Rate of Degradation and Efficiency of Conversion of Phytoplankton Debris by Marine Micro-Organisms

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Abstract
Detritus from the dinoflagellates Scrippsiella (= Peridinium) trochoideum and Isochrysis galbana and from the diatoms Skeletonema costatum, Thalassiosira angstii and Chaetoceros tricornutum incubated at 10.degree. C in seawater is colonized by a succession of microorganisms. Primary microbial decomposers in the incubation experiments were bacterial rods and cocci which reached a peak standing stock C of 1.86 .+-. 0.76% of the C supplied to the incubation media by the 3rd day of incubation. The bacteria were subsequently replaced by flagellates which attained a mean peak biomass of 12.5 .+-. 3.58% of the bacterial biomass by day [d] 6 before declining. Synchronous measurement of the utilization of dissolved and particulate components from the incubation media shows that there is a well-defined initial sequence of aggregation of particulate matter to form bacterio-particulate complexes, much as have been recorded for natural waters. During this phase, C is mainly utilized from the dissolved component of phytoplankton cell debris; the more refractory components including particulate debris are used more slowly. The dissolved organic component comprises a mean of 34.25% of the total C in the debris and has a 50% utilization time of only 1.56 d (37.44 h; the particulate component comprises 65.76% of the total C and has a 50% utilization time of as much as 11.56 d (277.4 h). Bacterial C conversion efficiency (bacterial C/detrital C used .times. 100) during the initial phases of colonization is 9.8% - a value similar to that recorded for bacterial conversion of dissolved components of macrophyte debris. The results suggest that the C conversion budget for the decomposition of phytoplankton cell debris is 100 g C yielding 4.644 g of bacterial C. This value for incorporation of C into bacteria from phytoplankton cell debris is much lower than might be anticipated from the absorption efficiency of selected labile components released in small quantities by living phytoplankton. The C conversion budget for whole phytoplankton debris thus suggests that as much as 30.8% of the C is mineralized and returned to the environment within 3 d by the bacteria which initially colonize the material; the more refractory 64.4%, comprising C in the particulate components of the cell debris, is mineralized within .apprx. 11 d by bacteria characteristically associated with the decomposition phase of a phytoplankton bloom.