Janet E. Kübler

NEW LIGHT ON THE SCALING OF METABOLIC RATE WITH THE SIZE OF ALGAE

The scaling of metabolic rate with the size of algae has been discussed and researched at length. The observation that algae usually have exponents b in the equation R = a·W−b (where R is the specific growth rate, W is the organism [cell] biomass, and a and b are constants) equal to or higher than the value of −0.25 for many other organisms is generally related to resource‐saturated (maximal) values of R. Recent work has shown that the exponent b for light‐limited growth is more negative than −0.25. This was predicted from considerations of the package effect in photon absorption, as modulated by the volume‐specific pigment content of the cells, and the photosynthetic unit size. Further work is needed to extrapolate these findings to fluctuating light environments. This minireview puts the recent work into a broader context and suggests how further work could quantify the roles of optical thickness and of spatial and temporal variations in the radiation field in determining metabolic rates.

FERTILIZATION SUCCESS CAN DRIVE PATTERNS OF PHASE DOMINANCE IN COMPLEX LIFE HISTORIES1

Many algal life cycles alternate between two free‐living generations. Life histories in which the two generations look identical (isomorphic) are common, particularly in the Rhodophyta. Reports of natural populations dominated by one generation of the life history have sought explanation in terms of phase‐specific differences in mortality and reproductive output, yet in many cases identification of these adaptations has been elusive or inconsistent with predictions. We hypothesized that the gametophyte‐to‐sporophyte ratio of ecologically equivalent isomorphs could result from variation in fertilization rate. We developed two models to test this hypothesis: one representing a generalized isomorphic life history and the other specific to red algae with a Polysiphonia‐type life history. Fertilization rate affected the gametophyte‐to‐sporophyte ratio, especially at low fertilization rates. In the general model, gametophytes dominated the population regardless of fertilization rate unless egg production greatly exceeded meiospore production. In the red algal model, phase dominance depended on the combination of fertilization rate and the number of carpospores produced per fertilization. The generational composition of model multiphasic algal populations results from their inherent reproductive characteristics and the dynamic environment to which fertilization and mortality rates are tied.

Spatial distribution and reproductive phenology of sexual and asexual Mastocarpus papillatus (Rhodophyta)

Fierst J.L., Kübler J.E. and Dudgeon S.R. 2010. Spatial distribution and reproductive phenology of sexual and asexual Mastocarpus papillatus (Rhodophyta). Phycologia 49: 274–282. DOI: 10.2216/09-41.1 Species of the genus Mastocarpus exhibit two distinct life cycles, a sexual alternation of generations and an obligate, asexual direct life cycle that produces only female upright fronds. In the intertidal red alga, M. papillatus (Kützing) sexual fronds dominate southern populations and asexual fronds dominate northern populations along the northeast Pacific coast, a pattern of spatial separation called geographic parthenogenesis. Along the central coast of California, sexual and asexual variants occur in mixed populations, but it is not known whether they are spatially separated within the intertidal zone at a given site. We investigated reproductive phenologies and analyzed patterns of spatial distributions of sexual and asexual M. papillatus at three sites in this region. Sexual M. papillatus were aggregated lower on the shore at two sites and only reproduced during part of a year, while asexual M. papillatus occurred throughout the intertidal range at all sites and reproduced throughout the year. The distribution patterns of sexual and asexual M. papillatus are consistent with a hypothesis of shoreline topography influencing their dynamics of dispersal and colonization. Spatial and temporal partitioning may contribute to the long-term coexistence of sexual and asexual life histories in this, and other, species of Mastocarpus. The occurrence of geographic parthenogenesis at multiple spatial scales in M. papillatus provides an opportunity to gain insight into the phenomenon.

Predicting Effects of Ocean Acidification and Warming on Algae Lacking Carbon Concentrating Mechanisms

Seaweeds that lack carbon-concentrating mechanisms are potentially inorganic carbon-limited under current air equilibrium conditions. To estimate effects of increased atmospheric carbon dioxide concentration and ocean acidification on photosynthetic rates, we modeled rates of photosynthesis in response to pCO2, temperature, and their interaction under limiting and saturating photon flux densities. We synthesized the available data for photosynthetic responses of red seaweeds lacking carbon-concentrating mechanisms to light and temperature. The model was parameterized with published data and known carbonate system dynamics. The model predicts that direction and magnitude of response to pCO2 and temperature, depend on photon flux density. At sub-saturating light intensities, photosynthetic rates are predicted to be low and respond positively to increasing pCO2, and negatively to increasing temperature. Consequently, pCO2 and temperature are predicted to interact antagonistically to influence photosynthetic rates at low PFD. The model predicts that pCO2 will have a much larger effect than temperature at sub-saturating light intensities. However, photosynthetic rates under low light will not increase proportionately as pCO2 in seawater continues to rise. In the range of light saturation (Ik), both CO2 and temperature have positive effects on photosynthetic rate and correspondingly strong predicted synergistic effects. At saturating light intensities, the response of photosynthetic rates to increasing pCO2 approaches linearity, but the model also predicts increased importance of thermal over pCO2 effects, with effects acting additively. Increasing boundary layer thickness decreased the effect of added pCO2 and, for very thick boundary layers, overwhelmed the effect of temperature on photosynthetic rates. The maximum photosynthetic rates of strictly CO2-using algae are low, so even large percentage increases in rates with climate change will not contribute much to changing primary production in the habitats where they commonly live.

Hydrozoans and the Shape of Things to Come

The physiological mechanisms that regulate adaptive plasticity of clonal organisms are key to their success in changing environments. Here, we review the mechanisms that regulate morphological plasticity of colonial hydrozoans. There is a heritable, genetic basis to colony form, but environmentally-induced plasticity and self-reinforcing developmental physiology explain much of total phenotypic variance. Morphological development of colonial hydrozoans emerges from interactions among (1) behaviors which drive gastrovascular transport, (2) architecture of the gastrovascular system that determines hydrodynamic characteristics of vascular flow, and, (3) gene products that vary in response to physiological signals provided by gastrovascular transport. Several morphogenetic signaling mechanisms have been identified, including, reactive oxygen species and nutrient concentrations in the hydroplasm, and hydromechanical forces associated with gastrovascular transport. We present a conceptual model of the interacting forces that drive hydrozoan morphological development. Several avenues for future research are suggested by the synthesis of information from prior studies of hydrozoans. Elucidating the morphogenetic signaling pathways responsive to metabolites or hydromechanical forces and the epigenetic effect of vascular architecture on colony form may give new insight into the self-maintenance of indeterminately growing and continuously developing vascular systems.

Growth, ammonium metabolism, and photosynthetic properties of Ulva australis (Chlorophyta) under decreasing pH and ammonium enrichment

The responses of macroalgae to ocean acidification could be altered by availability of macronutrients, such as ammonium (NH4+). This study determined how the opportunistic macroalga, Ulva australis responded to simultaneous changes in decreasing pH and NH4+ enrichment. This was investigated in a week-long growth experiment across a range of predicted future pHs with ambient and enriched NH4+ treatments followed by measurements of relative growth rates (RGR), NH4+ uptake rates and pools, total chlorophyll, and tissue carbon and nitrogen content. Rapid light curves (RLCs) were used to measure the maximum relative electron transport rate (rETRmax) and maximum quantum yield of photosystem II (PSII) photochemistry (Fv/Fm). Photosynthetic capacity was derived from the RLCs and included the efficiency of light harvesting (α), slope of photoinhibition (β), and the light saturation point (Ek). The results showed that NH4+ enrichment did not modify the effects of pH on RGRs, NH4+ uptake rates and pools, total chlorophyll, rETRmax, α, β, Fv/Fm, tissue C and N, and the C:N ratio. However, Ek was differentially affected by pH under different NH4+ treatments. Ek increased with decreasing pH in the ambient NH4+ treatment, but not in the enriched NH4+ treatment. NH4+ enrichment increased RGRs, NH4+ pools, total chlorophyll, rETRmax, α, β, Fv/Fm, and tissue N, and decreased NH4+ uptake rates and the C:N ratio. Decreased pH increased total chlorophyll content, rETRmax, Fv/Fm, and tissue N content, and decreased the C:N ratio. Therefore, the results indicate that U. australis growth is increased with NH4+ enrichment and not with decreasing pH. While decreasing pH influenced the carbon and nitrogen metabolisms of U. australis, it did not result in changes in growth.