Effects of the colonial cyanobacterium Trichodesmium spp. on bacterial activity

Abstract

We measured rates of particulate and dissolved primary production (DOCp) as well as bacterial production (BP) in order to determine the effect of freshly produced photosynthates on heterotrophic bacteria. During a 1 mo study carried out on samples from the southwestern lagoon of New Caledonia, we observed 2 different phytoplanktonic communities. The first was a mixed com- munity, dominated by diatoms and with a low abundance of Trichodesmium spp. (10% total phyto- plankton cells) and the second community was dominated by Trichodesmium spp. (up to 98% total phytoplankton cells). Chlorophyll a normalised total (particulate + dissolved) primary production rates were twice as high in the mixed community compared to the Trichodesmium spp. dominated community. The percentage of extracellular release (PER), calculated as the percentage of DOCp divided by total primary production (TOCp), reached 8% in the mixed community; in contrast, rates were much lower in the Trichodesmium spp. community and remained close to 1%. In terms of BP, activities were higher in the mixed community and were generally higher in light incubations than in the dark. In contrast, in the Trichodesmium spp. dominated community, bacterial activity was much lower and decreased with increasing Trichodesmium spp. abundance. This decrease in bacterial activity could not be explained by a decrease in bacterial abundance. This suggests that the propor- tion of active bacteria was much lower during the Trichodesmium spp. bloom.