Effects of cobalt and humic substances on the growth of bloom-forming marine phytoplankton

Abstract: In this thesis, the importance of cobalt and humic substances of terrestrial origin for the growth of some bloom-forming marine phytoplankton species is examined. Cobalt (Co) is considered an essential and potentially limiting trace metal for phytoplankton growth and is needed for the synthesis of vitamin B12. Since Co can be mobilised from soils at low pH values, acidification may have led to an increase in the river transport of Co to coastal waters. An increase in the humic transport to coastal waters has been reported during the last decades. The increased amounts of Co and humic substances may affect phytoplankton growth in a species-specific way. To evaluate the effects of Co on phytoplankton growth, experiments with monospecific algal cultures were performed. The three different phytoplankton taxa tested responded differently to Co additions. The toxic marine flagellate Chrysochromulina polylepis was positively affected by cobalt additions between 3 and 1000 nM. However, when added together with humic substances, cobalt did not stimulate the growth of this flagellate to the same extent. In other experiments, performed with a coastal phytoplankton community from the Swedish west coast and algal cultures, additions of humic substances increased bacterial numbers and production, primary production and phytoplankton biomass. The increased bacterial production supported an increase in the biomass of heterotrophic flagellates. Bacteria may have regenerated humic bound nitrogen making it available to the phytoplankton. There were also indications of a direct incorporation of humic substances by some phytoplankton species, which possibly influenced their growth. In conclusion, this thesis shows that the availability of Co and humic substances in coastal waters can have both positive and negative effects on the growth of bloom-forming phytoplankton species.