Response of the Diatom Phaeodactylum tricornutum to Photooxidative Stress Resulting from High Light Exposure

The response of microalgae to photooxidative stress resulting from high light exposure is a well-studied phenomenon. However, direct analyses of photosystem II (PSII) D1 protein (the main target of photoinhibition) in diatoms are scarce. In this study, the response of the diatom model species Phaeodactylum tricornutum to short-term exposure to high light was examined and the levels of D1 protein determined immunochemically. Low light (LL) acclimated cells (40 µmol photons m−2 s−1) subjected to high light (HL, 1,250 µmol photons m−2 s−1) showed rapid induction of non-photochemical quenching (NPQ) and ca. 20-fold increase in diatoxanthin (DT) concentration. This resulted from the conversion of diadinoxanthin (DD) to DT through the activation of the DD-cycle. D1 protein levels under LL decreased about 30% after 1 h of the addition of lincomycin (LINC), a chloroplast protein synthesis inhibitor, showing significant D1 degradation and repair under low irradiance. Exposure to HL lead to a 3.2-fold increase in D1 degradation rate, whereas average D1 repair rate was 1.3-x higher under HL than LL, leading to decreased levels of D1 protein under HL. There were significant effects of both HL and LINC on P. tricornutum maximum quantum yield of PSII (Fv/Fm), showing a reduction of active PSII reaction centres. Partial recovery of Fv/Fm in the dark demonstrates the photosynthetic resilience of this diatom to changes in the light regime. P. tricornutum showed high allocation of total protein to D1 and an active D1-repair cycle to limit photoinhibition

Photoprotection in sequestered plastids of sea slugs and respective algal sources

Some sea slugs are capable of retaining functional sequestered chloroplasts (kleptoplasts) for variable periods of time. The mechanisms supporting the maintenance of these organelles in animal hosts are still largely unknown. Non-photochemical quenching (NPQ) and the occurrence of a xanthophyll cycle were investigated in the sea slugs Elysia viridis and E. chlorotica using chlorophyll fluorescence measurements and pigment analysis. The photoprotective capacity of kleptoplasts was compared to that observed in their respective algal source, Codium tomentosum and Vaucheria litorea. A functional xanthophyll cycle and a rapidly reversible NPQ component were found in V. litorea and E. chlorotica but not in C. tomentosum and E. viridis. To our knowledge, this is the first report of the absence of a functional xanthophyll cycle in a green macroalgae. The absence of a functional xanthophyll cycle in C. tomentosum could contribute to the premature loss of photosynthetic activity and relatively short-term retention of kleptoplasts in E. viridis. On the contrary, E. chlorotica displays one of the longest functional examples of kleptoplasty known so far. We speculate that different efficiencies of photoprotection and repair mechanisms of algal food sources play a role in the longevity of photosynthetic activity in kleptoplasts retained by sea slugs

GROWTH AND FLUORESCENCE CHARACTERISTICS OF ULTRAPLANKTON ON A NORTH SOUTH TRANSECT IN THE EASTERN NORTH-ATLANTIC

In the summer of 1989 vertical profiles of chlorophyll a were taken in the North Atlantic. Stations were located along a transect following longitude 20-degrees-W, between 60 and 33-degrees-N. Maximum chlorophyll a levels were located near the surface in the north (2 mugl-1) but became gradually deeper towards the south (0.3 mug l-1 at a depth of 80-110m). Primary production varied only by a factor of 2.3 (226-533 mg C m-2 day-1), whereas the assimilation ratio showed far less variation (15.46 +/- 4.61 mg C mg Chl-1 day-1, n = 14). The northern part of the transect showed no thermal stratification, and nutrients were plentiful. Phytoplankton was distributed homogeneously, with a nearly constant signature of the cellular fluorescence characteristics as monitored by flow-cytometry. The dominant taxa were small prymnesiophytes (Emiliania huxleyi) and chroococcoid phycoerythrin-containing cyanobacteria. Cells were healthy, resulting in net growth rates (growth rate minus grazing) of up to 0.75 divisions per day. Of the total primary production, 68% was actually converted into new plant carbon. Midway along the transect (47-degrees-N) there was a sharp thermocline at a depth of 35 m. All nutrients (nitrogen, phosphate and silicate) were depleted in the upper water layer. Cyanobacteria dominated the phytoplankton community in numbers varying from 12,000 to 39,000 cells ml-1. Stabilization of the water column was apparently in progress, as could be derived from the gradual increase in mean cellular red fluorescence of the cyanobacteria with depth. Furthermore, net growth rates of the algae were relatively low, as was the proportion of primary production resulting in an actual increase of plant carbon (42%). At the southernmost station (33-degrees-N), the situation was typical of the oligotrophic subtropics: a sharp thermocline at 35 m, the 0.1% surface incident irradiance level at 150 m, and a nitracline at a depth of 100-120m. The most prominent (pico)plankters present were prochlorophytes, predominantly at the deep chlorophyll maximum (80-100 m, up to 95,000 cells ml-1). Cyanobacteria co-occurred but only in small numbers (maximum of 5500 ml-1), mainly in surface waters. The vertical (physical) stability of the water column was pronounced, and the mean cellular red fluorescence signal of cells found at the bottom of the euphotic zone was consequently higher than in surface waters. Most remarkable were the net growth rates of the various species suggesting a highly dynamic phytoplankton community. Extreme growth rates included positive as well as negative values, which were not always consistent with the greatest abundance of certain species. Despite these rapid changes, the actual increase in plant carbon was only 14% of the primary production

INFLUENCE OF PHYSICAL AND BIOLOGICAL PROCESSES ON THE CONCENTRATION OF O-2 AND CO2 IN THE ICE-COVERED WEDDELL SEA IN THE SPRING OF 1988

In October and November 1988, measurements of oxygen and total dissolved inorganic carbon (TCO2) concentrations were made in the northwestern Weddell Sea to the south and north of the marginal ice edge, in order to estimate the relative importance, regarding their variations, of both biological (photosynthesis and respiration) and physical (transport of O2 and CO2 by turbulent movements and by intrusion from the atmosphere) processes. In the ice-covered region, both respiration and upwelling determined the O2 and TCO2 variations, whilst in the open water just north of the marginal ice edge, photosynthetic activity was the most important factor controlling O2 and TCO2 levels. These findings underline the importance of the activity of the pelagic ecosystem in determining the concentration of O2 and CO2 not only in the ice-free but also in the ice-covered Antarctic Ocean

Bio-optical properties of the subtropical North Atlantic. I. Vertical variability

Studies of the physiological and optical properties of phytoplankton of the subtropical open ocean are rare. Yet, assessing the spatial and temporal variation in the photo-physiological parameters and chlorophyll-specific absorption coefficients of marine phytoplankton is essential if we are to account for the relative importance of pelagic systems in the global carbon economy. As a contribution to this general goal, the photosynthetic and bio-optical properties of phytoplankton were measured at 5 stations across a broad swath of the subtropical North Atlantic Ocean. Similar vertical patterns in pigment composition, chlorophyll-specific absorption coefficients and the photosynthesis-irradiance parameters were observed at all 5 stations. A considerable proportion of the vertical variation in the chlorophyll-specific absorption coefficients was caused by vertical changes in the proportion of the non-photosynthetic pigment (NPP) zeaxanthin relative to chlorophyll a. On the other hand, pigment packaging appeared to have a minor influence on the optical characteristics of the picoplankton-dominated region: chlorophyll-specific absorption coefficients observed at the red peak frequently exceeded 0.02 m2 (mg chl a)-1. Multiple, linear-regression analysis was used to examine the value of some commonly-measured environmental variables as potential predictors of the photosynthetic parameters αB and PmB. The results showed that indices of ambient irradiance, nutrient flux or nutrient concentration, separately or in combination, account for a significant fraction of the total variance in the photosynthetic parameters. The stability in the distribution patterns of both the photosynthetic parameters and absorptive characteristics of subtropical phytoplankton assemblages and in the principal factors governing the variability in these properties may facilitate the selection of appropriate input parameters for use in models of primary production for this hydrographic region