Microbial growth in turbulent suspension and its relation to marine aggregate formation

Microbial growth and respiration responses to seston of differing organic composition revealed two different mechanisms of marine aggregate formation: an almost immediate physicochemical aggregation of fine (<8 μm) particles and a longer-term (48 hrs) production of large, robust aggregates, mediated by bacterial activity. Suspensions of fine Spartina alterniflora fragments and a glacial till from Nova Scotia were used to stimulate bacterial and protozoan growth while maintained in turbulent suspension. Under high (6 to 11 mg·dm⁻³) DOC concentrations bacterial responses were rapid and, after 48 hrs, resulted in large, robust flocculated aggregates. With only the till in suspension, DOC levels were low (<3 mg·dm⁻³) yet the bacterial reponse was still rapid. Initial flocculation due to physical factors was evident in the till-only experiment and was either masked or not important in the high DOC experiments. Bacterial respiration per cell was 2 to 3 times as high under low DOC conditions as under high DOC and initial bacterial attachment to particle surfaces was significantly higher. Significant aggregate formation occurred after 48 hrs and appeared to be due to relict exudates from attached bacteria, in spite of low numbers of attached bacteria at the time of flocculation.