Randall S. Alberte

Photosynthetic responses of Zostera marina L. (Eelgrass) to in situ manipulations of light intensity

SummaryPhotosynthetic responses of the temperate seagrass, Zostera marina L., were examined by manipulations of photon flux density in an eelgrass bed in Great Harbor, Woods Hole, MA during August 1981. Sun reflectors and light shading screens were placed at shallow (1.3 m) and deep (5.5 m) stations in the eelgrass bed to increase (+35% to +40%) and decrease (-55%) ambient photon flux densities. The portion of the day that light intensities exceeding the light compensation point for Z. marina (Hcomp) and the light saturation point (Hsat) were determined to assess the impact of the reflectors and shades. The Hcomp and Hsat periods at the deep station shading screen were most strongly affected; Hcomp was reduced by 11% and Hsat was reduced by 52%. Light-saturated photosynthetic rates, dark respiration rates, leaf chlorophyll content, chlorophyll a/b, PSUO2 size, PSU density, leaf area, specific leaf area, leaf turnover times and leaf production rates were determined at the end of three sets of 1- to 2-week experiments. None of the measured parameters were affected by the photon flux density manipulations at the shallow station; however, at the deep station leaf production rates were significantly reduced under the shading screen and chlorophyll a/b ratios were higher at the reflector. These results indicate that adjustment to short-term changes in light regime in Z. marina is largely by leaf production rates. Further, the most dramatic changes in the periods of compensating or saturating photon flux densities had the greatest impact on the measured photosynthetic responses.

Flow, flapping, and photosynthesis ofNereocystis leutkeana: a functional comparison of undulate and flat blade morphologies

A number of species of macroalagae possess a flat, strap-like blade morphology in habitats exposed to rapidly-moving water whereas those at protected sites have a wider, undulate blade shape. We have explored the functional consequences of flat, narrow vs. wide, undulate blade morphologies in the giant bull kelpNereocystis luetkeana. Our study focused on the behavior of blades in ambient water currents and the consequences of that behavior to breakage and to photosynthesis. In flowing water, the narrow, flat blades flap with lower amplitude and collapse together into a more streamlined bundle than do wide, undulate blades, and hence experience lower drag per blade area at a given flow velocity. If the algae at current-swept sites had ruffled blades, drag forces would sometimes be sufficient to break the stipes. However, flat blades in a streamlined bundle experience more self-shading than do undulate blades, which remain spread out in water currents. Thus, there is a morphological trade-off between reducing drag and reducing self-shading. Photosynthetic14C-HCO3 uptake rates decrease in slow flow when the boundary layer along the blade surface across which diffusion takes place is relatively thick. However, blade flapping, which stirs water near the blade surface, enhances carbon uptake rates in slow water currents for both the undulate and the flat morphologies.

Effects of temperature on photosynthesis and respiration in eelgrass (Zostera marina L.)

Abstract The short-term temperature responses of the photosynthesis-irradiance (P-I) relationships and respiration of Zostera marina L. (eelgrass) leaves were determined at eight temperatures from 0 to 35°C for plants growing at 20–22°C in Great Harbor, Woods Hole, Massachusetts. Light-saturated, net photosynthesis increased with temperature up to an optimum of 25–30°C and decreased at 35°C. Dark respiration increased with temperature from 5 to 35°C. The initial slopes of the P-I curves were relatively constant between 5 and 30°C, but were greatest at 0 and least at 35°C. The photosynthetic saturation and compensation photon flux densities generally increased with increasing temperature. A Q10 value, determined over the temperature range 0–35°C, of 1.5–1.7 was obtained for light-saturated photosynthesis, while the value for dark respiration was 2.4. Ratios of maximum photosynthetic rates to respiration rates (P : R) were highest at 5°C and declined markedly at higher and lower temperatures. Calculations of daily carbon balances from P : R ratios and daily light regimes indicate that net positive leaf carbon balance could be maintained by Z. marina leaves in Great Harbor under winter temperature and light regimes, while high temperatures (⩾30°C) lead to negative daily carbon balances of leaves which could contribute to mortality or reduced growth of the plants.

The P700-chlorophyll a-protein. Isolation and some characteristics of the complex in higher plants.

Abstract A P700-chlorophyll a-protein complex has been purified from several higher plants by hydroxylapatite chromatography of Triton X-100-dissociated chloroplast membranes. The isolated material exhibits a red wavelength maximum at 677 nm, major spectral forms of chlorophyll a at 662, 669, 677, and 686 nm, a chlorophyll/P700 ratio of 40–45 1 , and contains only chlorophyll a and β-carotene of the photosynthetic pigments present in the chloroplast. The spectral characteristics and composition of the higher plant material are homologous to those of the P700-chlorophyll a-protein previously isolated from blue-green algae; however, unlike the blue-green algal component, cytochromes f and b6 are associated with the higher plant material. Evidence is presented that a chlorophyll a-protein termed “Complex I” which can be isolated from sodium dodecyl sulfate extracts of chloroplast membranes is a spectrally altered form of the eucaryotic P700-chlorophyll a-protein. The isolation procedure described in this paper is a more rapid technique for obtaining the heart of photosystem I than presently exists; furthermore, the P700 photooxidation and reduction kinetics in the fraction are improved over those in other isolated components showing the same enrichment of P700. It appears very probable that the heart of photosystem I is organized in the same manner in all chlorophyll a-containing organisms.

Impacts of CO2 Enrichment on Productivity and Light Requirements of Eelgrass.

Seagrasses, although well adapted for submerged existence, are CO2-limited and photosynthetically inefficient in seawater. This leads to high light requirements for growth and survival and makes seagrasses vulnerable to light limitation. We explored the long-term impact of increased CO2 availability on light requirements, productivity, and C allocation in eelgrass (Zostera marina L.). Enrichment of seawater CO2 increased photosynthesis 3-fold, but had no long-term impact on respiration. By tripling the rate of light-saturated photosynthesis, CO2 enrichment reduced the daily period of irradiance-saturated photosynthesis (Hsat) that is required for the maintenance of positive whole-plant C balance from 7 to 2.7 h, allowing plants maintained under 4 h of Hsat to perform like plants growing in unenriched seawater with 12 h of Hsat. Eelgrass grown under 4 h of Hsat without added CO2 consumed internal C reserves as photosynthesis rates and chlorophyll levels dropped. Growth ceased after 30 d. Leaf photosynthesis, respiration, chlorophyll, and sucrose-phosphate synthase activity of CO2-enriched plants showed no acclimation to prolonged enrichment. Thus, the CO2-stimulated improvement in photosynthesis reduced light requirements in the long term, suggesting that globally increasing CO2 may enhance seagrass survival in eutrophic coastal waters, where populations have been devastated by algal proliferation and reduced water-column light transparency.

Photoadaptation and growth of Zostera marina L. (eelgrass) transplants along a depth gradient

Photosynthetic and growth responses were assessed in Zostera marina L. transplants within and beyond the natural extent of an eelgrass meadow in Great Harbor, Woods Hole, MA. Transplant survival and rapid growth inshore of the shallow edge of the meadow (0.5 and 0.8 m water depth) indicated a periodic disturbance factor maintaining the shallow edge of the meadow. Transplant mortality, reduced growth, and a negative carbon balance of eelgrass transplanted offshore the deep edge of the meadow (7 and 10m) supported the hypothesis of light-limited eelgrass growth in the deep regions of the Great Harbor meadow. Photoadaptive responses occurred along the water depth gradient, and both photosynthesis and growth responses were used to assess the genetic vs. phenotypic components of eelgrass response to the water depth gradient. Reciprocal transplants between shallow (1.3 m) and deep (5.5 m) areas within the eelgrass meadow indicated photosynthetic and growth responses were primarily a result of growth habitat rather than genetic differentiation within the eelgrass meadow.

Eelgrass (Zostera marina L.) transplants in San Francisco Bay: Role of light availability on metabolism, growth and survival

Survival, metabolism and growth of Zostera marina L. transplants were examined along depth gradients in Keil Cove and Paradise Cove in the extremely turbid San Francisco Bay estuary. Water transparency was unusually high throughout 1989–1990 for San Francisco Bay. Transplant survival was strongly depth-dependent at Paradise Cove but not at Keil Cove. All transplants were lost below − 1.0 m depth within 1 year at Paradise Cove, but survived to depths of − 1.5 m at Keil Cove. Half the transplants growing in shallow water survived the first year at both sites. Shoot photosynthesis, respiration, growth, and sugar content did not differ between sites. Daily periods of irradiance-saturated photosynthesis (Hsat) were over 6 h all year. Seasonal photosynthetic acclimation to light availability maintained long Hsat periods and high ratios of daily whole-plant production to respiration through the winter, indicating a potential for net carbon gain throughout the year. Winter growth was 50% of the summer rate. Despite high initial losses, surviving transplants have persisted at both sites through 1994. Although eelgrass transplants can succeed in San Francisco Bay given sufficient light availability, the role of carbon reserves and transplant timing may influence transplant survival.

Thermal acclimation and whole-plant carbon balance in Zostera marina L. (eelgrass)

Thermal acclimation in eelgrass Zostera marina L. was investigated in laboratory experiments after growing plants at 10 and 20°C for 21 days under a 12:12 L:D regime. Metabolic rates showed significant shifts in short-term response to temperature in leaves and roots. Growth rates, tissue carbohydrate concentrations and metabolic rates measured at the two growth temperatures were statistically identical, indicating that thermal acclimation was essentially complete at these temperatures. When measured at pO2 values high enough to achieve capacity rates of respiration, thermal responses of respiration (Q10) were lower than previously reported while the thermal response of photosynthesis (measured at pO2 below air saturation) was similar to previous reports. Daily C budgets constructed from metabolic rate data indicated that Hsat periods required for photosynthesis to balance C demand can vary from 3 to > 12 h, depending on the ratios of net photosynthesis: respiration (Pnet: R) and shoot: root. Since Z. marina. shows evidence of thermal acclimation, seasonal changes in ambient temperature may not significantly affect Hsat requirements and whole-plant C balance. Rapid mortality at high temperatures during summer may result instead from thermal disruption of metabolism while internal C reserves may be important in meeting C demand during winter periods of low light availability, particularly among high-latitude populations.

Effects of anaerobiosis on root metabolism of Zostera marina (eelgrass): implications for survival in reducing sediments

The temperate seagrass Zostera marina L. typically grows in highly reducing sediments. Photosynthesis-mediated O2 supplied to below-ground tissues sustains aerobic respiration during photosynthetic periods. Roots, however, experience daily periods of anoxia and/or hypoxia at night and under conditions that reduce photosynthesis. Rhizosphere cores of Z. marina were collected in August 1984 from Great Harbor, Massachusetts, USA. We examined short-term anaerobic metabolism of [U-14C]sucrose in excised roots and roots of intact plants. Under anaerobic conditions roots showed appreciable labeling of CO2, ethanol and lactate, and slight labeling of alanine and other metabolites. Over 95% of the 14C-ethanol was recovered in the root exudate. Release of other metabolites from the roots was minimal. Ethanol was also released from hypoxic/anoxic roots of intact plants and none of this ethanol was transported to the shoot under any experimental conditions. Loss of ethanol from roots prevented tissue levels of this phytotoxin from increasing during anaerobiosis despite increased synthesis of ethanol. Anaerobic metabolism of [U-14C]glutamate in excised roots led to appreciable labelling of γ-aminobutyrate, which was known to accumulate in eelgrass roots. Roots recovered to fully aerobic metabolism within 4 h after re-establishment of aerobic conditions. The contributions of these root metabolic responses to the ability of Z. marina to grow in reducing marine sediments are related to light-regulated interactions of shoots and roots that likely dictate depth penetration, distribution and ecological success of eelgrass.

The antifouling activity of natural and synthetic phenolic acid sulphate esters

p-(sulphooxy) Cinnamic acid was isolated as a natural product for the first time from the seagrass Zostera marina (eelgrass) and was found to prevent attachment of marine bacteria and barnacles to artificial surfaces at nontoxic concentrations. Analogous synthetic sulphate esters had similar antifouling properties, while the non-sulphated phenolic acid precursors were ineffective. The antifouling properties of phenolic acid sulphates are consistent with an emerging pattern of biological activity exhibited by other sulphate esters isolated from a variety of marine organisms, and their low toxicity offers promise for the development of environmentally benign antifouling agents to protect structures in aquatic environments.