Aggregation data were collected using a
filtration aggregation 96-well assay. This assay was developed to allow
testing of multiple parameters with replicates in a 96-well plate. Filtration
rigs were built by sandwiching a 50-m mesh between two pipette tip
racks (Tip One; USA Scientific) and sealing this mesh in place with silicon.
The resultant apparatus was designated a “filtration rig.” The filtration rig
fit neatly in a 96-well plate and retained aggregates on the mesh, while
nonaggregated algal cells (diameter, 2 to 3 m) passed through into the
wells of the microtiter plate. In a typical aggregation experiment, 150 l of
algae at 1 107 cells/ml was pipetted into a round-bottom 96-well plate.
A 100-fold-concentrated bacterial suspension (1.5 l; original density,
1107 cells/ml) was added to the algae in the designated wells to give a 1:1
alga-to-bacterium ratio. Wells not receiving bacteria served as negative
controls for aggregation induced by the addition of bacteria. The plate was
sealed with Parafilm, and the contents were mixed by vortexing for 1 min.
Afiltration rig was placed on a round-bottom 96-well plate, and the 150l
containing the aggregates was pipetted from the first microtiter plate into
the top of the filtration rig using wide-bore pipette tips. The plate and rig
were centrifuged at 100g for 10 s to ensure that the entire volume in the
rig passed through the mesh into the second, lower 96-well plate. At this
point, the aggregates were retained on the mesh and the nonaggregated
cells had passed into the second plate. The algae in the second plate were
suspended and diluted into the linear range for chlorophyll fluorescence
measurement in a Spectromax M5 microplate reader. Chlorophyll fluorescence
was measured with an excitation wavelength of 488 nm, a 515-
nm-cutoff filter, and an emission wavelength of 685 nm. From these data,
the percentage of algae that had aggregated and remained on the mesh was
calculated from the fluorescence of the algae that were not aggregated
using the formula P[1(A/N)] · 100, where P is the percentage of algae
aggregated, A is the fluorescence of the filtrate from the aggregated sample,
and N is the fluorescence of the filtrate from the nonaggregated
sample.