Microbial production along the West Florida Shelf: Responses of bacteria and viruses to the presence and phase of Karenia brevis blooms

Bacterial abundance, production, protein and nucleic acid synthesis, growth, and viral abundance were measured in waters associated with three bloom stages of the “red tide” dinoflagellate Karenia brevis along the south West Florida Shelf (WFS). Measurements were taken: (1) when no bloom was present; (2) during the initiation stage of a bloom; and (3) during the maintenance stage of a bloom. Results indicate that the bacterial community was nutrient limited in the non-bloom period, with highest abundance and production rates occurring near and within estuaries. Abundance of virus like particles (VLPs) was higher within estuaries, but we hypothesize VLPs were not a high source of bacterial mortality, possibly due to high decay rates due to UV degradation or extracellular nucleases. High bacterial production, balanced protein to nucleic acid synthesis, and statistically similar bacteria abundances measured on consecutive days within the initiating bloom suggest a highly productive community with equally high mortality. VLP abundance declined during the first 48 h within both bloom stages, suggesting that viral genomes were either within host cells (not evident in water column samples), or bacterial mortality was due to mixotrophic grazing by K. brevis. Using a conservative grazing rate of 1 bacteria K. brevis−1 h−1, K. brevis grazing could account for >100% of bacterial mortality during an initiating bloom. Bacterial abundance and production were significantly decreased and protein to nucleic acid synthesis became unbalanced during the maintenance phase bloom. An increase in VLP abundance during the maintenance phase was most likely the cause of bacterial mortality as mixotrophic grazing could only account for ∼4% of the change in bacterial abundance. Together, these data suggest that the associated bacteria and viruses play a critical role in the formation and termination of K. brevis blooms.