Intertidal Biofilms Research Articles

Patterns in microphytobenthic primary productivity: Species‐specific variation inmigratory rhythms and photosynthetic efficiency in mixed‐species biofilms

The importance of temporal changes in the vertical distribution of microphytobenthic algae on the overall functioning of intertidal biofilms were investigated with low‐temperature scanning electron microscopy and high‐resolution single‐cell fluorescence imaging of photosystem II efficiency (estimated by the fluorescence parameter F’q/F’m) in intact cores maintained in tidal mesocosms. Early morning biofilms consisted of smaller naviculoid and nitzschioid taxa or euglenoid species. By midday, Gyrosigma balticum and Pleurosigma angulatum were dominant. Some taxa (e.g., Plagiotropis vitrea) disappeared from surface layers after midday. Species composition continued to change toward the end of the photoperiod, with G. balticum dominating in diatom‐rich biofilms. In Euglena‐rich biofilms, initial dense surface films of euglenids became progressively dominated by smaller diatoms. F’q/F’m (measured at a photosynthetically active photon flux density (PPFD) of 220 µmol m−2 s−1) of individual cells of all taxa declined significantly after midday, but increased toward dusk. There were significant differences in F’q/F’m between species, particularly after midday. F’q/F’m versus irradiance curves and relative electron transport rate (rETRmax) showed higher efficiencies and rETRmax for euglenids, whereas G. balticum, Nitzschia dubia, and small Nitzschia sp. were shade‐adapted with low values of F’q/F’m, rETRmax, and Esat. G. balticum, P. vitrea, and N. dubia showed rapid vertical migration away from the surface with increasing irradiance. Euglenids, P. angulatum, and N. dubia exhibited their highest rETRmax values at midday. Esat for algal cells was between 500 and 600 µmol m−2 s−1, except for N. dubia and small Nitzschia sp., which had an Esat of 300 µmol m−2 s−1. Differences in behavioral and photophysiological traits between microphytobenthic taxa could be a form of niche separation and need to be incorporated into conceptual models of daily patterns of production in intertidal biofilms.

Relationship between bacterial community profile in biofilm and attachment of the acorn barnacle Balanus amphitrite

The relationship between bacterial community profile in biofilm and attachment of the acorn barnacle Balanus amphitrite was investigated using a double-dish choice larval attachment bioassay and the DNA fingerprinting technique T-RFLP (terminal-restriction fragment length poly- morphism). Biofilms for bioassays were either developed at 3 intertidal heights (i.e. high, mid and low) for 6 d or at the mid-intertidal height for 3 to 12 d. A clear distinction among biofilm communi- ties at the 3 intertidal heights was revealed in the bacterial community profiles (determined by T- RFLP), biomass (determined by total organic carbon analysis), and abundance of bacteria and diatoms. Overall, cyprids of B. amphitrite preferred intertidal biofilms (i.e. 6 d old) over unfilmed sur- faces for attachment. Moreover, cyprids also preferred to attach on biofilms of mid-intertidal height over high-intertidal or subtidal heights. There was no correlation between cypris attachment and any of the 3 biofilm attributes (i.e. biomass, abundance of bacteria and diatoms). Therefore, it was con- cluded that changes in the bacterial community profile in the biofilm affect the attractiveness of the biofilm to barnacle larvae and, thus, may determine the behavior (accepting or rejecting a surface) of settling larvae. We hypothesize that the temporal and spatial changes in the microbial community profile lead to the temporal and spatial recruitment pattern of marine invertebrates at a microscopic scale.

Adaptations of tropical marine microphytobenthic assemblages along a gradient of light and nutrient availability in Suva Lagoon, Fiji

How are microphytobenthic biofilms adapted to the high incident irradiances and temperatures, low inorganic nutrient concentrations and high desiccation stresses on intertidal flats present in tropical environments? This study investigated biofilms subject to different environmental conditions in a range of tropical sites in Suva lagoon, Fiji. PAM fluorescence was used to measure photophysiological responses to the light climate. Biofilm colloidal carbohydrate, extracellular polymeric substances (EPS) and low molecular weight (MW) carbohydrate concentrations and diel carbohydrate production patterns were measured. Average biomass (Chl a) ranged from 15 to 36 mg m−2, and was highest in seagrass bed sediments, but biomass was not correlated with water column or sediment porewater nutrient concentrations. Biofilm photophysiology differed significantly along a combined gradient of light and nutrient availability, with F v/F m, relative ETRmax and E k of biofilms highest in mangrove and intertidal main island sites and lowest in subtidal coral reef flats. Subtidal biofilms showed photoinhibition at irradiances > 1000 µmol m−2. Significant correlations between Chl a and colloidal carbohydrate concentrations were present (except on intertidal sandflats), and tropical biofilms had higher ratios of colloidal carbohydrate and EPS to Chl a than temperate estuarine biofilms, probably due to a combination of high irradiance and low nutrient availability leading to the production of excess photoassimilates. The percentage of EPS present in the colloidal fraction was highest in coral sand biofilms (42%), which had the lowest nutrient concentrations, compared with other sites (25–32%). Intertidal biofilms predominantly consisted of large motile taxa and showed strong rhythms of vertical migration. During tidal emersion, high sediment temperatures (41 °C), irradiance (>2300 µmol m−2 s−1) and salinity (49‰) stimulated downward migration. In silty sediments, migration resulted in a reduction in photosynthetic activity during the midday period but, in sands with high light penetration (to a depth of > 1700 µm), high production rates of EPS (18.2 µg carbo. µg Chl a−1 h−1) and low MW carbohydrate exudates (40.2 µg carbo. µg Chl a−1 h−1) occurred. Vertical migration, high E k and high rates of photoassimilate dumping are all adaptations to living in the tropical intertidal zone. Seagrass and reef flat biofilms consisted of a diverse non-migratory flora of motile and non-motile taxa that were not subject to such extreme temperature and irradiance conditions. Low values of photosynthetic parameters and high colloidal and EPS content indicated that these biofilms were nutrient-limited.