Response of Photosynthetic Parameters to Varying Light Regimes in the Marine Environment

Supervisors

Brian Griffiths (CSIRO), Simon Wright (AAD), Tom Trull (UTas)

(PhD Only)

Project Outline

Estimates of primary productivity in the ocean are crucial to both models of CO2 flux and estimates of the food availability for other trophic levels. Primary productivity is difficult to measure directly and is generally estimated by combining estimates of the standing stock of phytoplankton with measurements of photosynthetic potential measured directly from photosynthesis vs irradiance experiments or indirectly from estimates of their photosynthetic competence using fast repetition rate fluorometry (FRRF) or pulse amplitude modulation fluorometry (PAM). Standing stock is conveniently measured continuously by monitoring fluorescence of chlorophyll a, or more accurately by HPLC analysis of all chlorophylls and carotenoid pigments which, when combined with interpretation using CHEMTAX software, can be used to estimate abundance of major components of the phytoplankton.

However estimates of productivity are confounded by variation of the other photosynthetic parameters, primarily in response to light. The fluorescence response per unit chlorophyll has been found to vary by up to a factor of five in a diurnal cycle. It also changes with nutrient status. Furthermore, the content of chlorophylls and carotenoids per cell has been shown to vary markedly with irradiance. Thus, the photosynthetic parameters measured are simultaneously affected by changes in ambient irradiance from diurnal and seasonal cycles, coupled with variation due to depth, all of which may be modified due to short term changes arising from passing clouds, waves and mixing as well as seasonal changes in mixed layer depth. Previously we have measured the response of cellular pigment content to differing (static) light environments, but little is known of the response to rapid changes of irradiance that are typical of the natural environment.

We propose to measure the response of photosynthetic parameters to changing light environments using a combination of laboratory and field studies. Laboratory studies will determine the response of cultured phytoplankton to changing irradiance - measuring cellular pigment content by a combination of HPLC and flow cytometry, fluorescence response using conventional fluorometry and FRRF, and photosynthetic rates using oxygen evolution and 14C incorporation. Measurements will be undertaken simulating clear sky conditions over diurnal cycles as well as the effects of rapidly changing irradiance simulating passing clouds and waves. These measurements will be undertaken using a range of cultured phytoplankton representing important Southern Ocean species typical of a range of light environments (i.e surface blooming vs deep populations). Cellular carbon content will also be measured.

It is proposed to couple these measurements with a series of shipboard experiments using natural populations incubated in 6 x 500 L minicosms to obtain measurements of nett community production over diurnal cycles. These will involve measurement of pigment content, fluorescence response, and productivity coupled with monitoring oxygen fluxes from each minicosm. Species composition will be determined by light and electron microscopy coupled with counts by flow cytometry.

This project should immediately improve our ability to estimate phytoplankton standing stock and composition as well as community productivity in the Southern Ocean. Estimates of standing stock will be improved by providing data on the extent of variation of cellular pigment composition, which will be used to constrain CHEMTAX calculations, as well as the fluorescence yield per unit chlorophyll. Estimates of primary productivity will be improved by determining how photosynthetic parameters respond to changes in the light environment. It will thus benefit both the CO2 flux and Antarctic Marine Ecosystem programs within the ACE CRC.

Contact

A/Prof Tom Trull or tel +61 3 6226 2988