2000 Annual Progress Report
The Importance of Organic Phosphorus in Promoting Cyanobacterial Blooms in Florida Bay: Competition Between Bacteria and Phytoplankton
Joseph N. Boyer, Ph.D.
Southeast Environmental Research Center
Miami, FL 33199
305-348-4076 (O), 305-348-4096 (fax), boyerj@fiu.edu
Introduction
The occurrence of extensive phytoplankton blooms in central Florida Bay has precipitated a significant amount of public, political, and scientific concern. Most vexing is the fact that these blooms are dominated by cyanobacteria, specifically Synechococcus elongatus. None of the current physical/chemical or nutrient limitation models accurately predict or explain these occurrences. There are significant differences in physiology between cyanobacteria and algae which may explain their dominance. Cyanobacteria have a much higher cell-specific production of alkaline phosphatase than do algae. Alkaline phosphatase is an enzyme which cleaves organic P making it available for cellular uptake. This enhanced enzyme capacity gives cyanobacteria an advantage over algae under P limited conditions in the presence of labile organic matter. These are the very conditions which are commonly found in central Florida Bay.
We proposed a two tiered approach
to elucidating a mechanism for cyanobacterial bloom development. The first step was to determine the relative
contribution of bacteria to the total community production. We hypothesized that spatial patterns in
primary production of bacteria and phytoplankton in Florida Bay were a product
of inorganic N and organic P availability.
We are using a running 10 year database of
nutrients and phytoplankton biomass (chlorophyll a) combined with new
measurements of primary production (pulse amplitude modulated (PAM)
fluorometry), bacterial production, bacterial biomass, and enzyme assays to
develop statistical models of substrate and nutrient competition among the
major components of the phyto- and bacterioplankton
community.
As this project involved use of new approaches to studying phytoplankton dynamics, our first efforts were put into methods development. Phytoplankton primary production was measured by coupling PAR measurements with pulse amplitude modulated (PAM) fluorometry. The advantages of PAM fluorometry include quick production measurements (no light/dark incubation assays), discrimination of major phytoplankton groups, and independence from using radioisotopes. This technique permits rapid measurement of: 1) the absorption cross section of Photosystem II (quantum yield); 2) the rate of photosynthetic electron transport and; 3) the level of photochemical quenching. PAM fluorometry will also be used to discriminate Chl a and quantum yield among the three major groups of phytoplankton: cyanobacteria, green algae, and diatoms. We compared PAM fluorometric determinations of Chl a with the standard acetone extraction method and found reasonable agreement. One of the problems we found with PAM fluorometry was one of instrument sensitivity. Sometimes the very low ambient phytoplankton biomass precluded the generation of acceptable light curves resulting in noisy data. We hope to address this problem with the addition of a new sensor having better sensitivity.
We have tested new stains for epifluorescence microscopy bacterial enumeration, specifically SYBR Green, Pico Green, and SYBR Gold. Our comparisons with the standard acridine orange technique have been very favorable for the SYBR stains: less fading of the sample and lower background fluorescence. The result is that smaller bacteria are more easily counted using SYBR Green and Gold. Part of the reason for testing this stain is that we hope to use flow cytometry to characterize microbial communities in the near future.
Data from these measurements are being used to develop functional response curves of microbial activities with physical/chemical water quality variables. We are relating enzyme activities with bacterial production and cell numbers in an effort to better characterize the microbial loop. Relationships among variables are being developed using a suite of multivariate statistics: principal component analysis, cluster analysis, and discriminant analysis.
Anticipated Activities (FY2001)
During the second year, we will be test the hypothesis that both the local source of labile DOM (both P and N) and the enhanced alkaline phosphatase activity of cyanobacteria favor the initiation and dominance of cyanobacterial blooms in central Florida Bay. We will incubate ambient water from each of the three zones of Florida Bay under three treatments: ambient light incubation, dark incubation, and DOM amendments. From these treatments we expect to quantify: 1) the effect of DOM on phytoplankton community structure; 2) the bacterial contribution to DOM amended community production; and 3) the effect of bioavailability of ambient and amended DOM on community structure.