Diatoms, which are early autotrophic colonisers, are an important constituent of the biofouling community in the marine environment. The effects of substratum and temporal variations on the fouling diatom community structure in a monsoon-influenced tropical estuary were studied. Fibreglass and glass coupons were exposed every month for a period of 4 days and the diatom population sampled at 24 h intervals, over a period of 14 months. The planktonic diatom community structure differed from the biofilm community. Pennate diatoms dominated the biofilms whilst centric diatoms were dominant in the water column. Among the biofilm diatoms, species belonging to the genera Navicula, Amphora, Nitzschia, Pleurosigma and Thalassionema were dominant. On certain occasions, the influence of planktonic blooms was also seen on the biofilm community. A comparative study of biofilms formed on the two substrata revealed significant differences in density and diversity. However species composition was almost constant. In addition to substratum variations, the biofilm diatom community structure also showed significant seasonal variations, which were attributed to physico-chemical and biological changes in both the water and substratum. Temporal variations in the tychopelagic diatoms of the water were also observed to exert an influence on the biofilm diatom community. Variations in diatom communities may determine the functional ecosystem of the benthic environment.

Natural biofilms constitute a complex network of microorganisms (bacteria, diatoms, protozoa, fungi) and their extracellular polymeric substances (EPS), which influence settlement in benthic invertebrates. We investigated the influence of diatom (Bacillariophyceae) films and EPS (>1000 molecular weight) on metamorphosis in the acorn barnacle Balanus amphitrite Darwin, a dominant fouling organism, using axenic and non-axenic films, and free and biofilm EPS, of 5 species of pennate diatoms: Amphora coffeaeformis and A. rostrata, and Navicula transitans var. derasa f. delicatula, N. crucicula and N. subinflata. Fourier-transformed infrared spectroscopy (FTIR) spectra revealed that the EPS produced by diatoms are of similar nature; however, the proportion of monosaccharides varies with species, indicating a potential role for influencing larval metamorphosis. Free EPS of axenic diatoms had no effect, whereas biofilm EPS induced larvae to metamorphose. Amphora spp. produced more biofilm EPS, but had relatively weaker effects than Navicula spp. Axenic diatom films also facilitated larval metamorphosis (depending on species and cell density) in some cases, suggesting that the cues provided by the diatoms themselves can also mediate invertebrate larval metamorphosis in the absence of microbial films. Non-axenic diatom films (which had higher cell densities) and biofilm EPS promoted metamorphosis to a greater degree than those of axenic diatoms. Enhancement of metamorphosis depended on diatom species and on their density in the films, as well as on the composition of their EPS. Differential responses of barnacle larvae to different diatom species and their EPS indicate that each diatom species provides a different set of physico-chemical signals to settling larvae. KEY WORDS: Diatoms · Bacillariophyceae · Extracellular polymeric substances · Biofilms · Larvae · Balanus amphitriteResale or republication not permitted without written consent of the publisher

Temporal variations in the diatom benthic propagule (DBP) community and its role in the phytoplankton community in a monsoon-affected tropical estuary, Zuari estuary, Goa (India) is presented. The DBP from the sediments was enumerated using an extinction dilution method (Most Probable Number method), which allows estimation of resting stages through examination of germinated vegetative cells in culture. The DBP community was dominated by planktonic species belonging to the genera Skeletonema, Fragilariopsis, Thalassiosira and Chaetoceros. Benthic propagules (BP) of S. costatum and Fragilariopsis sp. were dominant throughout the year. Between these two species, only S. costatum showed a linear relationship between the BP and planktonic cells indicating that this species is particularly important in coupling of pelagic and benthic ecosystems. During the onset and restart of monsoon after an intermittent break, water column was stratified, with a low salinity layer arising from riverine discharge and precipitation at the surface and relatively cold, saline, low oxygen waters at the bottom. Stratification favored blooming of S. costatum and Fragilariopsis sp. in nutrient rich surface and bottom waters respectively. The decline in these blooms ensuing nitrate depletion and salinity change resulted in an increased abundance of BP. Chaetoceros bloom was observed during the monsoon break as well as during nonmonsoon period and on both the occasions the decline in bloom was coupled with freshwater discharge. During the non-monsoon season, Thalassiosira blooms were encountered subsequent to high nitrate inputs. These findings suggest that in such shallow tropical regions, physical processes during monsoon (freshwater discharge) and non-monsoon seasons (currents, waves and tides) cause resuspension of diatom BP. Since light is not a limiting factor for germination in such regions, the blooming of resuspended BP depends upon nutrient availability.

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