The sinking rate and transparent exopolymer particle (TEP) production of Hemiaulus hauckii
Diatom-diazotroph associations contribute extensively to vertical flux in the modern
ocean, and are linked to extensive sedimentary layers in the paleo-record formed either
by rapidly sinking aggregates or more slowly sinking cells and chains. Although diatom
sinking rates have been extensively studied, the sinking rates of these diatom symbioses
are poorly documented. The effect of silicate and phosphate limitation on the sinking
rate of Hemiaulus hauckii and its symbiont Richelia intracellularis were determined in
semi-continuous cultures using a SETCOL. TEP (transparent exopolymer particle) was
measured concurrently for linkages to aggregate formation. Although cells occurred in
chains of 1-20 cells long, aggregate formation did not occur in semi-continuous cultures.
There was no significant effect of P or Si limitation on sinking rates compared to controls. Although H. hauckii possess a symbiont that lacks gas vesicles, sinking rates (0.04-0.72 m/day) were within the range predicted by size-rate relationships. A portion of each H. hauckii experimental measurement ascended as well with significant differences found between the lowered P 80% exchange and the lowered Si with 80% exchange (p=0.05) and between the lowered Si with 80% exchange and the Si control batch culture (p=0.039). TEP production did not change significantly between treatments. However, a fraction of TEP in each population was positively buoyant as well, with ascent rates between 0.32-0.49 m/day. This study confirms an earlier report of positive buoyancy in TEP as well as noting that a significant fraction of the Hemiaulus symbiosis can be ascending within the population.