J. Ramus

Ecosystem impacts of three sequential hurricanes (Dennis, Floyd, and Irene) on the United States' largest lagoonal estuary, Pamlico Sound, NC

Three sequential hurricanes, Dennis, Floyd, and Irene, affected coastal North Carolina in September and October 1999. These hurricanes inundated the region with up to 1 m of rainfall, causing 50- to 500-year flooding in the watershed of the Pamlico Sound, the largest lagoonal estuary in the United States and a key West Atlantic fisheries nursery. We investigated the ecosystem-level impacts on and responses of the Sound to the floodwater discharge. Floodwaters displaced three-fourths of the volume of the Sound, depressed salinity by a similar amount, and delivered at least half of the typical annual nitrogen load to this nitrogen-sensitive ecosystem. Organic carbon concentrations in floodwaters entering Pamlico Sound via a major tributary (the Neuse River Estuary) were at least 2-fold higher than concentrations under prefloodwater conditions. A cascading set of physical, chemical, and ecological impacts followed, including strong vertical stratification, bottom water hypoxia, a sustained increase in algal biomass, displacement of many marine organisms, and a rise in fish disease. Because of the Sounds long residence time (≈1 year), we hypothesize that the effects of the short-term nutrient enrichment could prove to be multiannual. A predicted increase in the frequency of hurricane activity over the next few decades may cause longer-term biogeochemical and trophic changes in this and other estuarine and coastal habitats.

Two components of onset and recovery during photoinhibition of Ulva rotundata

Short-term (up to 5 h) transfers of shade-adapted (100 μmol · m−2 · s−1) clonal tissue of the marine macroalga Ulva rotundata Blid. (Chlorophyta) to higher irradiances (1700, 850, and 350 μmol · m−2 · s−1) led to photoinhibition of room-temperature chlorophyll fluorescence and O2 evolution. The ratio of variable to maximum (Fv/Fm) and variable (Fv) fluorescence, and quantum yield (ϕ) declined with increasing irradiance and duration of exposure. This decline could be resolved into two components, consistent with the separation of photoinhibition into energy-dissipative processes (photoprotection) and damage to photosystem II (PSII) by excess excitation. The first component, a rapid decrease in Fv/Fm and in Fv, corresponds to an increase in initial (Fo) fluorescence and is highly sensitive to 1 mM chloramphenicol. This component is rapidly reversible under dim (40 μmol · m−2 · s−1) light, but is less reversible with increasing duration of exposure, and may reflect damage to PSII. The second (after 1 h exposure) component, a slower decline in Fv/Fm and Fv with declining Fo, appears to be associated with the photoprotective interconversion of violaxanthin to zeaxanthin and is sensitive to dithiothreitol. The accumulation of zeaxanthin in U. rotundata is very slow, and may account for the predominance of increases in Fo at high irradiances.

Ecological growth strategies in the seaweeds Gracilaria foliifera (Rhodophyceae) and Ulva sp. (Chlorophyceae): Soluble nitrogen and reserve carbohydrates

The seaweeds Gracilaria foliifera (Rhodophyceae) and Ulva sp. (Chlorophyceae) were grown in an outdoor continuous-flow system at both ambient incident light (I0) and 0.13 I0. During the winter, both species accumulated substantial soluble nitrogen reserves (up to 1020 μg-at N·g dry wt-1 in G. foliifera and 630 μg-at N·g dry wt-1 in Ulva sp.). The rate at which these N reserves were depleted was proportional to the growth rate. Seaweeds grown at 0.13 I0 had lower growth rates and higher levels of soluble tissue N than plants grown at I0. During the spring-summer growing season, peaks in tissue N followed nutrient peaks in the ambient seawater. Ulva sp. had higher nutrient uptake and growth rates than G. foliifera and showed greater fluctuations in soluble tissue N. This may characterize opportunistic seaweed species with high biomass turnover rates. At I0, the levels of starch (up to 340 mg·g dry wt-1 in G. foliifera and 170 mg·g dry wt-1 in Ulva sp.) were highest during the spring and summer. During this period, fluctuations in starch content were inversely related to growth rate and soluble tissue N. Seaweeds grown at 0.13 I0 did not accumulate starch. Neither species was found to overwinter with starch reserves.

THE PRODUCTION OF EXTRACELLULAR POLYSACCHARIDE BY THE UNICELLULAR RED ALGA PORPHYRIDIUM AERUGINEUM1,2

The unicellular red alga Porphyridium aerugineum was shown to be encapsulated by an amorphous, water‐soluble, polyanionic polysaccharide of high molecular weight. The encapsulating polysaccharide is qualitatively identical with polysaccharide found dissolved in large quantity in the culture medium. The kinetics of extracellular polysaccharide production as a function of cell age was studied. Rates of production (on a per cell basis) of both encapsulating and dissolved polysaccharides are greatest in stationary phase light‐grown cultures.

Uptake of inorganic nitrogen and seaweed surface area: Volume ratios

Surge uptake of ammonium and nitrate by four intertidal seaweed species was examined under standard conditions (16.5 or 20°C, 72 μE m−2 s−1), at both subsaturating and saturating initial nutrient concentrations. Uptake rates were positively correlated with the surface area: volume (SA:V) ratio of the thallus: Ulva curvata (Kutzing) DeToni > Fucus evanescens C. Agardh ≈ Gracilaria tikvahiae McLachlan > Codium decorticatum (Woodward) Howe. A positive correlation was also found for the relationship between SA:V and the initial slope of the curves of uptake rate vs. external concentration. These results support the predictions of an earlier funtional-form model, and suggest the usefulness of the SA:V ratio as a comparative index of nutrient uptake capability in co-occurring seaweeds.

Photoacclimation and photoinhibition in Ulva rotundata as influenced by nitrogen availability

Clonal tissue of the marine chlorophyte macroalga, Ulva rotundata Blid., was transferred from 100 to 1700 μmol photons · m−2 · s−1 under limiting (1.5 μM NH4+maximum, N/P=2) and sufficient (15 μM NH4+maximum, N/P=20) nitrogen supply at 18° C and 11 h light-13 h darkness daily. Photoinhibition was assayed by light-response curves (photosynthetic O2 exchange), and chlorophyll fluorescence at 77 K and room temperature. Daily surface-area growth rate (μSA) in N-sufficient plants increased sixfold over 3 d and was sustained at that level. During this period, respiration (Rd) doubled and light-saturated net photosynthesis capacity (Pm) increased by nearly 50%, indicating acclimation to high light. Quantum yield (ϕ) decreased by 25% on the first day, but recovered completely within one week. The ratio of variable to maximum fluorescence (Fv/Fm) also decreased markedly on the first day, because of an increase in initial fluorescence (Fo) and a decrease in Fm, and partially recovered over several days. Under the added stress of N deficiency, μSA accelerated fivefold over 4 d, despite chronic photoinhibition, then declined along with tissue-N. Respiration doubled, but Pm decreased by 50% over one week, indicating inability to acclimate to high light. Both ϕ and Fv/Fm decreased markedly on the first day and did not significantly recover. Changes in Fo, Fm and xanthophyll-cycle components indicate concurrent photodamage to photosystem II (PSII) and photoprotection by thermal deexcitation in the antenna pigments. Increasing μSA coincided with photoinhibition of PSII. Insufficient diel-carbon balance because of elevated Rd and declining Pm and tissue-N, rather than photochemical damage per se, was the apparent proximate cause of decelerating growth rate and subsequent tissue degeneration under N deficiency in U. rotundata.

Changes in photosynthetic pigment concentration in seaweeds as a function of water depth

We conducted a study of the relationship between changes in photosynthetic pigment content and water depth in Great Harbor near Woods Hole, Massachusetts, USA, on the green algae Ulva lactuca and Codium fragile and the red algae Porphyra umbilicalis and Chondrus crispus. A calibrated underwater photometer equipped with spectral band filters measured light attenuation by the water column. The depth required for a 10-fold diminution of photon flux was 3.6, 5.3, 6.0 and 6.0 m for red, blue, yellow and green light, respectively. Seaweeds were attached to vertically buoyed lines and left to adapt for 7 days; then, with their positions reversed, they were allowed to readapt for 7 days. All species showed greater photosynthetic pigment content with increased depth. Further, the ratio of phycobiliproteins and chlorophyll b to chlorophyll a increased with depth. Changes in pigment content were reversible and occurred in the absence of cell division. There was a net loss of pigments near the surface (high irradiance), and subsequent synthesis when seaweeds were transferred to a position deep in the water column (low irradiance). In contrast, seaweeds which were found in intertidal habitats changed only their pigment concentration, and not pigment ratio, a phenomena analogous to higher plant sun and shade adaptation. Therefore, seaweeds modify their photon-gathering photosynthetic antennae to ambient light fields in the water column by both intensity adaptation and complementary chromatic adaptation.

Correlation of changes in pigment content with photosynthetic capacity of seaweeds as a function of water depth

We conducted a study of the relationship between changes in photosynthetic pigment content and photosynthetic capacity as a function of water depth in Great Harbor near Woods Hole, Massachusetts, USA, on the green algae Ulva lactuca and Codium fragile and the red algae Porphyra umbilicalis and Chondrus crispus. Seaweeds were attached to vertically buoyed lines at 0.5 and 10 m and were allowed to adapt to the ambient light field. All species showed greater pigment content with depth, and the ratio of accessory pigments to chlorophyll a increased with depth. Seaweed samples from 0.5 and 10 m were placed in tandem pairs of stoppered bottles and hung at prescribed depths. The rates of O2 evolution were calculated from changes in dissolved O2 content, both as a function of biomass and chlorophyll a concentration. Our results indicate that intensity and/or chromatic adaptation enhance the photosynthetic capacity of a seaweed in limiting light conditions. The strategy of seaweeds in manipulating their photon-gathering antennae is not to maximize photosynthetic rate, but rather to optimize the photosynthetic rate. They can change pigment rations, or simply increase the total amount of pigment, or both. Further, if a seaweed is optically thick, as are Codium fragile and Chondrus crispus, it does not matter what color it is. We conclude that the red algae are phylogenetically no better adapted to utilize the ambient light at great depth than their green counterparts. The ambient light conditions alone do not determine the limit for the vertical distribution of the red algae relative to the green algae.

Fluorescence quenching during photosynthesis and photoinhibition of Ulva rotundata blid.

The relationships between photoinhibition and photoprotection in high and low-light-grown Ulva were examined by a combination of chlorophyll-fluorescence-monitoring techniques. Tissues were exposed to a computer-controlled sequence of 5-min exposures to red light, followed by 5-min darkness, with stepwise increases in photon flux. Coefficients of chlorophyll fluorescence quenching (1−qP and NPQ) were calculated following a saturating pulse of white light near the end of each 5-min light treatment. Dark-adapted chlorophyll fluorescence parameters (F0 and FV/FM) were calculated from a saturating pulse at the end of each 5-min dark period. Low-light-grown Ulva showed consistently higher 1−qP, i.e. higher reduction status of Q (high primary acceptor of photosystem II), and lower capacity for nonphotochemical quenching (NPQ) at saturating light than did high-light-grown plants. Consequently, low-light plants rapidly displayed photoinhibitory damage (increased F0) at light saturation in seawater. Removal of dissolved inorganic carbon from seawater also led to photoinhibitory damage of high-light-grown Ulva at light saturation, and addition of saturating amounts of dissolved inorganic carbon protected low-light-grown plants against photoinhibitory damage. A large part of NPQ was abolished by treatment with 3 mM dithiothreitol and the processes so inhibited were evidently photoprotective, because dithiothreitol treatment accelerated photoinhibitory damage in both low- and high-light-grown Ulva. The extent of photoinhibitory damage in Ulva was exacerbated by treatment with chloramphenicol (1 mM) without much effect on chlorophyll-quenching parameters, evidently because this inhibitor of chloroplast protein synthesis reduced the rate of repair processes.

ALCIAN BLUE: A QUANTITATIVE AQUEOUS ASSAY FOR ALGAL ACID AND SULFATED POLYSACCHARIDES1

Alcian Blue, a cationic copper phthalocyanine dye, complexes with the anionic carboxyl and half‐ester sulfate groups of acidic algal polysaccharide in aqueous solution to form an insoluble precipitate. The quantity of dye removed from solution is proportional to the quantity of polyanion in solution, and this principle forms the basis for the quantitative determination of acid and/or sulfated algal polysaccharides. The assay is linear between 0 and 100 μg/ml agar, alginic acid, carrageenan, pectin and Porphyridium aerugineum Geit. polysaccharide. In addition, the technique is used to determine the anion density of acid polysaccharides on a molar or weight equivalency basis.