
In higher latitudes, the Earth's magnetic field points down, and at the magnetic
pole, it points straight down. I would expect to find fewer magnetotactic
bacteria at the equator than near the magnetic poles, as the expense of
making the magnetosomes is only worthwhile if they can be used to find the
oxic-anoxic boundary (i.e. moving up and down is important, north and south
is not).
I would also expect bugs from south of the equator to swim in the opposite
direction from those north of the equator. One could use this as a test to see
if the bugs learn which way to go, or whether it is innate in their genes.
So with the fridge magnet, whose poles are less than a millimeter apart, you
won't see a lot of movement, since the bugs just move from one pole to the
other.
The Earth's magnetic field is very weak, so the strength of the magnet should
not matter much. You could try the modified rubber magnet, and it might work
as well as the strong NdFeB magnet. Some NdFeB magnets might be strong enough
to actually drag the bacteria around by their magnetosomes, as opposed to making
them swim in a particular direction. You could test this by reversing the magnet.
Swimmers should swim away. Dragged bugs would still be attracted.