SUPPORT OF BACTERIOPLANKTON PRODUCTION BY DISSOLVED HUMIC SUBSTANCES FROM 3 MARINE ENVIRONMENTS

Dissolved humic substances from 3 marine environments with varying vascular plant influence were shown to contain biologically labile components. Biomass of marine bacterioplankton increased in grazer-free incubations in the presence of humic substance supplements equivalent to 2-fold and 5-fold natural concentrations. Humic substances were present in highest concentration in seawater collected from a mangrove swamp site; these humics had the highest vascular plant influence (estimated at 66 % based on lignin phenol analysis) and yielded the least bacterial growth on a carbon basis. From 3 to 11 % of the humic substances from all 3 sites were utilized by marine bacterioplankton over a 5 d period. Results of this study suggest that some fraction of the humic substances pool can have a relatively short turnover time in seawater, and point out the importance of identifying which specific components of the dissolved organic carbon pool are available for supporting marine bacterioplankton production.