Influence of dissolved humic materials on carbon assimilation and alkaline phosphatase activity in natural algal‐bacterial assemblages

Abstract

SUMMARY. Mixed natural assemblages of algae and bacteria exhibited lower rates of 14C assimilation and high rates of dissimilation of recent photosynthate when amended with low concentrations (7.2 mg 1‐1) of unfractioned dissolved humic materials (DHM). The extent of the inhibition or stimulation was greatest in the smaller (1–5 μm) assemblage particles. In different algal‐bacterial assemblages, additions of DHM markedly enhanced community alkaline phosphatase activity (APA), particularly under low light regimes, DHM of low apparent molecular weight was much more stimulatory to both 14C assimilation and APA than DHM of high apparent molecular weight, supporting the belief that DHM molecular weight is an important determinant of DHM interactive capacity. Higher concentrations of D HM (either unfractionated, or molecular weight fractionated) produced greater APA responses. Addition of phosphate enhanced the disparity in rates of 14C assimilation of samples incubated under low and high light regimes, increased the rates of 14C assimilation, and depressed APA. There were indications of interactions between DHM and phosphorus in several experiments. Two hypotheses were invoked to explain increases in APA in response to DHM: (1) increased competition between algae and bacteria for phosphate following bacterial release from substrate limitation, or (2) DHM may have acted as a sequestering agent for organophosphorus compounds, and in so doing, gradually depleted available phosphate. In either case, it is clear that DHM alters phosphorus cycling. This DHM characteristic may be ecologically as important as its ability to complex trace metals.