Effects of silicon-limitation on growth and morphology of Triparma laevis NIES-2565 (Parmales, Heterokontophyta)

Rice husk as a potential source of silicate to oceanic phytoplankton

Global oceans are witnessing changes in the phytoplankton community composition due to various environmental stressors such as rising temperature, stratification, nutrient limitation, and ocean acidification. The Arabian Sea is undergoing changes in its phytoplankton community composition, especially during winter, with the diatoms being replaced by harmful algal blooms (HABs) of dinoflagellates. Recent studies have already highlighted dissolved silicate (DSi) limitation and change in Silicon (Si)/Nitrogen (N) ratios as the factors responsible for the observed changes in the phytoplankton community in the Arabian Sea. Our investigation also revealed Si/N < 1 in the northern Arabian Sea, indicating DSi limitation, especially during winter. Here, we demonstrate that rice husk with its phytoliths is an important source of bioavailable DSi for oceanic phytoplankton. Our experiment showed that a rice husk can release ∼12 μM of DSi in 15 days and can release DSi for ∼20 days. The DSi availability increased diatom abundance up to ∼9 times. The major benefitted diatom species from DSi enrichment were Nitzshia spp., Striatella spp., Navicula spp., Dactiliosolen spp., and Leptocylindrus spp. The increase in diatom abundance was accompanied by an increase in fucoxanthin and dimethyl sulphide (DMS), an anti-greenhouse gas. Thus, the rice husk with its buoyancy and slow DSi release has the potential to reduce HABs, and increase diatoms and fishery resources in addition to carbon dioxide (CO₂) sequestration in DSi-limited oceanic regions such as the Arabian Sea. Rice husk if released at the formation site of the Subantarctic mode water in the Southern Ocean could supply DSi to the thermocline in the global oceans thereby increasing diatom blooms and consequently the biotic carbon sequestration potential of the entire ocean.

Biogenic silica accumulation in picoeukaryotes: Novel players in the marine silica cycle

It is well known that the biological control of oceanic silica cycling is dominated by diatoms, with sponges and radiolarians playing additional roles. Recent studies have revealed that some smaller marine organisms (e.g. the picocyanobacterium Synechococcus) also take up silicic acid (dissolved silica, dSi) and accumulate silica, despite not exhibiting silicon dependent cellular structures. Here, we show biogenic silica (bSi) accumulation in five strains of picoeukaryotes (<2-3 μm), including three novel isolates from the Baltic Sea, and two marine species (Ostreococcus tauri and Micromonas commoda), in cultures grown with added dSi (100 μM). Average bSi accumulation in these novel biosilicifiers was between 30 and 92 amol Si cell^-1 . Growth rate and cell size of the picoeukaryotes were not affected by dSi addition. Still, the purpose of bSi accumulation in these smaller eukaryotic organisms lacking silicon dependent structures remains unclear. In line with the increasing recognition of picoeukaryotes in biogeochemical cycling, our findings suggest that they can also play a significant role in silica cycling.

Deciphering the diversity and distribution of chromophytic phytoplankton in the Bohai Sea and the Yellow Sea via RuBisCO genes (rbcL)

Present study investigated composition and distribution of chromophytic phytoplankton in the Bohai Sea (BS) and the Yellow Sea (YS) by using rbcL genes. Bacillariophyceae, Haptophyceae and Pelagophyceae were the most abundant phytoplankton groups. Distinct phytoplankton communities were observed in the BS and the YS: offshore stations were dominated by bloom forming genera Thalassiosira and Skeletonema, while brown tide-forming species including Chrysochromulina spp. and Aureococcus anophagefferens were commonly found in the nearshore areas. Redundancy analysis showed that phosphate, temperature and silicic acid play key roles in structuring chromophytic phytoplankton, such as phytoplankton at nearshore stations were affected by nutrient runoff from adjacent rivers (Yellow River). Anthropogenic activities in the Bohai Sea and seasonal circulation of ocean currents may also contribute to shaping chromophytic phytoplankton communities. This study provides data support and foundational observations of chromophytic phytoplankton in the BS and the YS, and their responses to environmental gradients and human activities.

Evolution of Phytoplankton in Relation to Their Physiological Traits

Defining the physiological traits that characterise phytoplankton involves comparison with related organisms in benthic habitats. Comparison of survival time in darkness under natural conditions requires more information. Gas vesicles and flagella as mechanisms of upward movement relative to surrounding water, allowing periodic vertical migration, are not confined to plankton, although buoyancy changes related to compositional changes of a large central vacuole may be restricted to plankton. Benthic microalgae have the same range of photosynthetic pigments as phytoplankton; it is not clear if there are differences in the rate of regulation and acclimation of photosynthetic machinery to variations in irradiance for phytoplankton and for microphytobenthos. There are inadequate data to determine if responses to variations in frequency or magnitude of changes in the supply of inorganic carbon, nitrogen or phosphorus differ between phytoplankton and benthic microalgae. Phagophotomixotrophy and osmophotomixotrophy occur in both phytoplankton and benthic microalgae. Further progress in identifying physiological traits specific to phytoplankton requires more experimentation on benthic microalgae that are closely related to planktonic microalgae, with attention to whether the benthic algae examined have, as far as can be determined, never been planktonic during their evolution or are derived from planktonic ancestors.

Harmful algal blooms under changing climate and constantly increasing anthropogenic actions: the review of management implications

The present review reports all management approaches (physical, chemical, and biological) traditionally adopted in mitigating the global impact of harmful cyanobacterial blooms (cyanoHABs). It recognizes that each mitigation strategy shows characteristic associated limitations and notes that no remedial step has provided a sustainable solution to HABs on a global scale. It emphasizes that the putative anthropogenic N&P inputs reduction through improved wastewater treatment and regulation of point and non-point sources-agricultural fertilizers only offer a short term solution. These approaches are rather preventive than curative hence, do not address concerns relating to the recovery of already-eutrophic and hypereutrophic systems. It raises new concerns on the implications of non-agricultural pollutants such as hydrocarbon fractions in bloom accretions often neglected while addressing HAB triggers. It also accesses the global impacts of HABs as it pertains to socio-economic implications in the geographically diverse world. It, therefore, proposes that Integrated Management Intervention involving the merging of two or more mitigation steps be administered across the aquatic continua as a prudent management solution to complement the current N&P dual management paradigm. It stresses that the contemporaneous adoption of management options with both preventive and curative measures is a key to sustainable HAB management. This review provides sufficient advances and current scenarios for approaching cyanoHABs. Further, it advocates that future research perspectives tackle the mitigation design beyond the short-term nutrient regulations and the parochial attention to the point and non-point N&P input sources.

Ontogenetic analysis of siliceous cell wall formation in Triparma laevis f. inornata (Parmales, Stramenopiles)

Triparma laevis f. inornata is a unicellular alga belonging to the Bolidophyceae, which is most closely related to diatoms. Like diatoms, T. laevis f. inornata has a siliceous cell wall. The cell wall of T. laevis f. inornata consists of four round plates (three shields and one ventral plate) and one dorsal and three girdle plates. But, unlike diatoms, T. laevis f. inornata cells can grow when concentrations of silica are depleted. We took advantage of this ability, using TEM to study the ontogeny of the siliceous plate, pattern center formation, and development. Two types of pattern centers (annulus and sternum) were observed in the early and middle stage of plate formation. During their formation, the annuli were initially crescent-shaped but eventually their ends fused to make a ring. Only outward silica deposition of the branching ribs occurred on the growing annulus until it became a ring, resulting in an unfilled circle inside the annulus. The pattern center of the shield plate was always an annulus, but in ventral plates both annulus and sternum were observed. The annuli and sterna in T. laevis f. inornata round plates were very similar to the annuli and sterna in diatom valves. These results suggested that the round plates of Parmales are homologous to diatom valves. This information on the plate ontogeny of T. laevis f. inornata provides new insights into the evolution of the siliceous cell wall in the Parmales and diatoms.

Acquisition and metabolism of carbon in the Ochrophyta other than diatoms

The acquisition and assimilation of inorganic C have been investigated in several of the 15 clades of the Ochrophyta other than diatoms, with biochemical, physiological and genomic data indicating significant mechanistic variation. Form ID Rubiscos in the Ochrophyta are characterized by a broad range of kinetics values. In spite of relatively high K0.5CO2 and low CO2 : O2 selectivity, diffusive entry of CO2 occurs in the Chrysophyceae and Synurophyceae. Eustigmatophyceae and Phaeophyceae, on the contrary, have CO2 concentrating mechanisms, usually involving the direct or indirect use of [Formula: see text] This variability is possibly due to the ecological contexts of the organism. In brown algae, C fixation generally takes place through a classical C3 metabolism, but there are some hints of the occurrence of C4 metabolism and low amplitude CAM in a few members of the Fucales. Genomic data show the presence of a number of potential C4 and CAM genes in Ochrophyta other than diatoms, but the other core functions of many of these genes give a very limited diagnostic value to their presence and are insufficient to conclude that C4 photosynthesis is present in these algae.This article is part of the themed issue 'The peculiar carbon metabolism in diatoms'.

Screening and Expression of a Silicon Transporter Gene (Lsi1) in Wild-Type Indica Rice Cultivars

Silicon (Si) is one of the most prevalent elements in the soil. It is beneficial for plant growth and development, and it contributes to plant defense against different stresses. The Lsi1 gene encodes a Si transporter that was identified in a mutant Japonica rice variety. This gene was not identified in fourteen Malaysian rice varieties during screening. Then, a mutant version of Lsi1 was substituted for the native version in the three most common Malaysian rice varieties, MR219, MR220, and MR276, to evaluate the function of the transgene. Real-time PCR was used to explore the differential expression of Lsi1 in the three transgenic rice varieties. Silicon concentrations in the roots and leaves of transgenic plants were significantly higher than in wild-type plants. Transgenic varieties showed significant increases in the activities of the enzymes SOD, POD, APX, and CAT; photosynthesis; and chlorophyll content; however, the highest chlorophyll A and B levels were observed in transgenic MR276. Transgenic varieties have shown a stronger root and leaf structure, as well as hairier roots, compared to the wild-type plants. This suggests that Lsi1 plays a key role in rice, increasing the absorption and accumulation of Si, then alters antioxidant activities, and improves morphological properties.

Mitotic spindle formation in Triparma laevis NIES-2565(Parmales, Heterokontophyta)

The parmalean algae possess a siliceous wall and represent the sister lineage of diatoms; they are thought to be a key group for understanding the evolution of diatoms. Diatoms possess well-characterized and unique mitotic structures, but the mitotic apparatus of Parmales is still unknown. We observed the microtubule (MT) array during interphase and mitosis in Triparma laevis using TEM. The interphase cells had four or five centrioles (∼80 nm in length), from which MTs emanated toward the cytoplasm. In prophase, the bundle of MTs arose at an extranuclear site. The position of centrioles with respect to an MT bundle changed during its elongation. Centrioles were observed on the lateral side of a shorter MT bundle (∼590 nm) and on either side of an extended MT bundle (∼700 nm). In metaphase, the spindle consisted of two types of MTs-MT bundle that passed through a cytoplasmic tunnel in the center of the nucleus and single MTs (possibly kinetochore MTs) that extended from the poles into the nucleus. The nuclear envelope disappeared only at the regions where the kinetochore MTs penetrated. In telophase, daughter chromosomes migrated toward opposite poles, and the MT bundle was observed between segregating chromosomes. These observations showed that MT nucleation does not always occur at the periphery of centrioles through cell cycle and that the spindle of T. laevis has a similar configuration to that of diatoms.

A possible mechanism of biological silicification in plants

Plants are significant exponents of biological silicification. While not all plants are generally considered as biosilicifiers the extent to which all plants deposit biogenic silica is largely unknown. There are plants which are known as silica accumulators though even in these plants the extent and degree to which their tissues are silicified is neither appreciated nor understood. An elucidation of the mechanism of silicification in biota is complicated by a lack of known bio-organic chemistry of silicic acid, the starting point in this process. Herein I argue the case that biological silicification is an entirely passive process. It is passive from the point of view that its underlying mechanisms and processes do not require us to invoke any as yet undiscovered silicon biochemistry. It is also passive in that although silicification confers clear biological/ecological advantages under certain conditions, it is actually non-essential in all plants and potentially, at least, toxic in some.