Light-Regulated Transcription of Genes Encoding Peridinin Chlorophyll a Proteins and the Major Intrinsic Light-Harvesting Complex Proteins in the Dinoflagellate Amphidinium carterae Hulburt (Dinophycae)1 : Changes in Cytosine Methylation Accompany Photoadaptation

In the dinoflagellate Amphidinium carterae, photoadaptation involves changes in the transcription of genes encoding both of the major classes of light-harvesting proteins, the peridinin chlorophyll a proteins (PCPs) and the major a/c-containing intrinsic light-harvesting proteins (LHCs). PCP and LHC transcript levels were increased up to 86- and 6-fold higher, respectively, under low-light conditions relative to cells grown at high illumination. These increases in transcript abundance were accompanied by decreases in the extent of methylation of CpG and CpNpG motifs within or near PCP- and LHC-coding regions. Cytosine methylation levels in A. carterae are therefore nonstatic and may vary with environmental conditions in a manner suggestive of involvement in the regulation of gene expression. However, chemically induced undermethylation was insufficient in activating transcription, because treatment with two methylation inhibitors had no effect on PCP mRNA or protein levels. Regulation of gene activity through changes in DNA methylation has traditionally been assumed to be restricted to higher eukaryotes (deuterostomes and green plants); however, the atypically large genomes of dinoflagellates may have generated the requirement for systems of this type in a relatively “primitive” organism. Dinoflagellates may therefore provide a unique perspective on the evolution of eukaryotic DNA-methylation systems.

Photosynthesis-irradiance response at physiological level: A mechanistic model

A mechanistic model is developed to present the photosynthetic response of phytoplankton to irradiance at the physiological level. The model is operated on photosynthetic units (PSU), and each PSU is assumed to have two states: reactive and activated. Light absorption that drives a reactive PSU into the activated state results from the effective absorption of the PSU. Transitions between the two states are asymmetrical in rate. A PSU in the reactive state becomes activated much faster than it recovers from the activated state to the reactive one. The turnover time for an activated PSU to transit into the reactive one is defined by the turnover time of the electron transport chain. The present model yields a photosynthesis-irradiance curve (PE-curve) in a hyperbola, which is described by three physiological parameters: effective cross-section (sigma (PSII)), turnover time of electron transport chain (tau) and number of PSUs (N). The PE-curve has an initial slope of sigma (PSII) x N, a half-saturated irradiance of 1/(sigma (PSII)), and a maximal photosynthetic rate of Nlc at the saturated irradiance. The PE-curve from the present model is comparable to the empirical function based on the target theory described by the Poisson distribution. (C) 2001 Academic Press.

Mechanisms contributing to the deep chlorophyll maximum in Lake Superior

The seasonal appearance of a deep chlorophyll maximum (DCM) in Lake Superior is a striking phenomenon that is widely observed; however its mechanisms of formation and maintenance are not well understood. As this phenomenon may be the reflection of an ecological driver, or a driver itself, a lack of understanding its driving forces limits the ability to accurately predict and manage changes in this ecosystem. Key mechanisms generally associated with DCM dynamics (i.e. ecological, physiological and physical phenomena) are examined individually and in concert to establish their role. First the prevailing paradigm, “the DCM is a great place to live”, is analyzed through an integration of the results of laboratory experiments and field measurements. The analysis indicates that growth at this depth is severely restricted and thus not able to explain the full magnitude of this phenomenon. Additional contributing mechanisms like photoadaptation, settling and grazing are reviewed with a one-dimensional mathematical model of chlorophyll and particulate organic carbon. Settling has the strongest impact on the formation and maintenance of the DCM, transporting biomass to the metalimnion and resulting in the accumulation of algae, i.e. a peak in the particulate organic carbon profile. Subsequently, shade adaptation becomes manifest as a chlorophyll maximum deeper in the water column where light conditions particularly favor the process. Shade adaptation mediates the magnitude, shape and vertical position of the chlorophyll peak. Growth at DCM depth shows only a marginal contribution, while grazing has an adverse effect on the extent of the DCM. The observed separation of the carbon biomass and chlorophyll maximum should caution scientists to equate the DCM with a large nutrient pool that is available to higher trophic levels. The ecological significance of the DCM should not be separated from the underlying carbon dynamics. When evaluated in its entirety, the DCM becomes the projected image of a structure that remains elusive to measure but represents the foundation of all higher trophic levels. These results also offer guidance in examine ecosystem perturbations such as climate change. For example, warming would be expected to prolong the period of thermal stratification, extending the late summer period of suboptimal (phosphorus-limited) growth and attendant transport of phytoplankton to the metalimnion. This reduction in epilimnetic algal production would decrease the supply of algae to the metalimnion, possibly reducing the supply of prey to the grazer community. This work demonstrates the value of modeling to challenge and advance our understanding of ecosystem dynamics, steps vital to reliable testing of management alternatives.

Variation in Coral Photosynthesis, Respiration and Growth Characteristics in Contrasting Light Microhabitats: An Analogue to Plants in Forest Gaps and Understoreys?

1. The often complex architecture of coral reefs forms a diversity of light microhabitats. Analogous to patterns in forest plants, light variation may drive strategies for efficient light utilization and metabolism in corals. 2. We investigated the spatial distribution of light regimes in a spur-and-groove reef environment and examine the photophysiology of the coral Montipora monasteriata (Forskal 1775), a species with a wide habitat distribution. Specifically, we examined the variation in tissue and skeletal thickness, and photosynthetic and metabolic responses among contrasting light microhabitats. 3. Daily irradiances reaching corals in caves and under overhangs were 1-5 and 30-40% of those in open habitats at similar depth (3-5 m), respectively. Daily rates of net photosynthesis of corals in cave habitats approximated zero, suggesting more than two orders of magnitude variation in scope for growth across habitats. 4. Three mechanisms of photoadaptation or acclimation were observed in cave and overhang habitats: (1) a 20-50% thinner tissue layer and 40-60% thinner skeletal plates, maximizing light interception per unit mass; (2) a two- to threefold higher photosynthetic efficiency per unit biomass; and (3) low rates of dark respiration. 5. Specimens from open and cave habitats displayed a high capacity to acclimate to downshifts or upshifts in irradiance, respectively. However, specimens in caves displayed limited acclimation to further irradiance reduction, indicating that these live near their irradiance limit. 6. Analogous to patterns for some plant species in forest gaps, the morphological plasticity and physiological flexibility of M. monasteriata enable it to occupy light habitats that vary by more than two orders of magnitude.