Living on Cold Substrata: New Insights and Approaches in the Study of Microphytobenthos Ecophysiology and Ecology in Kongsfjorden

Organisms in shallow waters at high latitudes are under pressure due to climate change. These areas are typically inhabited by microphytobenthos (MPB) communities, composed mainly of diatoms. Only sparse information is available on the ecophysiology and acclimation processes within MPBs from Arctic regions. The physico-chemical environment and the ecology and ecophysiology of benthic diatoms in Kongsfjorden (Svalbard, Norway) are addressed in this review. MPB biofilms cover extensive areas of sediment. They show high rates of primary production, stabilise sediment surfaces against erosion under hydrodynamic forces,and affect the exchange of oxygen and nutrients across the sediment-water interface. Additionally, this phototrophic community represents a key component in the functioning of the Kongsfjorden trophic web, particularly as a major food source for benthic suspension- or deposit-feeders. MPB in Kongsfjorden is confronted with pronounced seasonal variations in solar radiation, low temperatures, and hyposaline (meltwater) conditions in summer, as well as long periods of ice and snow cover in winter. From the few data available, it seems that these organisms can easily cope with these environmental extremes. The underlying physiological mechanisms that allow growth and photosynthesis to continue under widely varying abiotic parameters, along with vertical migration and heterotrophy, and biochemical features such as a pronounced fatty-acid metabolism and silicate incorporation are discussed. Existing gaps in our knowledge of benthic diatoms in Kongsfjorden, such as the chemical ecology of biotic interactions, need to be filled. In addition, since many of the underlying molecular acclimation mechanisms are poorly understood, modern approaches based on transcriptomics, proteomics, and/or metabolomics, in conjunction with cell biological and biochemical techniques, are urgently needed. Climate change models for the Arctic predict other multifactorial stressors, such as an increase in precipitation and permafrost thawing, with consequences for the shallow-water regions. Both precipitation and permafrost thawing are likely to increase nutrient-enriched, turbid freshwater runoff and may locally counteract the expected increase in coastal radiation availability. So far, complex interactions among factors, as well as the full genetic diversity and physiological plasticity of Arctic benthic diatoms, have only rarely been considered. The limited existing information is described and discussed in this review

Primary production and spatial distribution of subtidal microphytobenthos in a temperate coastal system, the Bay of Brest, France

The main objective of this study was to define the primary production and the spatial and temporal distribution of the subtidal microphytobenthic community in a temperate coastal ecosystem, the Bay of Brest. The productivity of the microphytobenthos (MPB) was estimated in winter, spring and late summer, by a series of in situ benthic chamber incubations. Oxygen (O-2) and dissolved inorganic carbon (DIC) fluxes were measured at the sediment-water interface in light and dark conditions to determine the net and gross primary production present. Functional regression of the O-2 and DIC data demonstrated that the community photosynthetic quotient (CPQ) for the benthic community was 1. A maximal gross production (P-max) of between 0.4 and 0.8 mmol O-2 mg chl a(-1) h(-1) was estimated for the MPB in the Bay. E-K values were low, ranging from 57.8 to 83.4 mu mol photons m(-2) s(-1) and can be considered an adaptation of the MPB to the reduced light levels reaching the sediment-water interface. Two sampling campaigns were undertaken in winter and late summer to measure the biomass of the benthic microalgal community in the Bay. Productivity estimates were combined with this biomass to give the MPB production in all areas of the Bay. Principal components analysis revealed that stations sampled were grouped primarily as a function of their depth, highlighting the importance above all of light availability, and their sediment type, with highest biomass concentrations found in bare muddy sediments. Hierarchical classification allowed the determination of four groups of stations in the Bay defined by their biotic and abiotic differences. The importance of the gastropod Crepidulafornicata in conditioning the benthic structural and biochemical environment was also highlighted. Geographical information systems based mapping allowed the representation of the spatial and temporal distribution of biomass and primary production and consequently a determination of the overall MPB production. Average seasonal production estimates for the Bay of Brest ranged from 57 in winter to 111 mg C m(-2) day(-1) in late summer and represented from 12-20% of total primary production