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

Detritus from the dinoflagellates Scrippsiella (= Peridinurn) trochoidea and Isochrysis galbana and from the diatoms Skeletonema costaturn, Thalassiosira angstii and Chaetoceros tricornuturn incubated at 10 "C in seawater is colonised by a succession of micro-organisms. Primary microbial decomposers in the incubation experiments were bacterial rods and cocci which reached a peak standing stock carbon of 1.86 f 0.76 % of the carbon supplied to the incubation media by the third day of incubation. The bacteria were subsequently replaced by flagellates which attained a mean peak biomass of 12.5 f 3.58 % of the bacterial biomass by Day 6 before declining. Synchronous measurement of the utilisation 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 bacterioparticulate complexes, much as have been recorded for natural waters. During this phase, carbon is mainly utilised from the dissolved component of phytoplankton cell debris, whilst the more refractory components including particulate debris is used more slowly. The dissolved organic component comprises a mean of 34 24 % of the total carbon in the debris and has a 50 % utilisation time of only 1.56 d (37.44 h), whereas the particulate component comprises 65 76 % of the total carbon and has a 50 % utilisation time of as much as 11.56 d (277.4 h). Bacterial carbon conversion efficiency (bacterial carbon/detrital carbon used X 100) during the initial phases of colonisation is 9.8 Sb -a value similar to that recorded for bacterial conversion of dissolved components of macrophyte debris. The results suggest that the carbon conversion budget for the decomposition of phytoplankton cell debris is 100 g carbon yielding 4 -644 g of bacterial carbon. This value for incorporation of carbon 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 carbon conversion budget for whole phytoplankton debris thus suggests that as much as 30.8 % of the carbon is mineralised and returned to the environment within 3 d by the bacteria which initially colonise the material, whereas the more refractory 64.4 %, comprising carbon in the particulate components of the cell debris, is mineralised within approximately 11 d by bacteria characteristically associated with the decomposition phase of a phytoplankton bloom.