engleski

Observations of planktonic Nitzschia and Navicula from the South Adriatic Sea

Members of the genera Nitzschia Hassall and Navicula Bory are well-known pennate diatoms, however, their occurrence in the marine plankton is rarely reported. These diatoms usually adhere to surfaces with their raphe system and do not have specific morphological adaptations to planktonic existence. Although there are several species described from the water column, e.g. Navicula directa (W. Sm.) Bréb. or Nitzschia bicapitata Cleve their reported abundances are usually very low and there were not many studies dealing with these taxa. In routine observations of marine samples these diatoms are generally lumped into a large group of “unknown pennates” and there are scarce data available on their taxonomy or distribution. During BIOTA (Bio-tracing Adriatic Water Masses) cruise conducted in March 2016 in open waters of Southern Adriatic Pit we cultured 18 strains of small pennate diatoms obtained from phytoplankton net samples (surface water) and from seawater samples collected at range of different depths (30m, 100m, 150m, 250m, 400m). All cultured strains were investigated with the same methodological approach – examination of material under light (LM), scanning (SEM) and transmission (TEM) electron microscopy. A unique hallmark of all cultured strains was their lightly silicified frustule, making it difficult to discern them under light microscope. However, we were able to group our strains into three different Nitzschia and two Navicula morphotypes. Morphotypes varied in size (from about 20 to 25μm), shape of the valves (linearlanceolate, sigmoid or asymmetrical with/without capitate poles) and specific perforation pattern within striae. The poster shows LM, SEM and TEM observations and morphometric details of five pennate taxa. Some of these show the resemblance with the cosmopolitan species such as Nitzschia commutata Grunow, but our preliminary morphological analyses revealed different discernible features. The presence of these diatoms in the plankton and their origin will be also discussed in context of possible adhering on marine snow particles or epiphytic living on frustules or setae of other diatoms such as Rhizosolenia Brightw. or Chaetoceros Ehrenb. and Bacteriastrum Shadbolt, respectively.

diatoms ; taxonomy ; morphology ; cultured strains

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